1224 lines
48 KiB
ReStructuredText
1224 lines
48 KiB
ReStructuredText
..
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If Passes.html is up to date, the following "one-liner" should print
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an empty diff.
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egrep -e '^<tr><td><a href="#.*">-.*</a></td><td>.*</td></tr>$' \
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-e '^ <a name=".*">.*</a>$' < Passes.html >html; \
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perl >help <<'EOT' && diff -u help html; rm -f help html
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open HTML, "<Passes.html" or die "open: Passes.html: $!\n";
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while (<HTML>) {
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m:^<tr><td><a href="#(.*)">-.*</a></td><td>.*</td></tr>$: or next;
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$order{$1} = sprintf("%03d", 1 + int %order);
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}
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open HELP, "../Release/bin/opt -help|" or die "open: opt -help: $!\n";
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while (<HELP>) {
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m:^ -([^ ]+) +- (.*)$: or next;
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my $o = $order{$1};
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$o = "000" unless defined $o;
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push @x, "$o<tr><td><a href=\"#$1\">-$1</a></td><td>$2</td></tr>\n";
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push @y, "$o <a name=\"$1\">-$1: $2</a>\n";
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}
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@x = map { s/^\d\d\d//; $_ } sort @x;
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@y = map { s/^\d\d\d//; $_ } sort @y;
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print @x, @y;
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EOT
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This (real) one-liner can also be helpful when converting comments to HTML:
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perl -e '$/ = undef; for (split(/\n/, <>)) { s:^ *///? ?::; print " <p>\n" if !$on && $_ =~ /\S/; print " </p>\n" if $on && $_ =~ /^\s*$/; print " $_\n"; $on = ($_ =~ /\S/); } print " </p>\n" if $on'
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====================================
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LLVM's Analysis and Transform Passes
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====================================
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.. contents::
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:local:
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Introduction
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============
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This document serves as a high level summary of the optimization features that
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LLVM provides. Optimizations are implemented as Passes that traverse some
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portion of a program to either collect information or transform the program.
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The table below divides the passes that LLVM provides into three categories.
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Analysis passes compute information that other passes can use or for debugging
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or program visualization purposes. Transform passes can use (or invalidate)
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the analysis passes. Transform passes all mutate the program in some way.
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Utility passes provides some utility but don't otherwise fit categorization.
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For example passes to extract functions to bitcode or write a module to bitcode
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are neither analysis nor transform passes. The table of contents above
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provides a quick summary of each pass and links to the more complete pass
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description later in the document.
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Analysis Passes
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===============
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This section describes the LLVM Analysis Passes.
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``-aa-eval``: Exhaustive Alias Analysis Precision Evaluator
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-----------------------------------------------------------
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This is a simple N^2 alias analysis accuracy evaluator. Basically, for each
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function in the program, it simply queries to see how the alias analysis
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implementation answers alias queries between each pair of pointers in the
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function.
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This is inspired and adapted from code by: Naveen Neelakantam, Francesco
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Spadini, and Wojciech Stryjewski.
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``-basic-aa``: Basic Alias Analysis (stateless AA impl)
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-------------------------------------------------------
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A basic alias analysis pass that implements identities (two different globals
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cannot alias, etc), but does no stateful analysis.
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``-basiccg``: Basic CallGraph Construction
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------------------------------------------
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Yet to be written.
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``-count-aa``: Count Alias Analysis Query Responses
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---------------------------------------------------
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A pass which can be used to count how many alias queries are being made and how
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the alias analysis implementation being used responds.
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.. _passes-da:
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``-da``: Dependence Analysis
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----------------------------
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Dependence analysis framework, which is used to detect dependences in memory
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accesses.
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``-debug-aa``: AA use debugger
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------------------------------
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This simple pass checks alias analysis users to ensure that if they create a
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new value, they do not query AA without informing it of the value. It acts as
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a shim over any other AA pass you want.
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Yes keeping track of every value in the program is expensive, but this is a
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debugging pass.
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``-domfrontier``: Dominance Frontier Construction
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-------------------------------------------------
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This pass is a simple dominator construction algorithm for finding forward
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dominator frontiers.
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``-domtree``: Dominator Tree Construction
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-----------------------------------------
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This pass is a simple dominator construction algorithm for finding forward
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dominators.
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``-dot-callgraph``: Print Call Graph to "dot" file
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--------------------------------------------------
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This pass, only available in ``opt``, prints the call graph into a ``.dot``
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graph. This graph can then be processed with the "dot" tool to convert it to
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postscript or some other suitable format.
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``-dot-cfg``: Print CFG of function to "dot" file
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-------------------------------------------------
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This pass, only available in ``opt``, prints the control flow graph into a
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``.dot`` graph. This graph can then be processed with the :program:`dot` tool
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to convert it to postscript or some other suitable format.
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``-dot-cfg-only``: Print CFG of function to "dot" file (with no function bodies)
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--------------------------------------------------------------------------------
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This pass, only available in ``opt``, prints the control flow graph into a
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``.dot`` graph, omitting the function bodies. This graph can then be processed
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with the :program:`dot` tool to convert it to postscript or some other suitable
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format.
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``-dot-dom``: Print dominance tree of function to "dot" file
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------------------------------------------------------------
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This pass, only available in ``opt``, prints the dominator tree into a ``.dot``
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graph. This graph can then be processed with the :program:`dot` tool to
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convert it to postscript or some other suitable format.
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``-dot-dom-only``: Print dominance tree of function to "dot" file (with no function bodies)
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-------------------------------------------------------------------------------------------
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This pass, only available in ``opt``, prints the dominator tree into a ``.dot``
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graph, omitting the function bodies. This graph can then be processed with the
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:program:`dot` tool to convert it to postscript or some other suitable format.
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``-dot-postdom``: Print postdominance tree of function to "dot" file
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--------------------------------------------------------------------
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This pass, only available in ``opt``, prints the post dominator tree into a
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``.dot`` graph. This graph can then be processed with the :program:`dot` tool
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to convert it to postscript or some other suitable format.
