//===- llvm/Analysis/DemandedBits.h - Determine demanded bits ---*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// 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_DEMANDEDBITS_H
#define LLVM_ANALYSIS_DEMANDEDBITS_H
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/IR/PassManager.h"
#include "llvm/Pass.h"
#include <optional>
namespace llvm {
class AssumptionCache;
class DominatorTree;
class Function;
class Instruction;
struct KnownBits;
class raw_ostream;
class DemandedBits {
public:
DemandedBits(Function &F, AssumptionCache &AC, DominatorTree &DT) :
F(F), AC(AC), DT(DT) {}
/// Return the bits demanded from instruction I.
///
/// For vector instructions individual vector elements are not distinguished:
/// A bit is demanded if it is demanded for any of the vector elements. The
/// size of the return value corresponds to the type size in bits of the
/// scalar type.
///
/// Instructions that do not have integer or vector of integer type are
/// accepted, but will always produce a mask with all bits set.
APInt getDemandedBits(Instruction *I);
/// Return the bits demanded from use U.
APInt getDemandedBits(Use *U);
/// Return true if, during analysis, I could not be reached.
bool isInstructionDead(Instruction *I);
/// Return whether this use is dead by means of not having any demanded bits.
bool isUseDead(Use *U);
void print(raw_ostream &OS);
/// Compute alive bits of one addition operand from alive output and known
/// operand bits
static APInt determineLiveOperandBitsAdd(unsigned OperandNo,
const APInt &AOut,
const KnownBits &LHS,
const KnownBits &RHS);
/// Compute alive bits of one subtraction operand from alive output and known
/// operand bits
static APInt determineLiveOperandBitsSub(unsigned OperandNo,
const APInt &AOut,
const KnownBits &LHS,
const KnownBits &RHS);
private:
void performAnalysis();
void determineLiveOperandBits(const Instruction *UserI,
const Value *Val, unsigned OperandNo,
const APInt &AOut, APInt &AB,
KnownBits &Known, KnownBits &Known2, bool &KnownBitsComputed);
Function &F;
AssumptionCache &AC;
DominatorTree &DT;
bool Analyzed = false;
// The set of visited instructions (non-integer-typed only).
SmallPtrSet<Instruction*, 32> Visited;
DenseMap<Instruction *, APInt> AliveBits;
// Uses with no demanded bits. If the user also has no demanded bits, the use
// might not be stored explicitly in this map, to save memory during analysis.
SmallPtrSet<Use *, 16> DeadUses;
};
class DemandedBitsWrapperPass : public FunctionPass {
private:
mutable std::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:
/// Provide the result type for this analysis pass.
using Result = DemandedBits;
/// Run the analysis pass over a function and produce demanded bits
/// information.
DemandedBits run(Function &F, FunctionAnalysisManager &AM);
};
/// 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 namespace llvm
#endif // LLVM_ANALYSIS_DEMANDEDBITS_H