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Johannes Doerfert via llvm-dev

2021-Jan-14 17:37 UTC

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

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
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Ryan Taylor via llvm-dev

2021-Jan-14 17:43 UTC

head link

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

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
> >
> > _______________________________________________
> > 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 - Jan 2021 - GVN removing loads that are affected by call

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

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