80 lines
3.3 KiB
C++
80 lines
3.3 KiB
C++
//===- SimpleLoopUnswitch.h - Hoist loop-invariant control flow -*- C++ -*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_TRANSFORMS_SCALAR_SIMPLELOOPUNSWITCH_H
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#define LLVM_TRANSFORMS_SCALAR_SIMPLELOOPUNSWITCH_H
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#include "llvm/Analysis/LoopAnalysisManager.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/IR/PassManager.h"
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#include "llvm/Transforms/Scalar/LoopPassManager.h"
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namespace llvm {
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/// This pass transforms loops that contain branches or switches on loop-
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/// invariant conditions to have multiple loops. For example, it turns the left
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/// into the right code:
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///
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/// for (...) if (lic)
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/// A for (...)
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/// if (lic) A; B; C
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/// B else
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/// C for (...)
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/// A; C
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///
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/// This can increase the size of the code exponentially (doubling it every time
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/// a loop is unswitched) so we only unswitch if the resultant code will be
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/// smaller than a threshold.
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///
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/// This pass expects LICM to be run before it to hoist invariant conditions out
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/// of the loop, to make the unswitching opportunity obvious.
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///
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/// There is a taxonomy of unswitching that we use to classify different forms
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/// of this transformaiton:
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///
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/// - Trival unswitching: this is when the condition can be unswitched without
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/// cloning any code from inside the loop. A non-trivial unswitch requires
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/// code duplication.
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///
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/// - Full unswitching: this is when the branch or switch is completely moved
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/// from inside the loop to outside the loop. Partial unswitching removes the
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/// branch from the clone of the loop but must leave a (somewhat simplified)
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/// branch in the original loop. While theoretically partial unswitching can
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/// be done for switches, the requirements are extreme - we need the loop
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/// invariant input to the switch to be sufficient to collapse to a single
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/// successor in each clone.
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///
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/// This pass always does trivial, full unswitching for both branches and
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/// switches. For branches, it also always does trivial, partial unswitching.
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///
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/// If enabled (via the constructor's `NonTrivial` parameter), this pass will
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/// additionally do non-trivial, full unswitching for branches and switches, and
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/// will do non-trivial, partial unswitching for branches.
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///
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/// Because partial unswitching of switches is extremely unlikely to be possible
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/// in practice and significantly complicates the implementation, this pass does
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/// not currently implement that in any mode.
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class SimpleLoopUnswitchPass : public PassInfoMixin<SimpleLoopUnswitchPass> {
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bool NonTrivial;
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public:
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SimpleLoopUnswitchPass(bool NonTrivial = false) : NonTrivial(NonTrivial) {}
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PreservedAnalyses run(Loop &L, LoopAnalysisManager &AM,
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LoopStandardAnalysisResults &AR, LPMUpdater &U);
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};
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/// Create the legacy pass object for the simple loop unswitcher.
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///
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/// See the documentaion for `SimpleLoopUnswitchPass` for details.
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Pass *createSimpleLoopUnswitchLegacyPass(bool NonTrivial = false);
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} // end namespace llvm
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#endif // LLVM_TRANSFORMS_SCALAR_SIMPLELOOPUNSWITCH_H
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