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Summary: This patch teaches InstCombine to accept a new parameter: maximum number of iterations over functions. InstCombine tries to simplify instructions by iterating over the whole function until the function stops changing. As a consequence, the last iteration before reaching a fixpoint visits all instructions in the worklist and never performs any rewrites. Bounding the number of iterations can have 2 benefits: * In case the users of the pass can make a good guess about the number of required iterations, we can save the time normally spent on the last iteration that doesn't change anything. * When the wants to use InstCombine as a cleanup pass, it may be enough to run just a few iterations and stop even before reaching a fixpoint. This can be also useful for implementing a lightweight pass pipeline (think `-O1`). This patch does not change the behavior of opt or Clang -- limiting the number of iterations is entirely opt-in. Reviewers: fhahn, davide, spatel, foad, nlopes, grosser, lebedev.ri, nikic, xbolva00 Reviewed By: spatel Subscribers: craig.topper, hiraditya, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D71145
77 lines
2.7 KiB
C++
77 lines
2.7 KiB
C++
//===- InstCombine.h - InstCombine pass -------------------------*- 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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/// \file
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///
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/// This file provides the primary interface to the instcombine pass. This pass
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/// is suitable for use in the new pass manager. For a pass that works with the
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/// legacy pass manager, use \c createInstructionCombiningPass().
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///
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_TRANSFORMS_INSTCOMBINE_INSTCOMBINE_H
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#define LLVM_TRANSFORMS_INSTCOMBINE_INSTCOMBINE_H
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#include "llvm/IR/Function.h"
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#include "llvm/IR/PassManager.h"
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#include "llvm/Transforms/InstCombine/InstCombineWorklist.h"
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namespace llvm {
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class InstCombinePass : public PassInfoMixin<InstCombinePass> {
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InstCombineWorklist Worklist;
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const bool ExpensiveCombines;
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const unsigned MaxIterations;
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public:
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static StringRef name() { return "InstCombinePass"; }
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explicit InstCombinePass(bool ExpensiveCombines = true);
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explicit InstCombinePass(bool ExpensiveCombines, unsigned MaxIterations);
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PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
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};
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/// The legacy pass manager's instcombine pass.
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///
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/// This is a basic whole-function wrapper around the instcombine utility. It
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/// will try to combine all instructions in the function.
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class InstructionCombiningPass : public FunctionPass {
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InstCombineWorklist Worklist;
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const bool ExpensiveCombines;
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const unsigned MaxIterations;
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public:
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static char ID; // Pass identification, replacement for typeid
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explicit InstructionCombiningPass(bool ExpensiveCombines = true);
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explicit InstructionCombiningPass(bool ExpensiveCombines,
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unsigned MaxIterations);
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void getAnalysisUsage(AnalysisUsage &AU) const override;
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bool runOnFunction(Function &F) override;
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};
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//===----------------------------------------------------------------------===//
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//
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// InstructionCombining - Combine instructions to form fewer, simple
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// instructions. This pass does not modify the CFG, and has a tendency to make
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// instructions dead, so a subsequent DCE pass is useful.
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//
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// This pass combines things like:
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// %Y = add int 1, %X
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// %Z = add int 1, %Y
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// into:
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// %Z = add int 2, %X
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//
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FunctionPass *createInstructionCombiningPass(bool ExpensiveCombines = true);
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FunctionPass *createInstructionCombiningPass(bool ExpensiveCombines,
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unsigned MaxIterations);
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}
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#endif
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