[llvm-dev] GVN / Alias Analysis issue with llvm.masked.scatter/gather intrinsics
Daniel Berlin via llvm-dev
llvm-dev at lists.llvm.org
Mon Aug 29 08:58:21 PDT 2016
this is definitely a bug in AA.
225 for (auto I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
226 const Value *Arg = *I;
227 if (!Arg->getType()->isPointerTy())
-> 228 continue;
229 unsigned CS2ArgIdx = std::distance(CS2.arg_begin(), I);
230 auto CS2ArgLoc = MemoryLocation::getForArgument(CS2,
CS2ArgIdx, TLI);
AliasAnalysis.cpp:228
It ignores every argument because they are vectors of pointers, not
pointers.
I'm surprised this has not broken anything before. It will never say two
intrinsics with vectors of pointers mod/ref each other.
On Mon, Aug 29, 2016 at 7:36 AM, Davide Italiano <davide at freebsd.org> wrote:
> + Daniel Berlin
>
> On Mon, Aug 29, 2016 at 3:42 PM, Chris Sakalis via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Hello everyone,
> >
> > I think I have found an gvn / alias analysis related bug, but before
> opening
> > an issue on the tracker I wanted to see if I am missing something. I have
> > the following testcase:
> >
> >> define spir_kernel void @test(<2 x i32*> %in1, <2 x i32*> %in2, i32*
> %out)
> >> {
> >> entry:
> >> ; Just some temporary storage
> >> %tmp.0 = alloca i32
> >> %tmp.1 = alloca i32
> >> %tmp.i = insertelement <2 x i32*> undef, i32* %tmp.0, i32 0
> >> %tmp = insertelement <2 x i32*> %tmp.i, i32* %tmp.1, i32 1
> >> ; Read from in1 and in2
> >> %in1.v = call <2 x i32> @llvm.masked.gather.v2i32(<2 x i32*> %in1, i32
> >> 1, <2 x i1> <i1 true, i1 true>, <2 x i32> undef) #1
> >> %in2.v = call <2 x i32> @llvm.masked.gather.v2i32(<2 x i32*> %in2, i32
> >> 1, <2 x i1> <i1 true, i1 true>, <2 x i32> undef) #1
> >> ; Store in1 to the allocas
> >> call void @llvm.masked.scatter.v2i32(<2 x i32> %in1.v, <2 x i32*>
> %tmp,
> >> i32 1, <2 x i1> <i1 true, i1 true>);
> >> ; Read in1 from the allocas
> >> %tmp.v.0 = call <2 x i32> @llvm.masked.gather.v2i32(<2 x i32*> %tmp,
> i32
> >> 1, <2 x i1> <i1 true, i1 true>, <2 x i32> undef) #1
> >> ; Store in2 to the allocas
> >> call void @llvm.masked.scatter.v2i32(<2 x i32> %in2.v, <2 x i32*>
> %tmp,
> >> i32 1, <2 x i1> <i1 true, i1 true>);
> >> ; Read in2 from the allocas
> >> %tmp.v.1 = call <2 x i32> @llvm.masked.gather.v2i32(<2 x i32*> %tmp,
> i32
> >> 1, <2 x i1> <i1 true, i1 true>, <2 x i32> undef) #1
> >> ; Store in2 to out for good measure
> >> %tmp.v.1.0 = extractelement <2 x i32> %tmp.v.1, i32 0
> >> %tmp.v.1.1 = extractelement <2 x i32> %tmp.v.1, i32 1
> >> store i32 %tmp.v.1.0, i32* %out
> >> %out.1 = getelementptr i32, i32* %out, i32 1
> >> store i32 %tmp.v.1.1, i32* %out.1
> >> ret void
> >> }
> >
> >
> > It uses a masked scatter operation to store a value to the two allocas
> and
> > then uses a masked gather operation to read that same value. This is done
> > twice in a row, with two different values. If I run this code through the
> > GVN pass, the second gather (%tmp.v.1) will be deemed to be the same as
> the
> > first gather (%tmp.v.0) and it will be removed. After some debugging, I
> > realized that this is happening because the Memory Dependence Analysis
> > returns %tmp.v.0 as the Def dependency for %tmp.v.1, even though the
> scatter
> > call in between changes the value stored at %tmp. This, in turn, is
> > happening because the alias analysis is returning NoModRef for the
> %tmp.v.1
> > and second scatter callsites. The resulting IR produces the wrong result:
> >
> >> define spir_kernel void @test(<2 x i32*> %in1, <2 x i32*> %in2, i32*
> %out)
> >> {
> >> entry:
> >> %tmp.0 = alloca i32
> >> %tmp.1 = alloca i32
> >> %tmp.i = insertelement <2 x i32*> undef, i32* %tmp.0, i32 0
> >> %tmp = insertelement <2 x i32*> %tmp.i, i32* %tmp.1, i32 1
> >> %in1.v = call <2 x i32> @llvm.masked.gather.v2i32(<2 x i32*> %in1, i32
> >> 1, <2 x i1> <i1 true, i1 true>, <2 x i32> undef) #1
> >> %in2.v = call <2 x i32> @llvm.masked.gather.v2i32(<2 x i32*> %in2, i32
> >> 1, <2 x i1> <i1 true, i1 true>, <2 x i32> undef) #1
> >> call void @llvm.masked.scatter.v2i32(<2 x i32> %in1.v, <2 x i32*>
> %tmp,
> >> i32 1, <2 x i1> <i1 true, i1 true>)
> >> %tmp.v.0 = call <2 x i32> @llvm.masked.gather.v2i32(<2 x i32*> %tmp,
> i32
> >> 1, <2 x i1> <i1 true, i1 true>, <2 x i32> undef) #1
> >> call void @llvm.masked.scatter.v2i32(<2 x i32> %in2.v, <2 x i32*>
> %tmp,
> >> i32 1, <2 x i1> <i1 true, i1 true>)
> >> ; The call to masked.gather is gone and now we are using the old value
> >> (%tmp.v.0)
> >> %tmp.v.1.0 = extractelement <2 x i32> %tmp.v.0, i32 0
> >> %tmp.v.1.1 = extractelement <2 x i32> %tmp.v.0, i32 1
> >> store i32 %tmp.v.1.0, i32* %out
> >> %out.1 = getelementptr i32, i32* %out, i32 1
> >> store i32 %tmp.v.1.1, i32* %out.1
> >> ret void
> >> }
> >
> >
> > The old value read from %tmp is used, instead of the new one. I tested
> this
> > code using `opt -gvn`, with LLVM 3.8.1. I also tried tip (84cb7f4) with
> the
> > same result.
> >
> > I should mention that if I replace the second scatter with stores, the
> issue
> > persists. The only way to avoid it is to replace all scatter/gather
> > intrinsics with equivalent sets of store/load, in which case the MemDep
> > returns the correct dependencies and the GVN pass will not remove the
> second
> > set of loads.
> >
> > So, my question is, is this a bug or am I doing something that I
> shouldn't
> > be in the IR? And if it is a bug, is it the AA analyses that return the
> > wrong result (I presume so) or should GVN handle such cases differently?
> >
> > Best regards,
> >
> > Chris
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
>
>
> --
> Davide
>
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare
>
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