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No test since the kinds of transforms this prevents seem to not really be relevant for SI's different addressing modes. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239261 91177308-0d34-0410-b5e6-96231b3b80d8
506 lines
19 KiB
C++
506 lines
19 KiB
C++
//===- NaryReassociate.cpp - Reassociate n-ary expressions ----------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass reassociates n-ary add expressions and eliminates the redundancy
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// exposed by the reassociation.
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//
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// A motivating example:
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//
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// void foo(int a, int b) {
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// bar(a + b);
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// bar((a + 2) + b);
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// }
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//
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// An ideal compiler should reassociate (a + 2) + b to (a + b) + 2 and simplify
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// the above code to
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//
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// int t = a + b;
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// bar(t);
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// bar(t + 2);
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//
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// However, the Reassociate pass is unable to do that because it processes each
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// instruction individually and believes (a + 2) + b is the best form according
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// to its rank system.
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//
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// To address this limitation, NaryReassociate reassociates an expression in a
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// form that reuses existing instructions. As a result, NaryReassociate can
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// reassociate (a + 2) + b in the example to (a + b) + 2 because it detects that
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// (a + b) is computed before.
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//
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// NaryReassociate works as follows. For every instruction in the form of (a +
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// b) + c, it checks whether a + c or b + c is already computed by a dominating
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// instruction. If so, it then reassociates (a + b) + c into (a + c) + b or (b +
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// c) + a and removes the redundancy accordingly. To efficiently look up whether
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// an expression is computed before, we store each instruction seen and its SCEV
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// into an SCEV-to-instruction map.
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//
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// Although the algorithm pattern-matches only ternary additions, it
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// automatically handles many >3-ary expressions by walking through the function
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// in the depth-first order. For example, given
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//
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// (a + c) + d
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// ((a + b) + c) + d
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//
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// NaryReassociate first rewrites (a + b) + c to (a + c) + b, and then rewrites
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// ((a + c) + b) + d into ((a + c) + d) + b.
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//
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// Finally, the above dominator-based algorithm may need to be run multiple
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// iterations before emitting optimal code. One source of this need is that we
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// only split an operand when it is used only once. The above algorithm can
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// eliminate an instruction and decrease the usage count of its operands. As a
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// result, an instruction that previously had multiple uses may become a
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// single-use instruction and thus eligible for split consideration. For
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// example,
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//
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// ac = a + c
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// ab = a + b
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// abc = ab + c
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// ab2 = ab + b
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// ab2c = ab2 + c
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//
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// In the first iteration, we cannot reassociate abc to ac+b because ab is used
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// twice. However, we can reassociate ab2c to abc+b in the first iteration. As a
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// result, ab2 becomes dead and ab will be used only once in the second
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// iteration.
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//
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// Limitations and TODO items:
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//
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// 1) We only considers n-ary adds for now. This should be extended and
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// generalized.
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//
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// 2) Besides arithmetic operations, similar reassociation can be applied to
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// GEPs. For example, if
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// X = &arr[a]
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// dominates
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// Y = &arr[a + b]
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// we may rewrite Y into X + b.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Analysis/TargetLibraryInfo.h"
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#include "llvm/Analysis/TargetTransformInfo.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Module.h"
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#include "llvm/IR/PatternMatch.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/Local.h"
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using namespace llvm;
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using namespace PatternMatch;
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#define DEBUG_TYPE "nary-reassociate"
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namespace {
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class NaryReassociate : public FunctionPass {
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public:
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static char ID;
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NaryReassociate(): FunctionPass(ID) {
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initializeNaryReassociatePass(*PassRegistry::getPassRegistry());
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}
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bool doInitialization(Module &M) override {
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DL = &M.getDataLayout();
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return false;
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}
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bool runOnFunction(Function &F) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addPreserved<DominatorTreeWrapperPass>();
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AU.addPreserved<ScalarEvolution>();
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AU.addPreserved<TargetLibraryInfoWrapperPass>();
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addRequired<ScalarEvolution>();
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AU.addRequired<TargetLibraryInfoWrapperPass>();
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AU.addRequired<TargetTransformInfoWrapperPass>();
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AU.setPreservesCFG();
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}
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private:
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// Runs only one iteration of the dominator-based algorithm. See the header
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// comments for why we need multiple iterations.
