https://llvm.org/bugs/show_bug.cgi?id=27894
Bug ID: 27894
Summary: failure to convert FP addition loop into
multiplication with fast-math
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Another example in the loop vectorizer code explosion series (bug 27881, bug
27826) can be seen with:
float multiply_the_hard_way(int n) {
float sum = 0.0f;
for (int i=0; i<n; i++)
sum += 7.0f;
return sum;
}
------------------------------------------------------------------------------
This may look ridiculous in simplified form, but the problem could easily exist
in real code or slightly more real code like bug 27881.
This may be considered a bug before it ever gets to the vectorizer as discussed
recently here:
http://lists.llvm.org/pipermail/llvm-dev/2016-May/099724.html
Ie, if these were integers, we'd convert this into an imul.
------------------------------------------------------------------------------
$ ./clang -O2 multiply_the_hard_way.c -S -ffast-math -emit-llvm -o -
; ModuleID = 'multiply_the_hard_way.c'
source_filename = "multiply_the_hard_way.c"
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.11.0"
; Function Attrs: norecurse nounwind readnone ssp uwtable
define float @multiple_the_hard_way(i32 %n) #0 {
entry:
%cmp5 = icmp sgt i32 %n, 0
br i1 %cmp5, label %for.body.preheader, label %for.cond.cleanup
for.body.preheader: ; preds = %entry
%min.iters.check = icmp ult i32 %n, 8
br i1 %min.iters.check, label %for.body.preheader15, label %min.iters.checked
for.body.preheader15: ; preds = %middle.block,
%min.iters.checked, %for.body.preheader
%i.07.ph = phi i32 [ 0, %min.iters.checked ], [ 0, %for.body.preheader ], [
%n.vec, %middle.block ]
%sum.06.ph = phi float [ 0.000000e+00, %min.iters.checked ], [ 0.000000e+00,
%for.body.preheader ], [ %8, %middle.block ]
br label %for.body
min.iters.checked: ; preds = %for.body.preheader
%n.vec = and i32 %n, -8
%cmp.zero = icmp eq i32 %n.vec, 0
br i1 %cmp.zero, label %for.body.preheader15, label %vector.body.preheader
vector.body.preheader: ; preds = %min.iters.checked
%0 = add i32 %n.vec, -8
%1 = lshr exact i32 %0, 3
%2 = add nuw nsw i32 %1, 1
%xtraiter = and i32 %2, 7
%3 = icmp ult i32 %0, 56
br i1 %3, label %middle.block.unr-lcssa, label %vector.body.preheader.new
vector.body.preheader.new: ; preds =
%vector.body.preheader
%unroll_iter = sub nsw i32 %2, %xtraiter
br label %vector.body
vector.body: ; preds = %vector.body,
%vector.body.preheader.new
%vec.phi = phi <4 x float> [ zeroinitializer, %vector.body.preheader.new ], [
%4, %vector.body ]
%vec.phi9 = phi <4 x float> [ zeroinitializer, %vector.body.preheader.new ],
[ %5, %vector.body ]
%niter = phi i32 [ %unroll_iter, %vector.body.preheader.new ], [
%niter.nsub.7, %vector.body ]
%4 = fadd fast <4 x float> %vec.phi, <float 5.600000e+01, float 5.600000e+01,
float 5.600000e+01, float 5.600000e+01>
%5 = fadd fast <4 x float> %vec.phi9, <float 5.600000e+01, float
5.600000e+01, float 5.600000e+01, float 5.600000e+01>
%niter.nsub.7 = add i32 %niter, -8
%niter.ncmp.7 = icmp eq i32 %niter.nsub.7, 0
br i1 %niter.ncmp.7, label %middle.block.unr-lcssa.loopexit, label
%vector.body, !llvm.loop !2
middle.block.unr-lcssa.loopexit: ; preds = %vector.body
%.lcssa20 = phi <4 x float> [ %5, %vector.body ]
%.lcssa19 = phi <4 x float> [ %4, %vector.body ]
br label %middle.block.unr-lcssa
middle.block.unr-lcssa: ; preds =
%middle.block.unr-lcssa.loopexit, %vector.body.preheader
%.lcssa16.ph = phi <4 x float> [ undef, %vector.body.preheader ], [
%.lcssa20, %middle.block.unr-lcssa.loopexit ]
%.lcssa.ph = phi <4 x float> [ undef, %vector.body.preheader ], [ %.lcssa19,
%middle.block.unr-lcssa.loopexit ]
%vec.phi.unr = phi <4 x float> [ zeroinitializer, %vector.body.preheader ], [
%.lcssa19, %middle.block.unr-lcssa.loopexit ]
