[llvm-dev] GVN removing loads that are affected by call
Johannes Doerfert via llvm-dev
llvm-dev at lists.llvm.org
Thu Jan 14 10:28:55 PST 2021
Can you share the attributes of the intrinsic declaration,
assuming removing `!tbaa` didn't help.
On 1/14/21 11:43 AM, Ryan Taylor wrote:
> 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
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