150 lines
4.8 KiB
C++
150 lines
4.8 KiB
C++
//===- llvm/Analysis/DemandedBits.h - Determine demanded bits ---*- 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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//
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// This pass implements a demanded bits analysis. A demanded bit is one that
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// contributes to a result; bits that are not demanded can be either zero or
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// one without affecting control or data flow. For example in this sequence:
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//
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// %1 = add i32 %x, %y
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// %2 = trunc i32 %1 to i16
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//
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// Only the lowest 16 bits of %1 are demanded; the rest are removed by the
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// trunc.
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_ANALYSIS_DEMANDED_BITS_H
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#define LLVM_ANALYSIS_DEMANDED_BITS_H
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#include "llvm/ADT/APInt.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/Optional.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/IR/PassManager.h"
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#include "llvm/Pass.h"
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namespace llvm {
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class AssumptionCache;
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class DominatorTree;
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class Function;
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class Instruction;
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struct KnownBits;
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class raw_ostream;
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class DemandedBits {
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public:
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DemandedBits(Function &F, AssumptionCache &AC, DominatorTree &DT) :
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F(F), AC(AC), DT(DT) {}
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/// Return the bits demanded from instruction I.
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///
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/// For vector instructions individual vector elements are not distinguished:
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/// A bit is demanded if it is demanded for any of the vector elements. The
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/// size of the return value corresponds to the type size in bits of the
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/// scalar type.
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///
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/// Instructions that do not have integer or vector of integer type are
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/// accepted, but will always produce a mask with all bits set.
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APInt getDemandedBits(Instruction *I);
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/// Return true if, during analysis, I could not be reached.
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bool isInstructionDead(Instruction *I);
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/// Return whether this use is dead by means of not having any demanded bits.
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bool isUseDead(Use *U);
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void print(raw_ostream &OS);
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/// Compute alive bits of one addition operand from alive output and known
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/// operand bits
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static APInt determineLiveOperandBitsAdd(unsigned OperandNo,
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const APInt &AOut,
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const KnownBits &LHS,
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const KnownBits &RHS);
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/// Compute alive bits of one subtraction operand from alive output and known
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/// operand bits
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static APInt determineLiveOperandBitsSub(unsigned OperandNo,
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const APInt &AOut,
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const KnownBits &LHS,
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const KnownBits &RHS);
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private:
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void performAnalysis();
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void determineLiveOperandBits(const Instruction *UserI,
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const Value *Val, unsigned OperandNo,
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const APInt &AOut, APInt &AB,
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KnownBits &Known, KnownBits &Known2, bool &KnownBitsComputed);
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Function &F;
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AssumptionCache ∾
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DominatorTree &DT;
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bool Analyzed = false;
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// The set of visited instructions (non-integer-typed only).
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SmallPtrSet<Instruction*, 32> Visited;
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DenseMap<Instruction *, APInt> AliveBits;
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// Uses with no demanded bits. If the user also has no demanded bits, the use
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// might not be stored explicitly in this map, to save memory during analysis.
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SmallPtrSet<Use *, 16> DeadUses;
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};
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class DemandedBitsWrapperPass : public FunctionPass {
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private:
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mutable Optional<DemandedBits> DB;
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public:
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static char ID; // Pass identification, replacement for typeid
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DemandedBitsWrapperPass();
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bool runOnFunction(Function &F) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override;
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/// Clean up memory in between runs
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void releaseMemory() override;
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DemandedBits &getDemandedBits() { return *DB; }
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void print(raw_ostream &OS, const Module *M) const override;
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};
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/// An analysis that produces \c DemandedBits for a function.
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class DemandedBitsAnalysis : public AnalysisInfoMixin<DemandedBitsAnalysis> {
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friend AnalysisInfoMixin<DemandedBitsAnalysis>;
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static AnalysisKey Key;
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public:
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/// Provide the result type for this analysis pass.
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using Result = DemandedBits;
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/// Run the analysis pass over a function and produce demanded bits
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/// information.
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DemandedBits run(Function &F, FunctionAnalysisManager &AM);
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};
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/// Printer pass for DemandedBits
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class DemandedBitsPrinterPass : public PassInfoMixin<DemandedBitsPrinterPass> {
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raw_ostream &OS;
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public:
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explicit DemandedBitsPrinterPass(raw_ostream &OS) : OS(OS) {}
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PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
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};
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/// Create a demanded bits analysis pass.
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FunctionPass *createDemandedBitsWrapperPass();
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} // end namespace llvm
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#endif // LLVM_ANALYSIS_DEMANDED_BITS_H
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