//===- NaryReassociate.h - Reassociate n-ary expressions --------*- 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 reassociates n-ary add expressions and eliminates the redundancy
// exposed by the reassociation.
//
// A motivating example:
//
// void foo(int a, int b) {
// bar(a + b);
// bar((a + 2) + b);
// }
//
// An ideal compiler should reassociate (a + 2) + b to (a + b) + 2 and simplify
// the above code to
//
// int t = a + b;
// bar(t);
// bar(t + 2);
//
// However, the Reassociate pass is unable to do that because it processes each
// instruction individually and believes (a + 2) + b is the best form according
// to its rank system.
//
// To address this limitation, NaryReassociate reassociates an expression in a
// form that reuses existing instructions. As a result, NaryReassociate can
// reassociate (a + 2) + b in the example to (a + b) + 2 because it detects that
// (a + b) is computed before.
//
// NaryReassociate works as follows. For every instruction in the form of (a +
// b) + c, it checks whether a + c or b + c is already computed by a dominating
// instruction. If so, it then reassociates (a + b) + c into (a + c) + b or (b +
// c) + a and removes the redundancy accordingly. To efficiently look up whether
// an expression is computed before, we store each instruction seen and its SCEV
// into an SCEV-to-instruction map.
//
// Although the algorithm pattern-matches only ternary additions, it
// automatically handles many >3-ary expressions by walking through the function
// in the depth-first order. For example, given
//
// (a + c) + d
// ((a + b) + c) + d
//
// NaryReassociate first rewrites (a + b) + c to (a + c) + b, and then rewrites
// ((a + c) + b) + d into ((a + c) + d) + b.
//
// Finally, the above dominator-based algorithm may need to be run multiple
// iterations before emitting optimal code. One source of this need is that we
// only split an operand when it is used only once. The above algorithm can
// eliminate an instruction and decrease the usage count of its operands. As a
// result, an instruction that previously had multiple uses may become a
// single-use instruction and thus eligible for split consideration. For
// example,
//
// ac = a + c
// ab = a + b
// abc = ab + c
// ab2 = ab + b
// ab2c = ab2 + c
//
// In the first iteration, we cannot reassociate abc to ac+b because ab is used
// twice. However, we can reassociate ab2c to abc+b in the first iteration. As a
// result, ab2 becomes dead and ab will be used only once in the second
// iteration.
//
// Limitations and TODO items:
//
// 1) We only considers n-ary adds and muls for now. This should be extended
// and generalized.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_TRANSFORMS_SCALAR_NARYREASSOCIATE_H
#define LLVM_TRANSFORMS_SCALAR_NARYREASSOCIATE_H
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/ValueHandle.h"
namespace llvm {
class AssumptionCache;
class BinaryOperator;
class DataLayout;
class DominatorTree;
class Function;
class GetElementPtrInst;
class Instruction;
class ScalarEvolution;
class SCEV;
class TargetLibraryInfo;
class TargetTransformInfo;
class Type;
class Value;
class NaryReassociatePass : public PassInfoMixin<NaryReassociatePass> {
public:
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
// Glue for old PM.
bool runImpl(Function &F, AssumptionCache *AC_, DominatorTree *DT_,
ScalarEvolution *SE_, TargetLibraryInfo *TLI_,
TargetTransformInfo *TTI_);
private:
// Runs only one iteration of the dominator-based algorithm. See the header
// comments for why we need multiple iterations.
bool doOneIteration(Function &F);
// Reassociates I for better CSE.
Instruction *tryReassociate(Instruction *I, const SCEV *&OrigSCEV);
// Reassociate GEP for better CSE.
Instruction *tryReassociateGEP(GetElementPtrInst *GEP);
// Try splitting GEP at the I-th index and see whether either part can be
// CSE'ed. This is a helper function for tryReassociateGEP.
//
// \p IndexedType The element type indexed by GEP's I-th index. This is
// equivalent to
// GEP->getIndexedType(GEP->getPointerOperand(), 0-th index,
// ..., i-th index).
GetElementPtrInst *tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
unsigned I, Type *IndexedType);
// Given GEP's I-th index = LHS + RHS, see whether &Base[..][LHS][..] or
// &Base[..][RHS][..] can be CSE'ed and rewrite GEP accordingly.
GetElementPtrInst *tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
unsigned I, Value *LHS,
Value *RHS, Type *IndexedType);
// Reassociate binary operators for better CSE.
Instruction *tryReassociateBinaryOp(BinaryOperator *I);
// A helper function for tryReassociateBinaryOp. LHS and RHS are explicitly
// passed.
Instruction *tryReassociateBinaryOp(Value *LHS, Value *RHS,
BinaryOperator *I);
// Rewrites I to (LHS op RHS) if LHS is computed already.
Instruction *tryReassociatedBinaryOp(const SCEV *LHS, Value *RHS,
BinaryOperator *I);
// Tries to match Op1 and Op2 by using V.
bool matchTernaryOp(BinaryOperator *I, Value *V, Value *&Op1, Value *&Op2);
// Gets SCEV for (LHS op RHS).
const SCEV *getBinarySCEV(BinaryOperator *I, const SCEV *LHS,
const SCEV *RHS);
// Returns the closest dominator of \c Dominatee that computes
// \c CandidateExpr. Returns null if not found.
Instruction *findClosestMatchingDominator(const SCEV *CandidateExpr,
Instruction *Dominatee);
// Try to match \p I as signed/unsigned Min/Max and reassociate it. \p
// OrigSCEV is set if \I matches Min/Max regardless whether resassociation is
// done or not. If reassociation was successful newly generated instruction is
// returned, otherwise nullptr.
template <typename PredT>
Instruction *matchAndReassociateMinOrMax(Instruction *I,
const SCEV *&OrigSCEV);
// Reassociate Min/Max.
template <typename MaxMinT>
Value *tryReassociateMinOrMax(Instruction *I, MaxMinT MaxMinMatch, Value *LHS,
Value *RHS);
// GetElementPtrInst implicitly sign-extends an index if the index is shorter
// than the pointer size. This function returns whether Index is shorter than
// GEP's pointer size, i.e., whether Index needs to be sign-extended in order
// to be an index of GEP.
bool requiresSignExtension(Value *Index, GetElementPtrInst *GEP);
AssumptionCache *AC;
const DataLayout *DL;
DominatorTree *DT;
ScalarEvolution *SE;
TargetLibraryInfo *TLI;
TargetTransformInfo *TTI;
// A lookup table quickly telling which instructions compute the given SCEV.
// Note that there can be multiple instructions at different locations
// computing to the same SCEV, so we map a SCEV to an instruction list. For
// example,
//
// if (p1)
// foo(a + b);
// if (p2)
// bar(a + b);
DenseMap<const SCEV *, SmallVector<WeakTrackingVH, 2>> SeenExprs;
};
} // end namespace llvm
#endif // LLVM_TRANSFORMS_SCALAR_NARYREASSOCIATE_H