[llvm-dev] GVN removing loads that are affected by call
Johannes Doerfert via llvm-dev
llvm-dev at lists.llvm.org
Thu Jan 14 11:00:37 PST 2021
Nothing comes to mind given the information.
~ Johannes
On 1/14/21 12:56 PM, Ryan Taylor wrote:
> argmemonly, nounwind, writeonly
>
> -fno-strict-aliasing did not help.
>
> On Thu, Jan 14, 2021 at 1:29 PM Johannes Doerfert <
> johannesdoerfert at gmail.com> wrote:
>
>> 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
>>>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
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