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``-dot-postdom-only``: Print postdominance tree of function to "dot" file (with no function bodies)
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---------------------------------------------------------------------------------------------------
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This pass, only available in ``opt``, prints the post dominator tree into a
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``.dot`` graph, omitting the function bodies. This graph can then be processed
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with the :program:`dot` tool to convert it to postscript or some other suitable
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format.
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``-globalsmodref-aa``: Simple mod/ref analysis for globals
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----------------------------------------------------------
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This simple pass provides alias and mod/ref information for global values that
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do not have their address taken, and keeps track of whether functions read or
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write memory (are "pure"). For this simple (but very common) case, we can
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provide pretty accurate and useful information.
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``-instcount``: Counts the various types of ``Instruction``\ s
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--------------------------------------------------------------
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This pass collects the count of all instructions and reports them.
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``-intervals``: Interval Partition Construction
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-----------------------------------------------
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This analysis calculates and represents the interval partition of a function,
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or a preexisting interval partition.
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In this way, the interval partition may be used to reduce a flow graph down to
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its degenerate single node interval partition (unless it is irreducible).
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``-iv-users``: Induction Variable Users
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---------------------------------------
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Bookkeeping for "interesting" users of expressions computed from induction
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variables.
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``-lazy-value-info``: Lazy Value Information Analysis
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-----------------------------------------------------
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Interface for lazy computation of value constraint information.
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``-libcall-aa``: LibCall Alias Analysis
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---------------------------------------
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LibCall Alias Analysis.
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``-lint``: Statically lint-checks LLVM IR
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-----------------------------------------
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This pass statically checks for common and easily-identified constructs which
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produce undefined or likely unintended behavior in LLVM IR.
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It is not a guarantee of correctness, in two ways. First, it isn't
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comprehensive. There are checks which could be done statically which are not
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yet implemented. Some of these are indicated by TODO comments, but those
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aren't comprehensive either. Second, many conditions cannot be checked
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statically. This pass does no dynamic instrumentation, so it can't check for
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all possible problems.
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Another limitation is that it assumes all code will be executed. A store
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through a null pointer in a basic block which is never reached is harmless, but
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this pass will warn about it anyway.
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Optimization passes may make conditions that this pass checks for more or less
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obvious. If an optimization pass appears to be introducing a warning, it may
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be that the optimization pass is merely exposing an existing condition in the
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code.
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This code may be run before :ref:`instcombine <passes-instcombine>`. In many
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cases, instcombine checks for the same kinds of things and turns instructions
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with undefined behavior into unreachable (or equivalent). Because of this,
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this pass makes some effort to look through bitcasts and so on.
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``-loops``: Natural Loop Information
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------------------------------------
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This analysis is used to identify natural loops and determine the loop depth of
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various nodes of the CFG. Note that the loops identified may actually be
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several natural loops that share the same header node... not just a single
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natural loop.
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``-memdep``: Memory Dependence Analysis
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---------------------------------------
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An analysis that determines, for a given memory operation, what preceding
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memory operations it depends on. It builds on alias analysis information, and
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tries to provide a lazy, caching interface to a common kind of alias
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information query.
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``-module-debuginfo``: Decodes module-level debug info
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------------------------------------------------------
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This pass decodes the debug info metadata in a module and prints in a
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(sufficiently-prepared-) human-readable form.
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For example, run this pass from ``opt`` along with the ``-analyze`` option, and
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it'll print to standard output.
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``-postdomfrontier``: Post-Dominance Frontier Construction
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----------------------------------------------------------
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This pass is a simple post-dominator construction algorithm for finding
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post-dominator frontiers.
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``-postdomtree``: Post-Dominator Tree Construction
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--------------------------------------------------
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This pass is a simple post-dominator construction algorithm for finding
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post-dominators.
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``-print-alias-sets``: Alias Set Printer
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----------------------------------------
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Yet to be written.
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``-print-callgraph``: Print a call graph
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----------------------------------------
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This pass, only available in ``opt``, prints the call graph to standard error
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in a human-readable form.
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``-print-callgraph-sccs``: Print SCCs of the Call Graph
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-------------------------------------------------------
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This pass, only available in ``opt``, prints the SCCs of the call graph to
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standard error in a human-readable form.
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``-print-cfg-sccs``: Print SCCs of each function CFG
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----------------------------------------------------
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This pass, only available in ``opt``, printsthe SCCs of each function CFG to
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standard error in a human-readable fom.
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``-print-dom-info``: Dominator Info Printer
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-------------------------------------------
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Dominator Info Printer.
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``-print-externalfnconstants``: Print external fn callsites passed constants
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----------------------------------------------------------------------------
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This pass, only available in ``opt``, prints out call sites to external
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functions that are called with constant arguments. This can be useful when
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looking for standard library functions we should constant fold or handle in
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alias analyses.
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``-print-function``: Print function to stderr
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---------------------------------------------
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The ``PrintFunctionPass`` class is designed to be pipelined with other
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``FunctionPasses``, and prints out the functions of the module as they are
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processed.
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``-print-module``: Print module to stderr
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-----------------------------------------
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This pass simply prints out the entire module when it is executed.
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.. _passes-print-used-types:
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``-print-used-types``: Find Used Types
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--------------------------------------
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This pass is used to seek out all of the types in use by the program. Note
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that this analysis explicitly does not include types only used by the symbol
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table.
|
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``-regions``: Detect single entry single exit regions
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-----------------------------------------------------
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The ``RegionInfo`` pass detects single entry single exit regions in a function,
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where a region is defined as any subgraph that is connected to the remaining
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graph at only two spots. Furthermore, a hierarchical region tree is built.
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.. _passes-scalar-evolution:
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``-scalar-evolution``: Scalar Evolution Analysis
|
|
------------------------------------------------
|
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The ``ScalarEvolution`` analysis can be used to analyze and catagorize scalar
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expressions in loops. It specializes in recognizing general induction
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variables, representing them with the abstract and opaque ``SCEV`` class.
|
|
Given this analysis, trip counts of loops and other important properties can be
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|
obtained.
