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llvm-mirror/include/llvm/Analysis/DemandedBits.h
Craig Topper c5d014c133 [ValueTracking] Introduce a KnownBits struct to wrap the two APInts for computeKnownBits
This patch introduces a new KnownBits struct that wraps the two APInt used by computeKnownBits. This allows us to treat them as more of a unit.

Initially I've just altered the signatures of computeKnownBits and InstCombine's simplifyDemandedBits to pass a KnownBits reference instead of two separate APInt references. I'll do similar to the SelectionDAG version of computeKnownBits/simplifyDemandedBits as a separate patch.

I've added a constructor that allows initializing both APInts to the same bit width with a starting value of 0. This reduces the repeated pattern of initializing both APInts. Once place default constructed the APInts so I added a default constructor for those cases.

Going forward I would like to add more methods that will work on the pairs. For example trunc, zext, and sext occur on both APInts together in several places. We should probably add a clear method that can be used to clear both pieces. Maybe a method to check for conflicting information. A method to return (Zero|One) so we don't write it out everywhere. Maybe a method for (Zero|One).isAllOnesValue() to determine if all bits are known. I'm sure there are many other methods we can come up with.

Differential Revision: https://reviews.llvm.org/D32376

llvm-svn: 301432
2017-04-26 16:39:58 +00:00

118 lines
3.3 KiB
C++

//===-- llvm/Analysis/DemandedBits.h - Determine demanded bits --*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements a demanded bits analysis. A demanded bit is one that
// contributes to a result; bits that are not demanded can be either zero or
// one without affecting control or data flow. For example in this sequence:
//
// %1 = add i32 %x, %y
// %2 = trunc i32 %1 to i16
//
// Only the lowest 16 bits of %1 are demanded; the rest are removed by the
// trunc.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_ANALYSIS_DEMANDED_BITS_H
#define LLVM_ANALYSIS_DEMANDED_BITS_H
#include "llvm/Pass.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/IR/PassManager.h"
namespace llvm {
class FunctionPass;
class Function;
class Instruction;
class DominatorTree;
class AssumptionCache;
struct KnownBits;
class DemandedBits {
public:
DemandedBits(Function &F, AssumptionCache &AC, DominatorTree &DT) :
F(F), AC(AC), DT(DT), Analyzed(false) {}
/// Return the bits demanded from instruction I.
APInt getDemandedBits(Instruction *I);
/// Return true if, during analysis, I could not be reached.
bool isInstructionDead(Instruction *I);
void print(raw_ostream &OS);
private:
Function &F;
AssumptionCache ∾
DominatorTree &DT;
void performAnalysis();
void determineLiveOperandBits(const Instruction *UserI,
const Instruction *I, unsigned OperandNo,
const APInt &AOut, APInt &AB,
KnownBits &Known, KnownBits &Known2);
bool Analyzed;
// The set of visited instructions (non-integer-typed only).
SmallPtrSet<Instruction*, 32> Visited;
DenseMap<Instruction *, APInt> AliveBits;
};
class DemandedBitsWrapperPass : public FunctionPass {
private:
mutable Optional<DemandedBits> DB;
public:
static char ID; // Pass identification, replacement for typeid
DemandedBitsWrapperPass();
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override;
/// Clean up memory in between runs
void releaseMemory() override;
DemandedBits &getDemandedBits() { return *DB; }
void print(raw_ostream &OS, const Module *M) const override;
};
/// An analysis that produces \c DemandedBits for a function.
class DemandedBitsAnalysis : public AnalysisInfoMixin<DemandedBitsAnalysis> {
friend AnalysisInfoMixin<DemandedBitsAnalysis>;
static AnalysisKey Key;
public:
/// \brief Provide the result typedef for this analysis pass.
typedef DemandedBits Result;
/// \brief Run the analysis pass over a function and produce demanded bits
/// information.
DemandedBits run(Function &F, FunctionAnalysisManager &AM);
};
/// \brief Printer pass for DemandedBits
class DemandedBitsPrinterPass : public PassInfoMixin<DemandedBitsPrinterPass> {
raw_ostream &OS;
public:
explicit DemandedBitsPrinterPass(raw_ostream &OS) : OS(OS) {}
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
};
/// Create a demanded bits analysis pass.
FunctionPass *createDemandedBitsWrapperPass();
} // End llvm namespace
#endif