From 8e08e73f0e12c9e669f2d7d0a5c9213df3046c01 Mon Sep 17 00:00:00 2001 From: Chris Lattner Date: Sun, 2 Jan 2011 20:24:21 +0000 Subject: [PATCH] If a loop iterates exactly once (has backedge count = 0) then don't mess with it. We'd rather peel/unroll it than convert all of its stores into memsets. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122711 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Transforms/Scalar/LoopIdiomRecognize.cpp | 6 ++++++ test/Transforms/LoopIdiom/basic.ll | 18 ++++++++++++++++++ 2 files changed, 24 insertions(+) diff --git a/lib/Transforms/Scalar/LoopIdiomRecognize.cpp b/lib/Transforms/Scalar/LoopIdiomRecognize.cpp index 8b022bab83a..57a502b1d25 100644 --- a/lib/Transforms/Scalar/LoopIdiomRecognize.cpp +++ b/lib/Transforms/Scalar/LoopIdiomRecognize.cpp @@ -159,6 +159,12 @@ bool LoopIdiomRecognize::runOnLoop(Loop *L, LPPassManager &LPM) { const SCEV *BECount = SE->getBackedgeTakenCount(L); if (isa(BECount)) return false; + // If this loop executes exactly one time, then it should be peeled, not + // optimized by this pass. + if (const SCEVConstant *BECst = dyn_cast(BECount)) + if (BECst->getValue()->getValue() == 0) + return false; + // We require target data for now. TD = getAnalysisIfAvailable(); if (TD == 0) return false; diff --git a/test/Transforms/LoopIdiom/basic.ll b/test/Transforms/LoopIdiom/basic.ll index 3fa6eb900b4..e8e0f38004c 100644 --- a/test/Transforms/LoopIdiom/basic.ll +++ b/test/Transforms/LoopIdiom/basic.ll @@ -188,4 +188,22 @@ for.end: ; preds = %for.body, %entry ; CHECK-NOT: store } +; This is a loop should not be transformed, it only executes one iteration. +define void @test8(i64* %Ptr, i64 %Size) nounwind ssp { +bb.nph: ; preds = %entry + br label %for.body + +for.body: ; preds = %bb.nph, %for.body + %indvar = phi i64 [ 0, %bb.nph ], [ %indvar.next, %for.body ] + %PI = getelementptr i64* %Ptr, i64 %indvar + store i64 0, i64 *%PI + %indvar.next = add i64 %indvar, 1 + %exitcond = icmp eq i64 %indvar.next, 1 + br i1 %exitcond, label %for.end, label %for.body + +for.end: ; preds = %for.body, %entry + ret void +; CHECK: @test8 +; CHECK: store i64 0, i64* %PI +}