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This analysis is primarily useful for induction variable substitution and
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strength reduction.
|
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``-scev-aa``: ScalarEvolution-based Alias Analysis
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|
--------------------------------------------------
|
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Simple alias analysis implemented in terms of ``ScalarEvolution`` queries.
|
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This differs from traditional loop dependence analysis in that it tests for
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dependencies within a single iteration of a loop, rather than dependencies
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between different iterations.
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``ScalarEvolution`` has a more complete understanding of pointer arithmetic
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than ``BasicAliasAnalysis``' collection of ad-hoc analyses.
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``-stack-safety``: Stack Safety Analysis
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|
------------------------------------------------
|
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The ``StackSafety`` analysis can be used to determine if stack allocated
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|
variables can be considered safe from memory access bugs.
|
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This analysis' primary purpose is to be used by sanitizers to avoid unnecessary
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instrumentation of safe variables.
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``-targetdata``: Target Data Layout
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-----------------------------------
|
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Provides other passes access to information on how the size and alignment
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required by the target ABI for various data types.
|
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|
|
Transform Passes
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|
================
|
|
|
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This section describes the LLVM Transform Passes.
|
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|
|
``-adce``: Aggressive Dead Code Elimination
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-------------------------------------------
|
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ADCE aggressively tries to eliminate code. This pass is similar to :ref:`DCE
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|
<passes-dce>` but it assumes that values are dead until proven otherwise. This
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|
is similar to :ref:`SCCP <passes-sccp>`, except applied to the liveness of
|
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values.
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``-always-inline``: Inliner for ``always_inline`` functions
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-----------------------------------------------------------
|
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A custom inliner that handles only functions that are marked as "always
|
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inline".
|
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``-argpromotion``: Promote 'by reference' arguments to scalars
|
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--------------------------------------------------------------
|
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|
This pass promotes "by reference" arguments to be "by value" arguments. In
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practice, this means looking for internal functions that have pointer
|
|
arguments. If it can prove, through the use of alias analysis, that an
|
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argument is *only* loaded, then it can pass the value into the function instead
|
|
of the address of the value. This can cause recursive simplification of code
|
|
and lead to the elimination of allocas (especially in C++ template code like
|
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the STL).
|
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This pass also handles aggregate arguments that are passed into a function,
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|
scalarizing them if the elements of the aggregate are only loaded. Note that
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it refuses to scalarize aggregates which would require passing in more than
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three operands to the function, because passing thousands of operands for a
|
|
large array or structure is unprofitable!
|
|
|
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Note that this transformation could also be done for arguments that are only
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|
stored to (returning the value instead), but does not currently. This case
|
|
would be best handled when and if LLVM starts supporting multiple return values
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from functions.
|
|
|
|
``-bb-vectorize``: Basic-Block Vectorization
|
|
--------------------------------------------
|
|
|
|
This pass combines instructions inside basic blocks to form vector
|
|
instructions. It iterates over each basic block, attempting to pair compatible
|
|
instructions, repeating this process until no additional pairs are selected for
|
|
vectorization. When the outputs of some pair of compatible instructions are
|
|
used as inputs by some other pair of compatible instructions, those pairs are
|
|
part of a potential vectorization chain. Instruction pairs are only fused into
|
|
vector instructions when they are part of a chain longer than some threshold
|
|
length. Moreover, the pass attempts to find the best possible chain for each
|
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pair of compatible instructions. These heuristics are intended to prevent
|
|
vectorization in cases where it would not yield a performance increase of the
|
|
resulting code.
|
|
|
|
``-block-placement``: Profile Guided Basic Block Placement
|
|
----------------------------------------------------------
|
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|
|
This pass is a very simple profile guided basic block placement algorithm. The
|
|
idea is to put frequently executed blocks together at the start of the function
|
|
and hopefully increase the number of fall-through conditional branches. If
|
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there is no profile information for a particular function, this pass basically
|
|
orders blocks in depth-first order.
|
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|
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``-break-crit-edges``: Break critical edges in CFG
|
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--------------------------------------------------
|
|
|
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Break all of the critical edges in the CFG by inserting a dummy basic block.
|
|
It may be "required" by passes that cannot deal with critical edges. This
|
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transformation obviously invalidates the CFG, but can update forward dominator
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(set, immediate dominators, tree, and frontier) information.
|
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``-codegenprepare``: Optimize for code generation
|
|
-------------------------------------------------
|
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|
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This pass munges the code in the input function to better prepare it for
|
|
SelectionDAG-based code generation. This works around limitations in its
|
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basic-block-at-a-time approach. It should eventually be removed.
|
|
|
|
``-constmerge``: Merge Duplicate Global Constants
|
|
-------------------------------------------------
|
|
|
|
Merges duplicate global constants together into a single constant that is
|
|
shared. This is useful because some passes (i.e., TraceValues) insert a lot of
|
|
string constants into the program, regardless of whether or not an existing
|
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string is available.
|
|
|
|
.. _passes-dce:
|
|
|
|
``-dce``: Dead Code Elimination
|
|
-------------------------------
|
|
|
|
Dead code elimination is similar to :ref:`dead instruction elimination
|
|
<passes-die>`, but it rechecks instructions that were used by removed
|
|
instructions to see if they are newly dead.
|
|
|
|
``-deadargelim``: Dead Argument Elimination
|
|
-------------------------------------------
|
|
|
|
This pass deletes dead arguments from internal functions. Dead argument
|
|
elimination removes arguments which are directly dead, as well as arguments
|
|
only passed into function calls as dead arguments of other functions. This
|
|
pass also deletes dead arguments in a similar way.
|
|
|
|
This pass is often useful as a cleanup pass to run after aggressive
|
|
interprocedural passes, which add possibly-dead arguments.
|
|
|
|
``-deadtypeelim``: Dead Type Elimination
|
|
----------------------------------------
|
|
|
|
This pass is used to cleanup the output of GCC. It eliminate names for types
|
|
that are unused in the entire translation unit, using the :ref:`find used types
|
|
<passes-print-used-types>` pass.
|
|
|
|
.. _passes-die:
|
|
|
|
``-die``: Dead Instruction Elimination
|
|
--------------------------------------
|
|
|
|
Dead instruction elimination performs a single pass over the function, removing
|
|
instructions that are obviously dead.
