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

Thu Jan 14 09:37:29 PST 2021

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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