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Hal Finkel via llvm-dev

2016-Aug-30 20:11 UTC

[llvm-dev] GVN / Alias Analysis issue with llvm.masked.scatter/gather intrinsics

----- Original Message -----
> From: "Daniel Berlin" <dberlin at dberlin.org>
> To: "Philip Reames" <listmail at philipreames.com>,
"Davide Italiano"
> <davide at freebsd.org>, "Chandler Carruth" <chandlerc
at gmail.com>
> Cc: "Chris Sakalis" <chrissakalis at gmail.com>,
"David Majnemer"
> <david.majnemer at gmail.com>, "Hal Finkel" <hfinkel at
anl.gov>,
> "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Tuesday, August 30, 2016 3:07:01 PM
> Subject: Re: [llvm-dev] GVN / Alias Analysis issue with
> llvm.masked.scatter/gather intrinsics
> This is now committed and a test added to GVN.
Thanks! 

I suspect that, in practice, we'll get little benefit from handling this
until our AA passes learn how to deal with (i.e. look back through) pointer
vectors.

-Hal 
> On Mon, Aug 29, 2016 at 1:59 PM, Daniel Berlin < dberlin at dberlin.org
> > wrote:
> > Okay, so then it sounds like, for now, the right fix is to stop
> > marking masked.gather and masked.scatter with intrarg* options.
> 
> > On Mon, Aug 29, 2016, 1:26 PM Philip Reames <
> > listmail at philipreames.com > wrote:
> 
> > > We might have specification bug here, but we appear to implement
> > > what
> > > we specified. argmemonly is specified as only considering pointer
> > > typed arguments. It's behavior on vector-of-pointers is
> > > unspecified,
> > > but would seem to fall into the same case as inttoptr of an
> > > integer
> > > argument (i.e. explicitly undefined). We could consider changing
> > > that.
> > 
> 
> > > On 08/29/2016 11:59 AM, Daniel Berlin wrote:
> > 
> 
> > > > + a few others.
> > > 
> > 
> 
> > > > After following this rabbit hole a bit, there are a lot of
> > > > mutually
> > > > recursive calls, etc, that may or may not do the right thing
> > > > with
> > > > vectors of pointers.
> > > 
> > 
> 
> > > > I can fix *this* particular bug with the attached patch.
> > > 
> > 
> 
> > > > However, it's mostly papering over stuff. Nothing seems
to know
> > > > what
> > > > to do with a memorylocation that is a vector of pointers.
They
> > > > all
> > > > expect memorylocation to be a single pointer location.
> > > 
> > 
> 
> > > > I would chalk it up to "luck" that this patch
fixes the bug.
> > > 
> > 
> 
> > > > It's pretty clear that MemoryLocation doesn't fit
the needs of
> > > > a
> > > > lot
> > > > of stuff anymore (we hacked AA nodes into it, and lots of
stuff
> > > > now
> > > > tries to figure out the invariantess of the locations, blah
> > > > blah
> > > > blah), but it seems like a big job to figure out what to
> > > > replace
> > > > it
> > > > with that will work for these cases.
> > > 
> > 
> 
> > > > (I'm pretty positive if we just make it MemoryLocations,
and
> > > > have
> > > > everything loop over the locations, the compiler will get a
lot
> > > > larger and a lot slower)
> > > 
> > 
> 
> > > > On Mon, Aug 29, 2016 at 9:10 AM, Daniel Berlin <
> > > > dberlin at dberlin.org
> > > > > wrote:
> > > 
> > 
> 
> > > > > it would also, for that matter, say the same about an
array
> > > > > of
> > > > > pointers.
> > > > 
> > > 
> > 
> 
> > > > > It's not clear to me what will break if you change
this to
> > > > > isPtrOrPtrVectorTy.
> > > > 
> > > 
> > 
> 
> > > > > In fact, i know it doesn't fix this bug.
> > > > 
> > > 
> > 
> 
> > > > > It's a pretty deep rabbit hole of things not quite
prepared
> > > > > to
> > > > > understand vectors of pointers.
> > > > 
> > > 
> > 
> 
> > > > > (we prepare memorylocations of them, but memory
locations
> > > > > expect
> > > > > to
> > > > > be one thing, not a group of things, etc).
> > > > 
> > > 
> > 
> 
> > > > > On Mon, Aug 29, 2016 at 8:58 AM, Daniel Berlin <
> > > > > dberlin at dberlin.org
> > > > > > wrote:
> > > > 
> > > 
> > 
> 
> > > > > > 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
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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Daniel Berlin via llvm-dev

2016-Aug-30 20:26 UTC

head link

[llvm-dev] GVN / Alias Analysis issue with llvm.masked.scatter/gather intrinsics

Yeah, i just hope it doesn't regress scatter/gather vector code badly.
But at least it's correct now?