|
|
|
|
``-dse``: Dead Store Elimination
|
|
--------------------------------
|
|
|
|
A trivial dead store elimination that only considers basic-block local
|
|
redundant stores.
|
|
|
|
.. _passes-function-attrs:
|
|
|
|
``-function-attrs``: Deduce function attributes
|
|
-----------------------------------------------
|
|
|
|
A simple interprocedural pass which walks the call-graph, looking for functions
|
|
which do not access or only read non-local memory, and marking them
|
|
``readnone``/``readonly``. In addition, it marks function arguments (of
|
|
pointer type) "``nocapture``" if a call to the function does not create any
|
|
copies of the pointer value that outlive the call. This more or less means
|
|
that the pointer is only dereferenced, and not returned from the function or
|
|
stored in a global. This pass is implemented as a bottom-up traversal of the
|
|
call-graph.
|
|
|
|
``-globaldce``: Dead Global Elimination
|
|
---------------------------------------
|
|
|
|
This transform is designed to eliminate unreachable internal globals from the
|
|
program. It uses an aggressive algorithm, searching out globals that are known
|
|
to be alive. After it finds all of the globals which are needed, it deletes
|
|
whatever is left over. This allows it to delete recursive chunks of the
|
|
program which are unreachable.
|
|
|
|
``-globalopt``: Global Variable Optimizer
|
|
-----------------------------------------
|
|
|
|
This pass transforms simple global variables that never have their address
|
|
taken. If obviously true, it marks read/write globals as constant, deletes
|
|
variables only stored to, etc.
|
|
|
|
``-gvn``: Global Value Numbering
|
|
--------------------------------
|
|
|
|
This pass performs global value numbering to eliminate fully and partially
|
|
redundant instructions. It also performs redundant load elimination.
|
|
|
|
.. _passes-indvars:
|
|
|
|
``-indvars``: Canonicalize Induction Variables
|
|
----------------------------------------------
|
|
|
|
This transformation analyzes and transforms the induction variables (and
|
|
computations derived from them) into simpler forms suitable for subsequent
|
|
analysis and transformation.
|
|
|
|
This transformation makes the following changes to each loop with an
|
|
identifiable induction variable:
|
|
|
|
* All loops are transformed to have a *single* canonical induction variable
|
|
which starts at zero and steps by one.
|
|
* The canonical induction variable is guaranteed to be the first PHI node in
|
|
the loop header block.
|
|
* Any pointer arithmetic recurrences are raised to use array subscripts.
|
|
|
|
If the trip count of a loop is computable, this pass also makes the following
|
|
changes:
|
|
|
|
* The exit condition for the loop is canonicalized to compare the induction
|
|
value against the exit value. This turns loops like:
|
|
|
|
.. code-block:: c++
|
|
|
|
for (i = 7; i*i < 1000; ++i)
|
|
|
|
into
|
|
|
|
.. code-block:: c++
|
|
|
|
for (i = 0; i != 25; ++i)
|
|
|
|
* Any use outside of the loop of an expression derived from the indvar is
|
|
changed to compute the derived value outside of the loop, eliminating the
|
|
dependence on the exit value of the induction variable. If the only purpose
|
|
of the loop is to compute the exit value of some derived expression, this
|
|
transformation will make the loop dead.
|
|
|
|
This transformation should be followed by strength reduction after all of the
|
|
desired loop transformations have been performed. Additionally, on targets
|
|
where it is profitable, the loop could be transformed to count down to zero
|
|
(the "do loop" optimization).
|
|
|
|
``-inline``: Function Integration/Inlining
|
|
------------------------------------------
|
|
|
|
Bottom-up inlining of functions into callees.
|
|
|
|
.. _passes-instcombine:
|
|
|
|
``-instcombine``: Combine redundant instructions
|
|
------------------------------------------------
|
|
|
|
Combine instructions to form fewer, simple instructions. This pass does not
|
|
modify the CFG. This pass is where algebraic simplification happens.
|
|
|
|
This pass combines things like:
|
|
|
|
.. code-block:: llvm
|
|
|
|
%Y = add i32 %X, 1
|
|
%Z = add i32 %Y, 1
|
|
|
|
into:
|
|
|
|
.. code-block:: llvm
|
|
|
|
%Z = add i32 %X, 2
|
|
|
|
This is a simple worklist driven algorithm.
|
|
|
|
This pass guarantees that the following canonicalizations are performed on the
|
|
program:
|
|
|
|
#. If a binary operator has a constant operand, it is moved to the right-hand
|
|
side.
|
|
#. Bitwise operators with constant operands are always grouped so that shifts
|
|
are performed first, then ``or``\ s, then ``and``\ s, then ``xor``\ s.
|
|
#. Compare instructions are converted from ``<``, ``>``, ``≤``, or ``≥`` to
|
|
``=`` or ``≠`` if possible.
|
|
#. All ``cmp`` instructions on boolean values are replaced with logical
|
|
operations.
|
|
#. ``add X, X`` is represented as ``mul X, 2`` ⇒ ``shl X, 1``
|
|
#. Multiplies with a constant power-of-two argument are transformed into
|
|
shifts.
|
|
#. … etc.
|
|
|
|
This pass can also simplify calls to specific well-known function calls (e.g.
|
|
runtime library functions). For example, a call ``exit(3)`` that occurs within
|
|
the ``main()`` function can be transformed into simply ``return 3``. Whether or
|
|
not library calls are simplified is controlled by the
|
|
:ref:`-function-attrs <passes-function-attrs>` pass and LLVM's knowledge of
|
|
library calls on different targets.
|
|
|
|
.. _passes-aggressive-instcombine:
|
|
|
|
``-aggressive-instcombine``: Combine expression patterns
|
|
--------------------------------------------------------
|
|
|
|
Combine expression patterns to form expressions with fewer, simple instructions.
|
|
This pass does not modify the CFG.
|
|
|
|
For example, this pass reduce width of expressions post-dominated by TruncInst
|
|
into smaller width when applicable.
|
|
|
|
It differs from instcombine pass in that it contains pattern optimization that
|
|
requires higher complexity than the O(1), thus, it should run fewer times than
|
|
instcombine pass.