%vec.phi9.unr = phi <4 x float> [ zeroinitializer, %vector.body.preheader ],
[ %.lcssa20, %middle.block.unr-lcssa.loopexit ]
%lcmp.mod = icmp eq i32 %xtraiter, 0
br i1 %lcmp.mod, label %middle.block, label %vector.body.epil.preheader
vector.body.epil.preheader: ; preds =
%middle.block.unr-lcssa
br label %vector.body.epil
vector.body.epil: ; preds = %vector.body.epil,
%vector.body.epil.preheader
%vec.phi.epil = phi <4 x float> [ %6, %vector.body.epil ], [ %vec.phi.unr,
%vector.body.epil.preheader ]
%vec.phi9.epil = phi <4 x float> [ %7, %vector.body.epil ], [ %vec.phi9.unr,
%vector.body.epil.preheader ]
%epil.iter = phi i32 [ %epil.iter.sub, %vector.body.epil ], [ %xtraiter,
%vector.body.epil.preheader ]
%6 = fadd fast <4 x float> %vec.phi.epil, <float 7.000000e+00, float
7.000000e+00, float 7.000000e+00, float 7.000000e+00>
%7 = fadd fast <4 x float> %vec.phi9.epil, <float 7.000000e+00, float
7.000000e+00, float 7.000000e+00, float 7.000000e+00>
%epil.iter.sub = add i32 %epil.iter, -1
%epil.iter.cmp = icmp eq i32 %epil.iter.sub, 0
br i1 %epil.iter.cmp, label %middle.block.epilog-lcssa, label
%vector.body.epil, !llvm.loop !5
middle.block.epilog-lcssa: ; preds = %vector.body.epil
%.lcssa22 = phi <4 x float> [ %7, %vector.body.epil ]
%.lcssa21 = phi <4 x float> [ %6, %vector.body.epil ]
br label %middle.block
middle.block: ; preds =
%middle.block.unr-lcssa, %middle.block.epilog-lcssa
%.lcssa16 = phi <4 x float> [ %.lcssa16.ph, %middle.block.unr-lcssa ], [
%.lcssa22, %middle.block.epilog-lcssa ]
%.lcssa = phi <4 x float> [ %.lcssa.ph, %middle.block.unr-lcssa ], [
%.lcssa21, %middle.block.epilog-lcssa ]
%bin.rdx = fadd fast <4 x float> %.lcssa16, %.lcssa
%rdx.shuf = shufflevector <4 x float> %bin.rdx, <4 x float> undef, <4 x i32>
<i32 2, i32 3, i32 undef, i32 undef>
%bin.rdx12 = fadd fast <4 x float> %bin.rdx, %rdx.shuf
%rdx.shuf13 = shufflevector <4 x float> %bin.rdx12, <4 x float> undef, <4 x
i32> <i32 1, i32 undef, i32 undef, i32 undef>
%bin.rdx14 = fadd fast <4 x float> %bin.rdx12, %rdx.shuf13
%8 = extractelement <4 x float> %bin.rdx14, i32 0
%cmp.n = icmp eq i32 %n.vec, %n
br i1 %cmp.n, label %for.cond.cleanup, label %for.body.preheader15
for.cond.cleanup.loopexit: ; preds = %for.body
%add.lcssa = phi float [ %add, %for.body ]
br label %for.cond.cleanup
for.cond.cleanup: ; preds =
%for.cond.cleanup.loopexit, %middle.block, %entry
%sum.0.lcssa = phi float [ 0.000000e+00, %entry ], [ %8, %middle.block ], [
%add.lcssa, %for.cond.cleanup.loopexit ]
ret float %sum.0.lcssa
for.body: ; preds =
%for.body.preheader15, %for.body
%i.07 = phi i32 [ %inc, %for.body ], [ %i.07.ph, %for.body.preheader15 ]
%sum.06 = phi float [ %add, %for.body ], [ %sum.06.ph, %for.body.preheader15
]
%add = fadd fast float %sum.06, 7.000000e+00
%inc = add nuw nsw i32 %i.07, 1
%exitcond = icmp eq i32 %inc, %n
br i1 %exitcond, label %for.cond.cleanup.loopexit, label %for.body,
!llvm.loop !7
}
attributes #0 = { norecurse nounwind readnone ssp uwtable
"disable-tail-calls"="false" "less-precise-fpmad"="false"
"no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="true" "no-jump-tables"="false" "no-nans-fp-math"="true"
"stack-protector-buffer-size"="8" "target-cpu"="core2"
"target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+ssse3,+x87"
"unsafe-fp-math"="true" "use-soft-float"="false" }
!llvm.module.flags = !{!0}
!llvm.ident = !{!1}
!0 = !{i32 1, !"PIC Level", i32 2}
!1 = !{!"clang version 3.9.0 (trunk 270847)"}
!2 = distinct !{!2, !3, !4}
!3 = !{!"llvm.loop.vectorize.width", i32 1}
!4 = !{!"llvm.loop.interleave.count", i32 1}
!5 = distinct !{!5, !6}
!6 = !{!"llvm.loop.unroll.disable"}
!7 = distinct !{!7, !8, !3, !4}
!8 = !{!"llvm.loop.unroll.runtime.disable"}
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