On Tue, Aug 30, 2016 at 1:11 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>
> ------------------------------
>
> *From: *"Daniel Berlin" <dberlin at dberlin.org>
> *To: *"Philip Reames" <listmail at philipreames.com>,
"Davide Italiano" <
> davide at freebsd.org>, "Chandler Carruth" <chandlerc at
gmail.com>
> *Cc: *"Chris Sakalis" <chrissakalis at gmail.com>,
"David Majnemer" <
> david.majnemer at gmail.com>, "Hal Finkel" <hfinkel at
anl.gov>, "llvm-dev" <
> llvm-dev at lists.llvm.org>
> *Sent: *Tuesday, August 30, 2016 3:07:01 PM
> *Subject: *Re: [llvm-dev] GVN / Alias Analysis issue with
> llvm.masked.scatter/gather intrinsics
>
> This is now committed and a test added to GVN.
>
> Thanks!
>
> I suspect that, in practice, we'll get little benefit from handling
this
> until our AA passes learn how to deal with (i.e. look back through) pointer
> vectors.
>
>  -Hal
>
>
>
> On Mon, Aug 29, 2016 at 1:59 PM, Daniel Berlin <dberlin at
dberlin.org>
> wrote:
>
>> Okay, so then it sounds like, for now, the right fix is to stop marking
>> masked.gather and masked.scatter with intrarg* options.
>>
>> On Mon, Aug 29, 2016, 1:26 PM Philip Reames <listmail at
philipreames.com>
>> wrote:
>>
>>> We might have specification bug here, but we appear to implement
what we
>>> specified.  argmemonly is specified as only considering pointer
typed
>>> arguments.  It's behavior on vector-of-pointers is unspecified,
but would
>>> seem to fall into the same case as inttoptr of an integer argument
(i.e.
>>> explicitly undefined).  We could consider changing that.
>>>
>>>
>>>
>>> On 08/29/2016 11:59 AM, Daniel Berlin wrote:
>>>
>>> + a few others.
>>>
>>> After following this rabbit hole a bit, there are a lot of mutually
>>> recursive calls, etc, that may or may not do the right thing with
vectors
>>> of pointers.
>>> I can fix *this* particular bug with the attached patch.
>>>
>>> However, it's mostly papering over stuff.  Nothing seems to
know what to
>>> do with a memorylocation that is a vector of pointers. They all
expect
>>> memorylocation to be a single pointer location.
>>>
>>> I would chalk it up to "luck" that this patch fixes the
bug.
>>>
>>> It's pretty clear that MemoryLocation doesn't fit the needs
of a lot of
>>> stuff anymore (we hacked AA nodes into it, and lots of stuff now
tries to
>>> figure out the invariantess of the locations, blah blah blah), but
it seems
>>> like a big job to figure out what to replace it with that will work
for
>>> these cases.
>>>
>>> (I'm pretty positive if we just make it MemoryLocations, and
have
>>> everything loop over the locations, the compiler will get a lot
larger and
>>> a lot slower)
>>>
>>> On Mon, Aug 29, 2016 at 9:10 AM, Daniel Berlin <dberlin at
dberlin.org>
>>> wrote:
>>>
>>>> it would also, for that matter, say the same about an array of
>>>> pointers.
>>>>
>>>> It's not clear to me what will break if you change this to
>>>> isPtrOrPtrVectorTy.
>>>>
>>>> In fact, i know it doesn't fix this bug.
>>>> It's a pretty deep rabbit hole of things not quite prepared
to
>>>> understand vectors of pointers.
>>>>
>>>> (we prepare memorylocations of them, but memory locations
expect to be
>>>> one thing, not a group of things, etc).
>>>>
>>>>
>>>>
>>>>
>>>> On Mon, Aug 29, 2016 at 8:58 AM, Daniel Berlin <dberlin at
dberlin.org>
>>>> wrote:
>>>>
>>>>> 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
>>>>>>
>>>>>
>>>>>
>>>>
>>>
>>>
>
>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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Chris Sakalis via llvm-dev

2016-Aug-31 08:58 UTC

head link

[llvm-dev] GVN / Alias Analysis issue with llvm.masked.scatter/gather intrinsics

Thank you for the quick fix, I can no longer reproduce the issue. As far a
releases go, I am guessing that this is going to be in 4.0?