|
|
|
|
``-internalize``: Internalize Global Symbols
|
|
--------------------------------------------
|
|
|
|
This pass loops over all of the functions in the input module, looking for a
|
|
main function. If a main function is found, all other functions and all global
|
|
variables with initializers are marked as internal.
|
|
|
|
``-ipsccp``: Interprocedural Sparse Conditional Constant Propagation
|
|
--------------------------------------------------------------------
|
|
|
|
An interprocedural variant of :ref:`Sparse Conditional Constant Propagation
|
|
<passes-sccp>`.
|
|
|
|
``-jump-threading``: Jump Threading
|
|
-----------------------------------
|
|
|
|
Jump threading tries to find distinct threads of control flow running through a
|
|
basic block. This pass looks at blocks that have multiple predecessors and
|
|
multiple successors. If one or more of the predecessors of the block can be
|
|
proven to always cause a jump to one of the successors, we forward the edge
|
|
from the predecessor to the successor by duplicating the contents of this
|
|
block.
|
|
|
|
An example of when this can occur is code like this:
|
|
|
|
.. code-block:: c++
|
|
|
|
if () { ...
|
|
X = 4;
|
|
}
|
|
if (X < 3) {
|
|
|
|
In this case, the unconditional branch at the end of the first if can be
|
|
revectored to the false side of the second if.
|
|
|
|
.. _passes-lcssa:
|
|
|
|
``-lcssa``: Loop-Closed SSA Form Pass
|
|
-------------------------------------
|
|
|
|
This pass transforms loops by placing phi nodes at the end of the loops for all
|
|
values that are live across the loop boundary. For example, it turns the left
|
|
into the right code:
|
|
|
|
.. code-block:: c++
|
|
|
|
for (...) for (...)
|
|
if (c) if (c)
|
|
X1 = ... X1 = ...
|
|
else else
|
|
X2 = ... X2 = ...
|
|
X3 = phi(X1, X2) X3 = phi(X1, X2)
|
|
... = X3 + 4 X4 = phi(X3)
|
|
... = X4 + 4
|
|
|
|
This is still valid LLVM; the extra phi nodes are purely redundant, and will be
|
|
trivially eliminated by ``InstCombine``. The major benefit of this
|
|
transformation is that it makes many other loop optimizations, such as
|
|
``LoopUnswitch``\ ing, simpler. You can read more in the
|
|
:ref:`loop terminology section for the LCSSA form <loop-terminology-lcssa>`.
|
|
|
|
.. _passes-licm:
|
|
|
|
``-licm``: Loop Invariant Code Motion
|
|
-------------------------------------
|
|
|
|
This pass performs loop invariant code motion, attempting to remove as much
|
|
code from the body of a loop as possible. It does this by either hoisting code
|
|
into the preheader block, or by sinking code to the exit blocks if it is safe.
|
|
This pass also promotes must-aliased memory locations in the loop to live in
|
|
registers, thus hoisting and sinking "invariant" loads and stores.
|
|
|
|
Hoisting operations out of loops is a canonicalization transform. It enables
|
|
and simplifies subsequent optimizations in the middle-end. Rematerialization
|
|
of hoisted instructions to reduce register pressure is the responsibility of
|
|
the back-end, which has more accurate information about register pressure and
|
|
also handles other optimizations than LICM that increase live-ranges.
|
|
|
|
This pass uses alias analysis for two purposes:
|
|
|
|
#. Moving loop invariant loads and calls out of loops. If we can determine
|
|
that a load or call inside of a loop never aliases anything stored to, we
|
|
can hoist it or sink it like any other instruction.
|
|
|
|
#. Scalar Promotion of Memory. If there is a store instruction inside of the
|
|
loop, we try to move the store to happen AFTER the loop instead of inside of
|
|
the loop. This can only happen if a few conditions are true:
|
|
|
|
#. The pointer stored through is loop invariant.
|
|
#. There are no stores or loads in the loop which *may* alias the pointer.
|
|
There are no calls in the loop which mod/ref the pointer.
|
|
|
|
If these conditions are true, we can promote the loads and stores in the
|
|
loop of the pointer to use a temporary alloca'd variable. We then use the
|
|
:ref:`mem2reg <passes-mem2reg>` functionality to construct the appropriate
|
|
SSA form for the variable.
|
|
|
|
``-loop-deletion``: Delete dead loops
|
|
-------------------------------------
|
|
|
|
This file implements the Dead Loop Deletion Pass. This pass is responsible for
|
|
eliminating loops with non-infinite computable trip counts that have no side
|
|
effects or volatile instructions, and do not contribute to the computation of
|
|
the function's return value.
|
|
|
|
.. _passes-loop-extract:
|
|
|
|
``-loop-extract``: Extract loops into new functions
|
|
---------------------------------------------------
|
|
|
|
A pass wrapper around the ``ExtractLoop()`` scalar transformation to extract
|
|
each top-level loop into its own new function. If the loop is the *only* loop
|
|
in a given function, it is not touched. This is a pass most useful for
|
|
debugging via bugpoint.
|
|
|
|
``-loop-extract-single``: Extract at most one loop into a new function
|
|
----------------------------------------------------------------------
|
|
|
|
Similar to :ref:`Extract loops into new functions <passes-loop-extract>`, this
|
|
pass extracts one natural loop from the program into a function if it can.
|
|
This is used by :program:`bugpoint`.
|
|
|
|
``-loop-reduce``: Loop Strength Reduction
|
|
-----------------------------------------
|
|
|
|
This pass performs a strength reduction on array references inside loops that
|
|
have as one or more of their components the loop induction variable. This is
|
|
accomplished by creating a new value to hold the initial value of the array
|
|
access for the first iteration, and then creating a new GEP instruction in the
|
|
loop to increment the value by the appropriate amount.
|
|
|
|
.. _passes-loop-rotate:
|
|
|
|
``-loop-rotate``: Rotate Loops
|
|
------------------------------
|
|
|
|
A simple loop rotation transformation. A summary of it can be found in
|
|
:ref:`Loop Terminology for Rotated Loops <loop-terminology-loop-rotate>`.