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bool doOneIteration(Function &F);
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// Reassociates I for better CSE.
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Instruction *tryReassociate(Instruction *I);
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// Reassociate GEP for better CSE.
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Instruction *tryReassociateGEP(GetElementPtrInst *GEP);
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// Try splitting GEP at the I-th index and see whether either part can be
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// CSE'ed. This is a helper function for tryReassociateGEP.
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//
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// \p IndexedType The element type indexed by GEP's I-th index. This is
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// equivalent to
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// GEP->getIndexedType(GEP->getPointerOperand(), 0-th index,
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// ..., i-th index).
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GetElementPtrInst *tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
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unsigned I, Type *IndexedType);
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// Given GEP's I-th index = LHS + RHS, see whether &Base[..][LHS][..] or
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// &Base[..][RHS][..] can be CSE'ed and rewrite GEP accordingly.
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GetElementPtrInst *tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
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unsigned I, Value *LHS,
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Value *RHS, Type *IndexedType);
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// Reassociate Add for better CSE.
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Instruction *tryReassociateAdd(BinaryOperator *I);
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// A helper function for tryReassociateAdd. LHS and RHS are explicitly passed.
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Instruction *tryReassociateAdd(Value *LHS, Value *RHS, Instruction *I);
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// Rewrites I to LHS + RHS if LHS is computed already.
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Instruction *tryReassociatedAdd(const SCEV *LHS, Value *RHS, Instruction *I);
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// Returns the closest dominator of \c Dominatee that computes
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// \c CandidateExpr. Returns null if not found.
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Instruction *findClosestMatchingDominator(const SCEV *CandidateExpr,
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Instruction *Dominatee);
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// GetElementPtrInst implicitly sign-extends an index if the index is shorter
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// than the pointer size. This function returns whether Index is shorter than
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// GEP's pointer size, i.e., whether Index needs to be sign-extended in order
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// to be an index of GEP.
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bool requiresSignExtension(Value *Index, GetElementPtrInst *GEP);
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DominatorTree *DT;
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ScalarEvolution *SE;
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TargetLibraryInfo *TLI;
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TargetTransformInfo *TTI;
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const DataLayout *DL;
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// A lookup table quickly telling which instructions compute the given SCEV.
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// Note that there can be multiple instructions at different locations
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// computing to the same SCEV, so we map a SCEV to an instruction list. For
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// example,
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//
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// if (p1)
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// foo(a + b);
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// if (p2)
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// bar(a + b);
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DenseMap<const SCEV *, SmallVector<Instruction *, 2>> SeenExprs;
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};
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} // anonymous namespace
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char NaryReassociate::ID = 0;
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INITIALIZE_PASS_BEGIN(NaryReassociate, "nary-reassociate", "Nary reassociation",
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false, false)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
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INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
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INITIALIZE_PASS_END(NaryReassociate, "nary-reassociate", "Nary reassociation",
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false, false)
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FunctionPass *llvm::createNaryReassociatePass() {
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return new NaryReassociate();
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}
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bool NaryReassociate::runOnFunction(Function &F) {
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if (skipOptnoneFunction(F))
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return false;
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DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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SE = &getAnalysis<ScalarEvolution>();
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TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
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TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
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bool Changed = false, ChangedInThisIteration;
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do {
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ChangedInThisIteration = doOneIteration(F);
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Changed |= ChangedInThisIteration;
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} while (ChangedInThisIteration);
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return Changed;
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}
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// Whitelist the instruction types NaryReassociate handles for now.
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static bool isPotentiallyNaryReassociable(Instruction *I) {
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switch (I->getOpcode()) {
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case Instruction::Add:
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case Instruction::GetElementPtr:
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return true;
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default:
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return false;
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}
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}
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bool NaryReassociate::doOneIteration(Function &F) {
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bool Changed = false;
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SeenExprs.clear();
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// Process the basic blocks in pre-order of the dominator tree. This order
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// ensures that all bases of a candidate are in Candidates when we process it.
