[llvm-dev] GVN removing loads that are affected by call
Ryan Taylor via llvm-dev
llvm-dev at lists.llvm.org
Fri Jan 15 17:32:17 PST 2021
So this seems to be an issue with the intrinsic setting NoCapture<0> (it
returns the pointer it takes). We have a similar intrinsic that does the
exact same thing but doesn't return the pointer. It also has NoCapture<1>
and doesn't cause this issue. Also, I managed to substitute memcpy and
there were no issues, it also has nocapture.
Are there some papers/citations which talk more about Pointer
Capture/Capture Tracking?
I saw this blog:
https://jonasdevlieghere.com/escape-analysis-capture-tracking-in-llvm/
And I read the CaptureTracking.h short description but I'd like a clearer
grasp.
Thanks,
Ryan
On Thu, Jan 14, 2021 at 2:03 PM Ryan Taylor <ryta1203 at gmail.com> wrote:
> Ok, thanks. I'll try to come up with a more generic test case.
>
> On Thu, Jan 14, 2021 at 2:00 PM Johannes Doerfert <
> johannesdoerfert at gmail.com> wrote:
>
>> 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
>> >>>>> llvm-dev at lists.llvm.org
>> >>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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