|
|
|
|
|
|
.. _passes-loop-simplify:
|
|
|
|
``-loop-simplify``: Canonicalize natural loops
|
|
----------------------------------------------
|
|
|
|
This pass performs several transformations to transform natural loops into a
|
|
simpler form, which makes subsequent analyses and transformations simpler and
|
|
more effective. A summary of it can be found in
|
|
:ref:`Loop Terminology, Loop Simplify Form <loop-terminology-loop-simplify>`.
|
|
|
|
Loop pre-header insertion guarantees that there is a single, non-critical entry
|
|
edge from outside of the loop to the loop header. This simplifies a number of
|
|
analyses and transformations, such as :ref:`LICM <passes-licm>`.
|
|
|
|
Loop exit-block insertion guarantees that all exit blocks from the loop (blocks
|
|
which are outside of the loop that have predecessors inside of the loop) only
|
|
have predecessors from inside of the loop (and are thus dominated by the loop
|
|
header). This simplifies transformations such as store-sinking that are built
|
|
into LICM.
|
|
|
|
This pass also guarantees that loops will have exactly one backedge.
|
|
|
|
Note that the :ref:`simplifycfg <passes-simplifycfg>` pass will clean up blocks
|
|
which are split out but end up being unnecessary, so usage of this pass should
|
|
not pessimize generated code.
|
|
|
|
This pass obviously modifies the CFG, but updates loop information and
|
|
dominator information.
|
|
|
|
``-loop-unroll``: Unroll loops
|
|
------------------------------
|
|
|
|
This pass implements a simple loop unroller. It works best when loops have
|
|
been canonicalized by the :ref:`indvars <passes-indvars>` pass, allowing it to
|
|
determine the trip counts of loops easily.
|
|
|
|
``-loop-unroll-and-jam``: Unroll and Jam loops
|
|
----------------------------------------------
|
|
|
|
This pass implements a simple unroll and jam classical loop optimisation pass.
|
|
It transforms loop from:
|
|
|
|
.. code-block:: c++
|
|
|
|
for i.. i+= 1 for i.. i+= 4
|
|
for j.. for j..
|
|
code(i, j) code(i, j)
|
|
code(i+1, j)
|
|
code(i+2, j)
|
|
code(i+3, j)
|
|
remainder loop
|
|
|
|
Which can be seen as unrolling the outer loop and "jamming" (fusing) the inner
|
|
loops into one. When variables or loads can be shared in the new inner loop, this
|
|
can lead to significant performance improvements. It uses
|
|
:ref:`Dependence Analysis <passes-da>` for proving the transformations are safe.
|
|
|
|
.. _passes-loop-unswitch:
|
|
|
|
``-loop-unswitch``: Unswitch loops
|
|
----------------------------------
|
|
|
|
This pass transforms loops that contain branches on loop-invariant conditions
|
|
to have multiple loops. For example, it turns the left into the right code:
|
|
|
|
.. code-block:: c++
|
|
|
|
for (...) if (lic)
|
|
A for (...)
|
|
if (lic) A; B; C
|
|
B else
|
|
C for (...)
|
|
A; C
|
|
|
|
This can increase the size of the code exponentially (doubling it every time a
|
|
loop is unswitched) so we only unswitch if the resultant code will be smaller
|
|
than a threshold.
|
|
|
|
This pass expects :ref:`LICM <passes-licm>` to be run before it to hoist
|
|
invariant conditions out of the loop, to make the unswitching opportunity
|
|
obvious.
|
|
|
|
``-loweratomic``: Lower atomic intrinsics to non-atomic form
|
|
------------------------------------------------------------
|
|
|
|
This pass lowers atomic intrinsics to non-atomic form for use in a known
|
|
non-preemptible environment.
|
|
|
|
The pass does not verify that the environment is non-preemptible (in general
|
|
this would require knowledge of the entire call graph of the program including
|
|
any libraries which may not be available in bitcode form); it simply lowers
|
|
every atomic intrinsic.
|
|
|
|
``-lowerinvoke``: Lower invokes to calls, for unwindless code generators
|
|
------------------------------------------------------------------------
|
|
|
|
This transformation is designed for use by code generators which do not yet
|
|
support stack unwinding. This pass converts ``invoke`` instructions to
|
|
``call`` instructions, so that any exception-handling ``landingpad`` blocks
|
|
become dead code (which can be removed by running the ``-simplifycfg`` pass
|
|
afterwards).
|
|
|
|
``-lowerswitch``: Lower ``SwitchInst``\ s to branches
|
|
-----------------------------------------------------
|
|
|
|
Rewrites switch instructions with a sequence of branches, which allows targets
|
|
to get away with not implementing the switch instruction until it is
|
|
convenient.
|
|
|
|
.. _passes-mem2reg:
|
|
|
|
``-mem2reg``: Promote Memory to Register
|
|
----------------------------------------
|
|
|
|
This file promotes memory references to be register references. It promotes
|
|
alloca instructions which only have loads and stores as uses. An ``alloca`` is
|
|
transformed by using dominator frontiers to place phi nodes, then traversing
|
|
the function in depth-first order to rewrite loads and stores as appropriate.
|
|
This is just the standard SSA construction algorithm to construct "pruned" SSA
|
|
form.
|
|
|
|
``-memcpyopt``: MemCpy Optimization
|
|
-----------------------------------
|
|
|
|
This pass performs various transformations related to eliminating ``memcpy``
|
|
calls, or transforming sets of stores into ``memset``\ s.
|
|
|
|
``-mergefunc``: Merge Functions
|
|
-------------------------------
|
|
|
|
This pass looks for equivalent functions that are mergable and folds them.
|
|
|
|
Total-ordering is introduced among the functions set: we define comparison
|
|
that answers for every two functions which of them is greater. It allows to
|
|
arrange functions into the binary tree.
|
|
|
|
For every new function we check for equivalent in tree.
|
|
|
|
If equivalent exists we fold such functions. If both functions are overridable,
|
|
we move the functionality into a new internal function and leave two
|
|
overridable thunks to it.
|
|
|
|
If there is no equivalent, then we add this function to tree.
|
|
|
|
Lookup routine has O(log(n)) complexity, while whole merging process has
|
|
complexity of O(n*log(n)).
