From 07005440ae148d1eb102493a1148e2818fab21e0 Mon Sep 17 00:00:00 2001 From: Zaara Syeda Date: Thu, 7 Apr 2022 12:27:05 -0400 Subject: [PATCH] [LSR] Optimize unused IVs to final values in the exit block Loop Strength Reduce sometimes optimizes away all uses of an induction variable from a loop but leaves the IV increments. When the only remaining use of the IV is the PHI in the exit block, this patch will call rewriteLoopExitValues to replace the exit block PHI with the final value of the IV to skip the updates in each loop iteration. Differential Revision: https://reviews.llvm.org/D118808 --- .../Transforms/Scalar/LoopStrengthReduce.cpp | 40 +++++++++++++ .../LoopStrengthReduce/remove_scev_indvars.ll | 57 +++++++++++++++++++ 2 files changed, 97 insertions(+) create mode 100644 llvm/test/Transforms/LoopStrengthReduce/remove_scev_indvars.ll diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp index fd20de28a009..d1e03292cb82 100644 --- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp +++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp @@ -113,6 +113,7 @@ #include "llvm/Transforms/Utils.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" #include "llvm/Transforms/Utils/Local.h" +#include "llvm/Transforms/Utils/LoopUtils.h" #include "llvm/Transforms/Utils/ScalarEvolutionExpander.h" #include #include @@ -5600,6 +5601,27 @@ void LSRInstance::Rewrite(const LSRUse &LU, const LSRFixup &LF, DeadInsts.emplace_back(OperandIsInstr); } +// Check if there are any loop exit values which are only used once within the +// loop which may potentially be optimized with a call to rewriteLoopExitValue. +static bool LoopExitValHasSingleUse(Loop *L) { + BasicBlock *ExitBB = L->getExitBlock(); + if (!ExitBB) + return false; + + for (PHINode &ExitPhi : ExitBB->phis()) { + if (ExitPhi.getNumIncomingValues() != 1) + break; + + BasicBlock *Pred = ExitPhi.getIncomingBlock(0); + Value *IVNext = ExitPhi.getIncomingValueForBlock(Pred); + // One use would be the exit phi node, and there should be only one other + // use for this to be considered. + if (IVNext->getNumUses() == 2) + return true; + } + return false; +} + /// Rewrite all the fixup locations with new values, following the chosen /// solution. void LSRInstance::ImplementSolution( @@ -6374,6 +6396,24 @@ static bool ReduceLoopStrength(Loop *L, IVUsers &IU, ScalarEvolution &SE, DeleteDeadPHIs(L->getHeader(), &TLI, MSSAU.get()); } } + // LSR may at times remove all uses of an induction variable from a loop. + // The only remaining use is the PHI in the exit block. + // When this is the case, if the exit value of the IV can be calculated using + // SCEV, we can replace the exit block PHI with the final value of the IV and + // skip the updates in each loop iteration. + if (L->isRecursivelyLCSSAForm(DT, LI) && LoopExitValHasSingleUse(L)) { + SmallVector DeadInsts; + const DataLayout &DL = L->getHeader()->getModule()->getDataLayout(); + SCEVExpander Rewriter(SE, DL, "lsr", false); + int Rewrites = rewriteLoopExitValues(L, &LI, &TLI, &SE, &TTI, Rewriter, &DT, + OnlyCheapRepl, DeadInsts); + if (Rewrites) { + Changed = true; + RecursivelyDeleteTriviallyDeadInstructionsPermissive(DeadInsts, &TLI, + MSSAU.get()); + DeleteDeadPHIs(L->getHeader(), &TLI, MSSAU.get()); + } + } if (SalvageableDVI.empty()) return Changed; diff --git a/llvm/test/Transforms/LoopStrengthReduce/remove_scev_indvars.ll b/llvm/test/Transforms/LoopStrengthReduce/remove_scev_indvars.ll new file mode 100644 index 000000000000..9a9ef194caea --- /dev/null +++ b/llvm/test/Transforms/LoopStrengthReduce/remove_scev_indvars.ll @@ -0,0 +1,57 @@ +; RUN: opt < %s -S -loop-reduce | FileCheck %s + +define void @testIVNext(i64* nocapture %a, i64 signext %m, i64 signext %n) { +entry: + br label %for.body + +for.body: + %indvars.iv.prol = phi i64 [ %indvars.iv.next.prol, %for.body ], [ %m, %entry ] + %i = phi i64 [ %i.next, %for.body ], [ 0, %entry ] + %uglygep138 = getelementptr i64, i64* %a, i64 %i + store i64 55, i64* %uglygep138, align 4 + %indvars.iv.next.prol = add nuw nsw i64 %indvars.iv.prol, 1 + %i.next = add i64 %i, 1 + %i.cmp.not = icmp eq i64 %i.next, %n + br i1 %i.cmp.not, label %for.exit, label %for.body + +; CHECK: entry: +; CHECK: %0 = add i64 %n, %m +; CHECK-NOT : %indvars.iv.next.prol +; CHECK-NOT: %indvars.iv.prol +; CHECK: %indvars.iv.unr = phi i64 [ %0, %for.exit ] +for.exit: + %indvars.iv.next.prol.lcssa = phi i64 [ %indvars.iv.next.prol, %for.body ] + br label %exit + +exit: + %indvars.iv.unr = phi i64 [ %indvars.iv.next.prol.lcssa, %for.exit ] + ret void +} + +define void @testIV(i64* nocapture %a, i64 signext %m, i64 signext %n) { +entry: + br label %for.body + +for.body: + %iv.prol = phi i64 [ %iv.next.prol, %for.body ], [ %m, %entry ] + %i = phi i64 [ %i.next, %for.body ], [ 0, %entry ] + %uglygep138 = getelementptr i64, i64* %a, i64 %i + store i64 55, i64* %uglygep138, align 4 + %iv.next.prol = add nuw nsw i64 %iv.prol, 1 + %i.next = add i64 %i, 1 + %i.cmp.not = icmp eq i64 %i.next, %n + br i1 %i.cmp.not, label %for.exit, label %for.body + +; CHECK: entry: +; CHECK: %0 = add i64 %n, %m +; CHECK: %1 = add i64 %0, -1 +; CHECK-NOT: %iv.next.prol +; CHECK-NOT: %iv.prol +; CHECK: %indvars.iv.unr = phi i64 [ %1, %for.exit ] +for.exit: + %iv.prol.lcssa = phi i64 [ %iv.prol, %for.body ] + br label %exit +exit: + %indvars.iv.unr = phi i64 [%iv.prol.lcssa, %for.exit] + ret void +}