[llvm-dev] GVN removing loads that are affected by call

[Ryan Taylor via llvm-dev]

This is right before GVN:

define i32 @foo(<4 x i16> %p, <4 x i16> %p1, i16* nocapture %res)
local_unnamed_addr #0 !dbg !6 {
entry:
  %temp = alloca i64, align 8
  %tmpcast = bitcast i64* %temp to [4 x i16]*
  %0 = bitcast i64* %temp to i8*, !dbg !8
  call void @llvm.lifetime.start.p0i8(i64 8, i8* nonnull %0) #3, !dbg !8
  store i64 0, i64* %temp, align 8, !dbg !9
  %1 = bitcast i64* %temp to <4 x i16>*, !dbg !10
  %2 = call <4 x i16>* @llvm.XXX.intrinsic(<4 x i16>* nonnull %1, <4 x i16>
%p, i32 0), !dbg !11, !tbaa !12
  %arrayidx = bitcast i64* %temp to i16*, !dbg !16
  %3 = load i16, i16* %arrayidx, align 8, !dbg !16, !tbaa !17
  br label %for.body, !dbg !19

for.body:                                         ; preds = %entry
  %arrayidx1 = getelementptr inbounds [4 x i16], [4 x i16]* %tmpcast, i32
0, i32 1, !dbg !20
  %4 = load i16, i16* %arrayidx1, align 2, !dbg !20, !tbaa !17
  %cmp3 = icmp sgt i16 %3, %4, !dbg !21
  %spec.select = select i1 %cmp3, i16 %4, i16 %3, !dbg !22
  %arrayidx1.1 = getelementptr inbounds [4 x i16], [4 x i16]* %tmpcast, i32
0, i32 2, !dbg !20
  %5 = load i16, i16* %arrayidx1.1, align 2, !dbg !20, !tbaa !17
  %cmp3.1 = icmp sgt i16 %spec.select, %5, !dbg !21
  %spec.select.1 = select i1 %cmp3.1, i16 %5, i16 %spec.select, !dbg !22
  %arrayidx1.2 = getelementptr inbounds [4 x i16], [4 x i16]* %tmpcast, i32
0, i32 3, !dbg !20
  %6 = load i16, i16* %arrayidx1.2, align 2, !dbg !20, !tbaa !17
  %cmp3.2 = icmp sgt i16 %spec.select.1, %6, !dbg !21
  %spec.select.2 = select i1 %cmp3.2, i16 %6, i16 %spec.select.1, !dbg !22
  store i16 %spec.select.2, i16* %res, align 2, !dbg !23, !tbaa !17
  %7 = tail call <4 x i16>* @llvm.XXX.intrinsic(<4 x i16>* %2, <4 x i16>
%p1, i32 0), !dbg !24, !tbaa !12
  %8 = load i16, i16* %arrayidx, align 8, !dbg !25, !tbaa !17
  br label %for.body12, !dbg !26

for.body12:                                       ; preds = %for.body
  %arrayidx14 = getelementptr inbounds [4 x i16], [4 x i16]* %tmpcast, i32
0, i32 1, !dbg !27
  %9 = load i16, i16* %arrayidx14, align 2, !dbg !27, !tbaa !17
  %cmp16 = icmp sgt i16 %8, %9, !dbg !28
  %spec.select39 = select i1 %cmp16, i16 %9, i16 %8, !dbg !29
  %arrayidx14.1 = getelementptr inbounds [4 x i16], [4 x i16]* %tmpcast,
i32 0, i32 2, !dbg !27
  %10 = load i16, i16* %arrayidx14.1, align 2, !dbg !27, !tbaa !17
  %cmp16.1 = icmp sgt i16 %spec.select39, %10, !dbg !28
  %spec.select39.1 = select i1 %cmp16.1, i16 %10, i16 %spec.select39, !dbg
!29
  %arrayidx14.2 = getelementptr inbounds [4 x i16], [4 x i16]* %tmpcast,
i32 0, i32 3, !dbg !27
  %11 = load i16, i16* %arrayidx14.2, align 2, !dbg !27, !tbaa !17
  %cmp16.2 = icmp sgt i16 %spec.select39.1, %11, !dbg !28
  %spec.select39.2 = select i1 %cmp16.2, i16 %11, i16 %spec.select39.1,
!dbg !29
  %conv24 = sext i16 %spec.select39.2 to i32, !dbg !30
  call void @llvm.lifetime.end.p0i8(i64 8, i8* nonnull %0) #3, !dbg !31
  ret i32 %conv24, !dbg !32

On Thu, Jan 14, 2021 at 11:54 AM Roman Lebedev <lebedev.ri at gmail.com> wrote:

> It would be good to have an actual IR reproducer here.
>
> On Thu, Jan 14, 2021 at 7:51 PM Ryan Taylor via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > So given an intrinsic that has a pointer as in/out and IntrWriteMem
> property.
> >
> > call intrinsic(address a, ....);
> > loop over address a
> >   load from address a + offset
> > call intrinsic (address a, ...);
> > loop over address a
> >   load from address a + offset
> >
> > GVN is removing the second loads, despite the second call overwriting
> the memory starting at address a. AA has the intrinsics marked as unknown
> instructions but has all of these as mayAlias in a set. I'm not seeing this
> issue with -fno-unroll-loops.
> >
> > Thanks.
> >
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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