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for (auto Node = GraphTraits<DominatorTree *>::nodes_begin(DT);
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Node != GraphTraits<DominatorTree *>::nodes_end(DT); ++Node) {
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BasicBlock *BB = Node->getBlock();
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for (auto I = BB->begin(); I != BB->end(); ++I) {
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if (SE->isSCEVable(I->getType()) && isPotentiallyNaryReassociable(I)) {
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const SCEV *OldSCEV = SE->getSCEV(I);
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if (Instruction *NewI = tryReassociate(I)) {
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Changed = true;
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SE->forgetValue(I);
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I->replaceAllUsesWith(NewI);
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RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
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I = NewI;
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}
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// Add the rewritten instruction to SeenExprs; the original instruction
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// is deleted.
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const SCEV *NewSCEV = SE->getSCEV(I);
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SeenExprs[NewSCEV].push_back(I);
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// Ideally, NewSCEV should equal OldSCEV because tryReassociate(I)
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// is equivalent to I. However, ScalarEvolution::getSCEV may
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// weaken nsw causing NewSCEV not to equal OldSCEV. For example, suppose
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// we reassociate
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// I = &a[sext(i +nsw j)] // assuming sizeof(a[0]) = 4
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// to
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// NewI = &a[sext(i)] + sext(j).
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//
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// ScalarEvolution computes
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// getSCEV(I) = a + 4 * sext(i + j)
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// getSCEV(newI) = a + 4 * sext(i) + 4 * sext(j)
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// which are different SCEVs.
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//
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// To alleviate this issue of ScalarEvolution not always capturing
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// equivalence, we add I to SeenExprs[OldSCEV] as well so that we can
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// map both SCEV before and after tryReassociate(I) to I.
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//
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// This improvement is exercised in @reassociate_gep_nsw in nary-gep.ll.
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if (NewSCEV != OldSCEV)
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SeenExprs[OldSCEV].push_back(I);
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}
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}
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}
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return Changed;
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}
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Instruction *NaryReassociate::tryReassociate(Instruction *I) {
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switch (I->getOpcode()) {
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case Instruction::Add:
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return tryReassociateAdd(cast<BinaryOperator>(I));
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case Instruction::GetElementPtr:
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return tryReassociateGEP(cast<GetElementPtrInst>(I));
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default:
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llvm_unreachable("should be filtered out by isPotentiallyNaryReassociable");
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}
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}
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// FIXME: extract this method into TTI->getGEPCost.
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static bool isGEPFoldable(GetElementPtrInst *GEP,
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const TargetTransformInfo *TTI,
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const DataLayout *DL) {
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GlobalVariable *BaseGV = nullptr;
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int64_t BaseOffset = 0;
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bool HasBaseReg = false;
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int64_t Scale = 0;
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if (GlobalVariable *GV = dyn_cast<GlobalVariable>(GEP->getPointerOperand()))
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BaseGV = GV;
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else
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HasBaseReg = true;
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gep_type_iterator GTI = gep_type_begin(GEP);
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for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I, ++GTI) {
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if (isa<SequentialType>(*GTI)) {
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int64_t ElementSize = DL->getTypeAllocSize(GTI.getIndexedType());
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if (ConstantInt *ConstIdx = dyn_cast<ConstantInt>(*I)) {
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BaseOffset += ConstIdx->getSExtValue() * ElementSize;
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} else {
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// Needs scale register.
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if (Scale != 0) {
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// No addressing mode takes two scale registers.
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return false;
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}
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Scale = ElementSize;
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}
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} else {
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StructType *STy = cast<StructType>(*GTI);
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uint64_t Field = cast<ConstantInt>(*I)->getZExtValue();
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BaseOffset += DL->getStructLayout(STy)->getElementOffset(Field);
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}
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}
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unsigned AddrSpace = GEP->getPointerAddressSpace();
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return TTI->isLegalAddressingMode(GEP->getType()->getElementType(), BaseGV,
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BaseOffset, HasBaseReg, Scale, AddrSpace);
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}
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Instruction *NaryReassociate::tryReassociateGEP(GetElementPtrInst *GEP) {
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// Not worth reassociating GEP if it is foldable.
