From 11b279a832b26214d2441fec3abf98ab97b00bce Mon Sep 17 00:00:00 2001 From: Sanjoy Das Date: Wed, 18 Feb 2015 19:32:25 +0000 Subject: Partial fix for bug 22589 Don't spend the entire iteration space in the scalar loop prologue if computing the trip count overflows. This change also gets rid of the backedge check in the prologue loop and the extra check for overflowing trip-count. Differential Revision: http://reviews.llvm.org/D7715 llvm-svn: 229731 --- .../Transforms/LoopUnroll/tripcount-overflow.ll | 29 ++++++++++++++-------- 1 file changed, 18 insertions(+), 11 deletions(-) (limited to 'llvm/test/Transforms/LoopUnroll/tripcount-overflow.ll') diff --git a/llvm/test/Transforms/LoopUnroll/tripcount-overflow.ll b/llvm/test/Transforms/LoopUnroll/tripcount-overflow.ll index d59368578ec..052077cdd5d 100644 --- a/llvm/test/Transforms/LoopUnroll/tripcount-overflow.ll +++ b/llvm/test/Transforms/LoopUnroll/tripcount-overflow.ll @@ -1,19 +1,28 @@ ; RUN: opt < %s -S -unroll-runtime -unroll-count=2 -loop-unroll | FileCheck %s target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128" -; When prologue is fully unrolled, the branch on its end is unconditional. -; Unrolling it is illegal if we can't prove that trip-count+1 doesn't overflow, -; like in this example, where it comes from an argument. -; -; This test is based on an example from here: -; http://stackoverflow.com/questions/23838661/why-is-clang-optimizing-this-code-out -; +; This test case documents how runtime loop unrolling handles the case +; when the backedge-count is -1. + +; If %N, the backedge-taken count, is -1 then %0 unsigned-overflows +; and is 0. %xtraiter too is 0, signifying that the total trip-count +; is divisible by 2. The prologue then branches to the unrolled loop +; and executes the 2^32 iterations there, in groups of 2. + + +; CHECK: entry: +; CHECK-NEXT: %0 = add i32 %N, 1 +; CHECK-NEXT: %xtraiter = and i32 %0, 1 +; CHECK-NEXT: %lcmp.mod = icmp ne i32 %xtraiter, 0 +; CHECK-NEXT: br i1 %lcmp.mod, label %while.body.prol, label %entry.split + ; CHECK: while.body.prol: -; CHECK: br i1 +; CHECK: br label %entry.split + ; CHECK: entry.split: ; Function Attrs: nounwind readnone ssp uwtable -define i32 @foo(i32 %N) #0 { +define i32 @foo(i32 %N) { entry: br label %while.body @@ -26,5 +35,3 @@ while.body: ; preds = %while.body, %entry while.end: ; preds = %while.body ret i32 %i } - -attributes #0 = { nounwind readnone ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" } -- cgit v1.2.3