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//===- llvm/Analysis/LoopUnrollAnalyzer.h - Loop Unroll Analyzer-*- 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 file implements UnrolledInstAnalyzer class. It's used for predicting
// potential effects that loop unrolling might have, such as enabling constant
// propagation and other optimizations.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_ANALYSIS_LOOPUNROLLANALYZER_H
#define LLVM_ANALYSIS_LOOPUNROLLANALYZER_H
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/IR/InstVisitor.h"
// This class is used to get an estimate of the optimization effects that we
// could get from complete loop unrolling. It comes from the fact that some
// loads might be replaced with concrete constant values and that could trigger
// a chain of instruction simplifications.
//
// E.g. we might have:
// int a[] = {0, 1, 0};
// v = 0;
// for (i = 0; i < 3; i ++)
// v += b[i]*a[i];
// If we completely unroll the loop, we would get:
// v = b[0]*a[0] + b[1]*a[1] + b[2]*a[2]
// Which then will be simplified to:
// v = b[0]* 0 + b[1]* 1 + b[2]* 0
// And finally:
// v = b[1]
namespace llvm {
class UnrolledInstAnalyzer : private InstVisitor<UnrolledInstAnalyzer, bool> {
typedef InstVisitor<UnrolledInstAnalyzer, bool> Base;
friend class InstVisitor<UnrolledInstAnalyzer, bool>;
struct SimplifiedAddress {
Value *Base = nullptr;
ConstantInt *Offset = nullptr;
};
public:
UnrolledInstAnalyzer(unsigned Iteration,
DenseMap<Value *, Value *> &SimplifiedValues,
ScalarEvolution &SE, const Loop *L)
: SimplifiedValues(SimplifiedValues), SE(SE), L(L) {
IterationNumber = SE.getConstant(APInt(64, Iteration));
}
// Allow access to the initial visit method.
using Base::visit;
private:
/// A cache of pointer bases and constant-folded offsets corresponding
/// to GEP (or derived from GEP) instructions.
///
/// In order to find the base pointer one needs to perform non-trivial
/// traversal of the corresponding SCEV expression, so it's good to have the
/// results saved.
DenseMap<Value *, SimplifiedAddress> SimplifiedAddresses;
/// SCEV expression corresponding to number of currently simulated
/// iteration.
const SCEV *IterationNumber;
/// While we walk the loop instructions, we build up and maintain a mapping
/// of simplified values specific to this iteration. The idea is to propagate
/// any special information we have about loads that can be replaced with
/// constants after complete unrolling, and account for likely simplifications
/// post-unrolling.
DenseMap<Value *, Value *> &SimplifiedValues;
ScalarEvolution &SE;
const Loop *L;
bool simplifyInstWithSCEV(Instruction *I);
bool visitInstruction(Instruction &I);
bool visitBinaryOperator(BinaryOperator &I);
bool visitLoad(LoadInst &I);
bool visitCastInst(CastInst &I);
bool visitCmpInst(CmpInst &I);
bool visitPHINode(PHINode &PN);
};
}
#endif
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