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if (isGEPFoldable(GEP, TTI, DL))
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return nullptr;
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gep_type_iterator GTI = gep_type_begin(*GEP);
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for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I) {
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if (isa<SequentialType>(*GTI++)) {
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if (auto *NewGEP = tryReassociateGEPAtIndex(GEP, I - 1, *GTI)) {
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return NewGEP;
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}
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}
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}
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return nullptr;
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}
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bool NaryReassociate::requiresSignExtension(Value *Index,
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GetElementPtrInst *GEP) {
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unsigned PointerSizeInBits =
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DL->getPointerSizeInBits(GEP->getType()->getPointerAddressSpace());
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return cast<IntegerType>(Index->getType())->getBitWidth() < PointerSizeInBits;
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}
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GetElementPtrInst *
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NaryReassociate::tryReassociateGEPAtIndex(GetElementPtrInst *GEP, unsigned I,
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Type *IndexedType) {
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Value *IndexToSplit = GEP->getOperand(I + 1);
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if (SExtInst *SExt = dyn_cast<SExtInst>(IndexToSplit))
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IndexToSplit = SExt->getOperand(0);
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if (AddOperator *AO = dyn_cast<AddOperator>(IndexToSplit)) {
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// If the I-th index needs sext and the underlying add is not equipped with
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// nsw, we cannot split the add because
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// sext(LHS + RHS) != sext(LHS) + sext(RHS).
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if (requiresSignExtension(IndexToSplit, GEP) && !AO->hasNoSignedWrap())
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return nullptr;
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Value *LHS = AO->getOperand(0), *RHS = AO->getOperand(1);
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// IndexToSplit = LHS + RHS.
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if (auto *NewGEP = tryReassociateGEPAtIndex(GEP, I, LHS, RHS, IndexedType))
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return NewGEP;
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// Symmetrically, try IndexToSplit = RHS + LHS.
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if (LHS != RHS) {
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if (auto *NewGEP =
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tryReassociateGEPAtIndex(GEP, I, RHS, LHS, IndexedType))
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return NewGEP;
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}
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}
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return nullptr;
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}
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GetElementPtrInst *
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NaryReassociate::tryReassociateGEPAtIndex(GetElementPtrInst *GEP, unsigned I,
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Value *LHS, Value *RHS,
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Type *IndexedType) {
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// Look for GEP's closest dominator that has the same SCEV as GEP except that
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// the I-th index is replaced with LHS.
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SmallVector<const SCEV *, 4> IndexExprs;
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for (auto Index = GEP->idx_begin(); Index != GEP->idx_end(); ++Index)
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IndexExprs.push_back(SE->getSCEV(*Index));
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// Replace the I-th index with LHS.
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IndexExprs[I] = SE->getSCEV(LHS);
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const SCEV *CandidateExpr = SE->getGEPExpr(
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GEP->getSourceElementType(), SE->getSCEV(GEP->getPointerOperand()),
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IndexExprs, GEP->isInBounds());
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auto *Candidate = findClosestMatchingDominator(CandidateExpr, GEP);
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if (Candidate == nullptr)
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return nullptr;
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PointerType *TypeOfCandidate = dyn_cast<PointerType>(Candidate->getType());
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// Pretty rare but theoretically possible when a numeric value happens to
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// share CandidateExpr.
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if (TypeOfCandidate == nullptr)
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return nullptr;
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// NewGEP = (char *)Candidate + RHS * sizeof(IndexedType)
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uint64_t IndexedSize = DL->getTypeAllocSize(IndexedType);
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Type *ElementType = TypeOfCandidate->getElementType();
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uint64_t ElementSize = DL->getTypeAllocSize(ElementType);
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// Another less rare case: because I is not necessarily the last index of the
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// GEP, the size of the type at the I-th index (IndexedSize) is not
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// necessarily divisible by ElementSize. For example,
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//
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// #pragma pack(1)
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// struct S {
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// int a[3];
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// int64 b[8];
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// };
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// #pragma pack()
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//
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// sizeof(S) = 100 is indivisible by sizeof(int64) = 8.