|
|
|
|
Read
|
|
:doc:`this <MergeFunctions>`
|
|
article for more details.
|
|
|
|
``-mergereturn``: Unify function exit nodes
|
|
-------------------------------------------
|
|
|
|
Ensure that functions have at most one ``ret`` instruction in them.
|
|
Additionally, it keeps track of which node is the new exit node of the CFG.
|
|
|
|
``-partial-inliner``: Partial Inliner
|
|
-------------------------------------
|
|
|
|
This pass performs partial inlining, typically by inlining an ``if`` statement
|
|
that surrounds the body of the function.
|
|
|
|
``-prune-eh``: Remove unused exception handling info
|
|
----------------------------------------------------
|
|
|
|
This file implements a simple interprocedural pass which walks the call-graph,
|
|
turning invoke instructions into call instructions if and only if the callee
|
|
cannot throw an exception. It implements this as a bottom-up traversal of the
|
|
call-graph.
|
|
|
|
``-reassociate``: Reassociate expressions
|
|
-----------------------------------------
|
|
|
|
This pass reassociates commutative expressions in an order that is designed to
|
|
promote better constant propagation, GCSE, :ref:`LICM <passes-licm>`, PRE, etc.
|
|
|
|
For example: 4 + (x + 5) ⇒ x + (4 + 5)
|
|
|
|
In the implementation of this algorithm, constants are assigned rank = 0,
|
|
function arguments are rank = 1, and other values are assigned ranks
|
|
corresponding to the reverse post order traversal of current function (starting
|
|
at 2), which effectively gives values in deep loops higher rank than values not
|
|
in loops.
|
|
|
|
``-reg2mem``: Demote all values to stack slots
|
|
----------------------------------------------
|
|
|
|
This file demotes all registers to memory references. It is intended to be the
|
|
inverse of :ref:`mem2reg <passes-mem2reg>`. By converting to ``load``
|
|
instructions, the only values live across basic blocks are ``alloca``
|
|
instructions and ``load`` instructions before ``phi`` nodes. It is intended
|
|
that this should make CFG hacking much easier. To make later hacking easier,
|
|
the entry block is split into two, such that all introduced ``alloca``
|
|
instructions (and nothing else) are in the entry block.
|
|
|
|
``-sroa``: Scalar Replacement of Aggregates
|
|
------------------------------------------------------
|
|
|
|
The well-known scalar replacement of aggregates transformation. This transform
|
|
breaks up ``alloca`` instructions of aggregate type (structure or array) into
|
|
individual ``alloca`` instructions for each member if possible. Then, if
|
|
possible, it transforms the individual ``alloca`` instructions into nice clean
|
|
scalar SSA form.
|
|
|
|
.. _passes-sccp:
|
|
|
|
``-sccp``: Sparse Conditional Constant Propagation
|
|
--------------------------------------------------
|
|
|
|
Sparse conditional constant propagation and merging, which can be summarized
|
|
as:
|
|
|
|
* Assumes values are constant unless proven otherwise
|
|
* Assumes BasicBlocks are dead unless proven otherwise
|
|
* Proves values to be constant, and replaces them with constants
|
|
* Proves conditional branches to be unconditional
|
|
|
|
Note that this pass has a habit of making definitions be dead. It is a good
|
|
idea to run a :ref:`DCE <passes-dce>` pass sometime after running this pass.
|
|
|
|
.. _passes-simplifycfg:
|
|
|
|
``-simplifycfg``: Simplify the CFG
|
|
----------------------------------
|
|
|
|
Performs dead code elimination and basic block merging. Specifically:
|
|
|
|
* Removes basic blocks with no predecessors.
|
|
* Merges a basic block into its predecessor if there is only one and the
|
|
predecessor only has one successor.
|
|
* Eliminates PHI nodes for basic blocks with a single predecessor.
|
|
* Eliminates a basic block that only contains an unconditional branch.
|
|
|
|
``-sink``: Code sinking
|
|
-----------------------
|
|
|
|
This pass moves instructions into successor blocks, when possible, so that they
|
|
aren't executed on paths where their results aren't needed.
|
|
|
|
``-strip``: Strip all symbols from a module
|
|
-------------------------------------------
|
|
|
|
Performs code stripping. This transformation can delete:
|
|
|
|
* names for virtual registers
|
|
* symbols for internal globals and functions
|
|
* debug information
|
|
|
|
Note that this transformation makes code much less readable, so it should only
|
|
be used in situations where the strip utility would be used, such as reducing
|
|
code size or making it harder to reverse engineer code.
|
|
|
|
``-strip-dead-debug-info``: Strip debug info for unused symbols
|
|
---------------------------------------------------------------
|
|
|
|
.. FIXME: this description is the same as for -strip
|
|
|
|
performs code stripping. this transformation can delete:
|
|
|
|
* names for virtual registers
|
|
* symbols for internal globals and functions
|
|
* debug information
|
|
|
|
note that this transformation makes code much less readable, so it should only
|
|
be used in situations where the strip utility would be used, such as reducing
|
|
code size or making it harder to reverse engineer code.
|
|
|
|
``-strip-dead-prototypes``: Strip Unused Function Prototypes
|
|
------------------------------------------------------------
|
|
|
|
This pass loops over all of the functions in the input module, looking for dead
|
|
declarations and removes them. Dead declarations are declarations of functions
|
|
for which no implementation is available (i.e., declarations for unused library
|
|
functions).
|
|
|
|
``-strip-debug-declare``: Strip all ``llvm.dbg.declare`` intrinsics
|
|
-------------------------------------------------------------------
|
|
|
|
.. FIXME: this description is the same as for -strip
|
|
|
|
This pass implements code stripping. Specifically, it can delete:
|
|
|
|
#. names for virtual registers
|
|
#. symbols for internal globals and functions
|
|
#. debug information
|
|
|
|
Note that this transformation makes code much less readable, so it should only
|
|
be used in situations where the 'strip' utility would be used, such as reducing
|
|
code size or making it harder to reverse engineer code.
