[llvm-dev] GVN removing loads that are affected by call
Ryan Taylor via llvm-dev
llvm-dev at lists.llvm.org
Thu Jan 14 09:43:11 PST 2021
Yes, this is for downstream/out of tree target so I'm not sure how you
could reproduce it either but I thought the IR might help a bit.
My guess is it's not a bug in GVN as much as an issue with the intrinsic
properties, or lack thereof. I put this in the first post but the Alias
Sets show:
AliasSet[0x75fbd0, 9] may alias, Mod/Ref Pointers: (i8* %1, 8), (i16*
%arrayidx, 2), (i16* %arrayidx1, 2), (i16* %arrayidx5, 2), (i16* %19, 2),
(i16* %arrayidx6, 2), (i16* %arrayidx14, 2), (i16* %arrayidx19, 2)
4 Unknown instructions: void <badref>, <4 x i16>* %6, <4 x i16>* %22,
void <badref>
On Thu, Jan 14, 2021 at 12:37 PM Johannes Doerfert <
johannesdoerfert at gmail.com> wrote:
> There is still not enough information here.
>
> My first guess. The `!tbaa` annotation on the `XXX.intrinsic` and the
> `load`
> basically encode there is no alias. Easy to verify, remove the ones on the
> intrinsic.
>
> ~ Johannes
>
>
> P.S. If this was a bug in GVN, and I assume it is not, a reproducer
> would help
> a lot. So a small IR sample that shows the problem and which we
> can run.
> This is a "redacted?" IR fragment in which I don't know what
> transformation
> is problematic. I also can not run it through GVN, which makes it
> impossible
> to reproduce.
>
>
> On 1/14/21 11:27 AM, Ryan Taylor via llvm-dev wrote:
> > 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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> > llvm-dev at lists.llvm.org
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>
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