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//
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// TODO: bail out on this case for now. We could emit uglygep.
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if (IndexedSize % ElementSize != 0)
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return nullptr;
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// NewGEP = &Candidate[RHS * (sizeof(IndexedType) / sizeof(Candidate[0])));
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IRBuilder<> Builder(GEP);
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Type *IntPtrTy = DL->getIntPtrType(TypeOfCandidate);
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if (RHS->getType() != IntPtrTy)
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RHS = Builder.CreateSExtOrTrunc(RHS, IntPtrTy);
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if (IndexedSize != ElementSize) {
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RHS = Builder.CreateMul(
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RHS, ConstantInt::get(IntPtrTy, IndexedSize / ElementSize));
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}
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GetElementPtrInst *NewGEP =
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cast<GetElementPtrInst>(Builder.CreateGEP(Candidate, RHS));
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NewGEP->setIsInBounds(GEP->isInBounds());
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NewGEP->takeName(GEP);
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return NewGEP;
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}
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Instruction *NaryReassociate::tryReassociateAdd(BinaryOperator *I) {
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Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
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if (auto *NewI = tryReassociateAdd(LHS, RHS, I))
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return NewI;
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if (auto *NewI = tryReassociateAdd(RHS, LHS, I))
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return NewI;
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return nullptr;
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}
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Instruction *NaryReassociate::tryReassociateAdd(Value *LHS, Value *RHS,
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Instruction *I) {
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Value *A = nullptr, *B = nullptr;
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// To be conservative, we reassociate I only when it is the only user of A+B.
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if (LHS->hasOneUse() && match(LHS, m_Add(m_Value(A), m_Value(B)))) {
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// I = (A + B) + RHS
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// = (A + RHS) + B or (B + RHS) + A
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const SCEV *AExpr = SE->getSCEV(A), *BExpr = SE->getSCEV(B);
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const SCEV *RHSExpr = SE->getSCEV(RHS);
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if (BExpr != RHSExpr) {
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if (auto *NewI = tryReassociatedAdd(SE->getAddExpr(AExpr, RHSExpr), B, I))
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return NewI;
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}
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if (AExpr != RHSExpr) {
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if (auto *NewI = tryReassociatedAdd(SE->getAddExpr(BExpr, RHSExpr), A, I))
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return NewI;
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}
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}
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return nullptr;
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}
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Instruction *NaryReassociate::tryReassociatedAdd(const SCEV *LHSExpr,
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Value *RHS, Instruction *I) {
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auto Pos = SeenExprs.find(LHSExpr);
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// Bail out if LHSExpr is not previously seen.
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if (Pos == SeenExprs.end())
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return nullptr;
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// Look for the closest dominator LHS of I that computes LHSExpr, and replace
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// I with LHS + RHS.
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auto *LHS = findClosestMatchingDominator(LHSExpr, I);
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if (LHS == nullptr)
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return nullptr;
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Instruction *NewI = BinaryOperator::CreateAdd(LHS, RHS, "", I);
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NewI->takeName(I);
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return NewI;
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}
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Instruction *
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NaryReassociate::findClosestMatchingDominator(const SCEV *CandidateExpr,
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Instruction *Dominatee) {
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auto Pos = SeenExprs.find(CandidateExpr);
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if (Pos == SeenExprs.end())
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return nullptr;
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auto &Candidates = Pos->second;
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// Because we process the basic blocks in pre-order of the dominator tree, a
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// candidate that doesn't dominate the current instruction won't dominate any
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// future instruction either. Therefore, we pop it out of the stack. This
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// optimization makes the algorithm O(n).
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while (!Candidates.empty()) {
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Instruction *Candidate = Candidates.back();
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if (DT->dominates(Candidate, Dominatee))
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return Candidate;
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Candidates.pop_back();
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}
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return nullptr;
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}
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