Best,

Chris

On Tue, Aug 30, 2016 at 9:26 PM, Daniel Berlin <dberlin at dberlin.org>
wrote:
> Yeah, i just hope it doesn't regress scatter/gather vector code badly.
> But at least it's correct now?
>
>
> On Tue, Aug 30, 2016 at 1:11 PM, Hal Finkel <hfinkel at anl.gov>
wrote:
>
>>
>> ------------------------------
>>
>> *From: *"Daniel Berlin" <dberlin at dberlin.org>
>> *To: *"Philip Reames" <listmail at philipreames.com>,
"Davide Italiano" <
>> davide at freebsd.org>, "Chandler Carruth" <chandlerc
at gmail.com>
>> *Cc: *"Chris Sakalis" <chrissakalis at gmail.com>,
"David Majnemer" <
>> david.majnemer at gmail.com>, "Hal Finkel" <hfinkel at
anl.gov>, "llvm-dev" <
>> llvm-dev at lists.llvm.org>
>> *Sent: *Tuesday, August 30, 2016 3:07:01 PM
>> *Subject: *Re: [llvm-dev] GVN / Alias Analysis issue with
>> llvm.masked.scatter/gather intrinsics
>>
>> This is now committed and a test added to GVN.
>>
>> Thanks!
>>
>> I suspect that, in practice, we'll get little benefit from handling
this
>> until our AA passes learn how to deal with (i.e. look back through)
pointer
>> vectors.
>>
>>  -Hal
>>
>>
>>
>> On Mon, Aug 29, 2016 at 1:59 PM, Daniel Berlin <dberlin at
dberlin.org>
>> wrote:
>>
>>> Okay, so then it sounds like, for now, the right fix is to stop
marking
>>> masked.gather and masked.scatter with intrarg* options.
>>>
>>> On Mon, Aug 29, 2016, 1:26 PM Philip Reames <listmail at
philipreames.com>
>>> wrote:
>>>
>>>> We might have specification bug here, but we appear to
implement what
>>>> we specified.  argmemonly is specified as only considering
pointer typed
>>>> arguments.  It's behavior on vector-of-pointers is
unspecified, but would
>>>> seem to fall into the same case as inttoptr of an integer
argument (i.e.
>>>> explicitly undefined).  We could consider changing that.
>>>>
>>>>
>>>>
>>>> On 08/29/2016 11:59 AM, Daniel Berlin wrote:
>>>>
>>>> + a few others.
>>>>
>>>> After following this rabbit hole a bit, there are a lot of
mutually
>>>> recursive calls, etc, that may or may not do the right thing
with vectors
>>>> of pointers.
>>>> I can fix *this* particular bug with the attached patch.
>>>>
>>>> However, it's mostly papering over stuff.  Nothing seems to
know what
>>>> to do with a memorylocation that is a vector of pointers. They
all expect
>>>> memorylocation to be a single pointer location.
>>>>
>>>> I would chalk it up to "luck" that this patch fixes
the bug.
>>>>
>>>> It's pretty clear that MemoryLocation doesn't fit the
needs of a lot of
>>>> stuff anymore (we hacked AA nodes into it, and lots of stuff
now tries to
>>>> figure out the invariantess of the locations, blah blah blah),
but it seems
>>>> like a big job to figure out what to replace it with that will
work for
>>>> these cases.
>>>>
>>>> (I'm pretty positive if we just make it MemoryLocations,
and have
>>>> everything loop over the locations, the compiler will get a lot
larger and
>>>> a lot slower)
>>>>
>>>> On Mon, Aug 29, 2016 at 9:10 AM, Daniel Berlin <dberlin at
dberlin.org>
>>>> wrote:
>>>>
>>>>> it would also, for that matter, say the same about an array
of
>>>>> pointers.
>>>>>
>>>>> It's not clear to me what will break if you change this
to
>>>>> isPtrOrPtrVectorTy.
>>>>>
>>>>> In fact, i know it doesn't fix this bug.
>>>>> It's a pretty deep rabbit hole of things not quite
prepared to
>>>>> understand vectors of pointers.
>>>>>
>>>>> (we prepare memorylocations of them, but memory locations
expect to be
>>>>> one thing, not a group of things, etc).
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Mon, Aug 29, 2016 at 8:58 AM, Daniel Berlin <dberlin
at dberlin.org>
>>>>> wrote:
>>>>>
>>>>>> 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
>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>>
>>
>>
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>
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llvm dev - Aug 2016 - GVN / Alias Analysis issue with llvm.masked.scatter/gather intrinsics

[llvm-dev] GVN / Alias Analysis issue with llvm.masked.scatter/gather intrinsics

[llvm-dev] GVN / Alias Analysis issue with llvm.masked.scatter/gather intrinsics

[llvm-dev] GVN / Alias Analysis issue with llvm.masked.scatter/gather intrinsics