101 lines
2.7 KiB
ReStructuredText
101 lines
2.7 KiB
ReStructuredText
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Known Bits Analysis
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===================
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The Known Bits Analysis pass makes information about the known values of bits
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available to other passes to enable transformations like those in the examples
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below. The information is lazily computed so you should only pay for what you
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use.
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Examples
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--------
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A simple example is that transforming::
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a + 1
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into::
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a | 1
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is only valid when the addition doesn't carry. In other words it's only valid
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if ``a & 1`` is zero.
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Another example is:
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.. code-block:: none
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%1:(s32) = G_CONSTANT i32 0xFF0
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%2:(s32) = G_AND %0, %1
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%3:(s32) = G_CONSTANT i32 0x0FF
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%4:(s32) = G_AND %2, %3
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We can use the constants and the definition of ``G_AND`` to determine the known
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bits:
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.. code-block:: none
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; %0 = 0x????????
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%1:(s32) = G_CONSTANT i32 0xFF0 ; %1 = 0x00000FF0
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%2:(s32) = G_AND %0, %1 ; %2 = 0x00000??0
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%3:(s32) = G_CONSTANT i32 0x0FF ; %3 = 0x000000FF
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%4:(s32) = G_AND %2, %3 ; %4 = 0x000000?0
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and then use this to simplify the expression:
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.. code-block:: none
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; %0 = 0x????????
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%5:(s32) = G_CONSTANT i32 0x0F0 ; %5 = 0x00000FF0
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%4:(s32) = G_AND %0, %5 ; %4 = 0x000000?0
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Note that ``%4`` still has the same known bits as before the transformation.
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Many transformations share this property. The main exception being when the
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transform causes undefined bits to become defined to either zero, one, or
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defined but unknown.
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Usage
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-----
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To use Known Bits Analysis in a pass, first include the header and register the
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dependency with ``INITIALIZE_PASS_DEPENDENCY``.
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.. code-block:: c++
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#include "llvm/CodeGen/GlobalISel/GISelKnownBits.h"
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...
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INITIALIZE_PASS_BEGIN(...)
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INITIALIZE_PASS_DEPENDENCY(GISelKnownBitsAnalysis)
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INITIALIZE_PASS_END(...)
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and require the pass in ``getAnalysisUsage``.
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.. code-block:: c++
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void MyPass::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<GISelKnownBitsAnalysis>();
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// Optional: If your pass preserves known bits analysis (many do) then
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// indicate that it's preserved for re-use by another pass here.
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AU.addPreserved<GISelKnownBitsAnalysis>();
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}
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Then it's just a matter of fetching the analysis and using it:
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.. code-block:: c++
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bool MyPass::runOnMachineFunction(MachineFunction &MF) {
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...
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GISelKnownBits &KB = getAnalysis<GISelKnownBitsAnalysis>().get(MF);
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...
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MachineInstr *MI = ...;
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KnownBits Known = KB->getKnownBits(MI->getOperand(0).getReg());
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if (Known.Zeros & 1) {
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// Bit 0 is known to be zero
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}
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...
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}
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There are many more API's beyond ``getKnownBits()``. See the `API reference
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<https://llvm.org/doxygen>`_ for more information
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