|
|
|
|
``-strip-nondebug``: Strip all symbols, except dbg symbols, from a module
|
|
-------------------------------------------------------------------------
|
|
|
|
.. FIXME: this description is the same as for -strip
|
|
|
|
This pass implements code stripping. Specifically, it can delete:
|
|
|
|
#. names for virtual registers
|
|
#. symbols for internal globals and functions
|
|
#. debug information
|
|
|
|
Note that this transformation makes code much less readable, so it should only
|
|
be used in situations where the 'strip' utility would be used, such as reducing
|
|
code size or making it harder to reverse engineer code.
|
|
|
|
``-tailcallelim``: Tail Call Elimination
|
|
----------------------------------------
|
|
|
|
This file transforms calls of the current function (self recursion) followed by
|
|
a return instruction with a branch to the entry of the function, creating a
|
|
loop. This pass also implements the following extensions to the basic
|
|
algorithm:
|
|
|
|
#. Trivial instructions between the call and return do not prevent the
|
|
transformation from taking place, though currently the analysis cannot
|
|
support moving any really useful instructions (only dead ones).
|
|
#. This pass transforms functions that are prevented from being tail recursive
|
|
by an associative expression to use an accumulator variable, thus compiling
|
|
the typical naive factorial or fib implementation into efficient code.
|
|
#. TRE is performed if the function returns void, if the return returns the
|
|
result returned by the call, or if the function returns a run-time constant
|
|
on all exits from the function. It is possible, though unlikely, that the
|
|
return returns something else (like constant 0), and can still be TRE'd. It
|
|
can be TRE'd if *all other* return instructions in the function return the
|
|
exact same value.
|
|
#. If it can prove that callees do not access theier caller stack frame, they
|
|
are marked as eligible for tail call elimination (by the code generator).
|
|
|
|
Utility Passes
|
|
==============
|
|
|
|
This section describes the LLVM Utility Passes.
|
|
|
|
``-deadarghaX0r``: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)
|
|
------------------------------------------------------------------------
|
|
|
|
Same as dead argument elimination, but deletes arguments to functions which are
|
|
external. This is only for use by :doc:`bugpoint <Bugpoint>`.
|
|
|
|
``-extract-blocks``: Extract Basic Blocks From Module (for bugpoint use)
|
|
------------------------------------------------------------------------
|
|
|
|
This pass is used by bugpoint to extract all blocks from the module into their
|
|
own functions.
|
|
|
|
``-instnamer``: Assign names to anonymous instructions
|
|
------------------------------------------------------
|
|
|
|
This is a little utility pass that gives instructions names, this is mostly
|
|
useful when diffing the effect of an optimization because deleting an unnamed
|
|
instruction can change all other instruction numbering, making the diff very
|
|
noisy.
|
|
|
|
.. _passes-verify:
|
|
|
|
``-verify``: Module Verifier
|
|
----------------------------
|
|
|
|
Verifies an LLVM IR code. This is useful to run after an optimization which is
|
|
undergoing testing. Note that llvm-as verifies its input before emitting
|
|
bitcode, and also that malformed bitcode is likely to make LLVM crash. All
|
|
language front-ends are therefore encouraged to verify their output before
|
|
performing optimizing transformations.
|
|
|
|
#. Both of a binary operator's parameters are of the same type.
|
|
#. Verify that the indices of mem access instructions match other operands.
|
|
#. Verify that arithmetic and other things are only performed on first-class
|
|
types. Verify that shifts and logicals only happen on integrals f.e.
|
|
#. All of the constants in a switch statement are of the correct type.
|
|
#. The code is in valid SSA form.
|
|
#. It is illegal to put a label into any other type (like a structure) or to
|
|
return one.
|
|
#. Only phi nodes can be self referential: ``%x = add i32 %x``, ``%x`` is
|
|
invalid.
|
|
#. PHI nodes must have an entry for each predecessor, with no extras.
|
|
#. PHI nodes must be the first thing in a basic block, all grouped together.
|
|
#. PHI nodes must have at least one entry.
|
|
#. All basic blocks should only end with terminator insts, not contain them.
|
|
#. The entry node to a function must not have predecessors.
|
|
#. All Instructions must be embedded into a basic block.
|
|
#. Functions cannot take a void-typed parameter.
|
|
#. Verify that a function's argument list agrees with its declared type.
|
|
#. It is illegal to specify a name for a void value.
|
|
#. It is illegal to have an internal global value with no initializer.
|
|
#. It is illegal to have a ``ret`` instruction that returns a value that does
|
|
not agree with the function return value type.
|
|
#. Function call argument types match the function prototype.
|
|
#. All other things that are tested by asserts spread about the code.
|
|
|
|
Note that this does not provide full security verification (like Java), but
|
|
instead just tries to ensure that code is well-formed.
|
|
|
|
.. _passes-view-cfg:
|
|
|
|
``-view-cfg``: View CFG of function
|
|
-----------------------------------
|
|
|
|
Displays the control flow graph using the GraphViz tool.
|
|
|
|
``-view-cfg-only``: View CFG of function (with no function bodies)
|
|
------------------------------------------------------------------
|
|
|
|
Displays the control flow graph using the GraphViz tool, but omitting function
|
|
bodies.
|
|
|
|
``-view-dom``: View dominance tree of function
|
|
----------------------------------------------
|
|
|
|
Displays the dominator tree using the GraphViz tool.
|
|
|
|
``-view-dom-only``: View dominance tree of function (with no function bodies)
|
|
-----------------------------------------------------------------------------
|
|
|
|
Displays the dominator tree using the GraphViz tool, but omitting function
|
|
bodies.
|
|
|
|
``-view-postdom``: View postdominance tree of function
|
|
------------------------------------------------------
|
|
|
|
Displays the post dominator tree using the GraphViz tool.
|
|
|
|
``-view-postdom-only``: View postdominance tree of function (with no function bodies)
|
|
-------------------------------------------------------------------------------------
|
|
|
|
Displays the post dominator tree using the GraphViz tool, but omitting function
|
|
bodies.
|
|
|
|
``-transform-warning``: Report missed forced transformations
|
|
------------------------------------------------------------
|
|
|
|
Emits warnings about not yet applied forced transformations (e.g. from
|
|
``#pragma omp simd``).
|