[llvm-dev] GVN removing loads that are affected by call
Ryan Taylor via llvm-dev
llvm-dev at lists.llvm.org
Thu Jan 14 10:56:25 PST 2021
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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