--- /dev/null
+//===- NaryReassociate.cpp - Reassociate n-ary expressions ----------------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// 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 respectively. 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.
+//
+// Limitations and TODO items:
+//
+// 1) We only considers n-ary adds for now. This should be extended and
+// generalized.
+//
+// 2) Besides arithmetic operations, similar reassociation can be applied to
+// GEPs. For example, if
+// X = &arr[a]
+// dominates
+// Y = &arr[a + b]
+// we may rewrite Y into X + b.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/PatternMatch.h"
+#include "llvm/Transforms/Scalar.h"
+using namespace llvm;
+using namespace PatternMatch;
+
+#define DEBUG_TYPE "nary-reassociate"
+
+namespace {
+class NaryReassociate : public FunctionPass {
+public:
+ static char ID;
+
+ NaryReassociate(): FunctionPass(ID) {
+ initializeNaryReassociatePass(*PassRegistry::getPassRegistry());
+ }
+
+ bool runOnFunction(Function &F) override;
+
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.addPreserved<DominatorTreeWrapperPass>();
+ AU.addRequired<DominatorTreeWrapperPass>();
+ // TODO: can we preserve ScalarEvolution?
+ AU.addRequired<ScalarEvolution>();
+ AU.setPreservesCFG();
+ }
+
+private:
+ // Reasssociates I to a better form.
+ Instruction *tryReassociateAdd(Instruction *I);
+ // A helper function for tryReassociateAdd. LHS and RHS are explicitly passed.
+ Instruction *tryReassociateAdd(Value *LHS, Value *RHS, Instruction *I);
+ // Rewrites I to LHS + RHS if LHS is computed already.
+ Instruction *tryReassociatedAdd(const SCEV *LHS, Value *RHS, Instruction *I);
+
+ DominatorTree *DT;
+ ScalarEvolution *SE;
+ // 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. For example,
+ // if (p1)
+ // foo(a + b);
+ // if (p2)
+ // bar(a + b);
+ DenseMap<const SCEV *, SmallVector<Instruction *, 2>> SeenExprs;
+};
+} // anonymous namespace
+
+char NaryReassociate::ID = 0;
+INITIALIZE_PASS_BEGIN(NaryReassociate, "nary-reassociate", "Nary reassociation",
+ false, false)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
+INITIALIZE_PASS_END(NaryReassociate, "nary-reassociate", "Nary reassociation",
+ false, false)
+
+FunctionPass *llvm::createNaryReassociatePass() {
+ return new NaryReassociate();
+}
+
+bool NaryReassociate::runOnFunction(Function &F) {
+ if (skipOptnoneFunction(F))
+ return false;
+
+ DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+ SE = &getAnalysis<ScalarEvolution>();
+
+ // Traverse the dominator tree in the depth-first order. This order makes sure
+ // all bases of a candidate are in Candidates when we process it.
+ bool Changed = false;
+ SeenExprs.clear();
+ for (auto Node = GraphTraits<DominatorTree *>::nodes_begin(DT);
+ Node != GraphTraits<DominatorTree *>::nodes_end(DT); ++Node) {
+ BasicBlock *BB = Node->getBlock();
+ for (auto I = BB->begin(); I != BB->end(); ++I) {
+ if (I->getOpcode() == Instruction::Add) {
+ if (Instruction *NewI = tryReassociateAdd(I)) {
+ I->replaceAllUsesWith(NewI);
+ I->eraseFromParent();
+ I = NewI;
+ }
+ // We should add the rewritten instruction because tryReassociateAdd may
+ // have invalidated the original one.
+ SeenExprs[SE->getSCEV(I)].push_back(I);
+ }
+ }
+ }
+ return Changed;
+}
+
+Instruction *NaryReassociate::tryReassociateAdd(Instruction *I) {
+ Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
+ if (auto *NewI = tryReassociateAdd(LHS, RHS, I))
+ return NewI;
+ if (auto *NewI = tryReassociateAdd(RHS, LHS, I))
+ return NewI;
+ return nullptr;
+}
+
+Instruction *NaryReassociate::tryReassociateAdd(Value *LHS, Value *RHS,
+ Instruction *I) {
+ Value *A = nullptr, *B = nullptr;
+ // To be conservative, we reassociate I only when it is the only user of A+B.
+ if (LHS->hasOneUse() && match(LHS, m_Add(m_Value(A), m_Value(B)))) {
+ // I = (A + B) + RHS
+ // = (A + RHS) + B or (B + RHS) + A
+ const SCEV *AExpr = SE->getSCEV(A), *BExpr = SE->getSCEV(B);
+ const SCEV *RHSExpr = SE->getSCEV(RHS);
+ if (auto *NewI = tryReassociatedAdd(SE->getAddExpr(AExpr, RHSExpr), B, I))
+ return NewI;
+ if (auto *NewI = tryReassociatedAdd(SE->getAddExpr(BExpr, RHSExpr), A, I))
+ return NewI;
+ }
+ return nullptr;
+}
+
+Instruction *NaryReassociate::tryReassociatedAdd(const SCEV *LHSExpr,
+ Value *RHS, Instruction *I) {
+ auto Pos = SeenExprs.find(LHSExpr);
+ // Bail out if LHSExpr is not previously seen.
+ if (Pos == SeenExprs.end())
+ return nullptr;
+
+ auto &LHSCandidates = Pos->second;
+ unsigned NumIterations = 0;
+ // Search at most 10 items to avoid running quadratically.
+ static const unsigned MaxNumIterations = 10;
+ for (auto LHS = LHSCandidates.rbegin();
+ LHS != LHSCandidates.rend() && NumIterations < MaxNumIterations;
+ ++LHS, ++NumIterations) {
+ if (DT->dominates(*LHS, I)) {
+ Instruction *NewI = BinaryOperator::CreateAdd(*LHS, RHS, "", I);
+ NewI->takeName(I);
+ return NewI;
+ }
+ }
+ return nullptr;
+}
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