[llvm] r243500 - ignore duplicate divisor uses when transforming into reciprocal multiplies (PR24141)

[Chandler Carruth]

FWIW, it was a regression we found in the wild, so I think it should be
merged.

On Tue, Jul 28, 2015 at 6:03 PM Hans Wennborg <hans at chromium.org> wrote:

> We should probably merge this to 3.7.
>
> Owen, you're the owner here. OK to merge?
>
> On Tue, Jul 28, 2015 at 4:28 PM, Sanjay Patel <spatel at rotateright.com>
> wrote:
> > Author: spatel
> > Date: Tue Jul 28 18:28:22 2015
> > New Revision: 243500
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=243500&view=rev
> > Log:
> > ignore duplicate divisor uses when transforming into reciprocal
> multiplies (PR24141)
> >
> > PR24141: https://llvm.org/bugs/show_bug.cgi?id=24141
> > contains a test case where we have duplicate entries in a node's uses()
> list.
> >
> > After r241826, we use CombineTo() to delete dead nodes when combining
> the uses into
> > reciprocal multiplies, but this fails if we encounter the just-deleted
> node again in
> > the list.
> >
> > The solution in this patch is to not add duplicate entries to the list
> of users that
> > we will subsequently iterate over. For the test case, this avoids
> triggering the
> > combine divisors logic entirely because there really is only one user of
> the divisor.
> >
> > Differential Revision: http://reviews.llvm.org/D11345
> >
> >
> > Modified:
> >     llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> >     llvm/trunk/test/CodeGen/X86/fdiv-combine.ll
> >
> > Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=243500&r1=243499&r2=243500&view=diff
> >
> ==============================================================================
> > --- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
> > +++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Tue Jul 28
> 18:28:22 2015
> > @@ -8261,11 +8261,11 @@ SDValue DAGCombiner::combineRepeatedFPDi
> >      return SDValue();
> >
> >    // Find all FDIV users of the same divisor.
> > -  SmallVector<SDNode *, 4> Users;
> > -  for (auto *U : N1->uses()) {
> > +  // Use a set because duplicates may be present in the user list.
> > +  SetVector<SDNode *> Users;
> > +  for (auto *U : N1->uses())
> >      if (U->getOpcode() == ISD::FDIV && U->getOperand(1) == N1)
> > -      Users.push_back(U);
> > -  }
> > +      Users.insert(U);
> >
> >    // Now that we have the actual number of divisor uses, make sure it
> meets
> >    // the minimum threshold specified by the target.
> >
> > Modified: llvm/trunk/test/CodeGen/X86/fdiv-combine.ll
> > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/fdiv-combine.ll?rev=243500&r1=243499&r2=243500&view=diff
> >
> ==============================================================================
> > --- llvm/trunk/test/CodeGen/X86/fdiv-combine.ll (original)
> > +++ llvm/trunk/test/CodeGen/X86/fdiv-combine.ll Tue Jul 28 18:28:22 2015
> > @@ -44,5 +44,24 @@ define double @div3_arcp(double %x, doub
> >    ret double %ret
> >  }
> >
> > +define void @PR24141() #0 {
> > +; CHECK-LABEL: PR24141:
> > +; CHECK:       callq
> > +; CHECK-NEXT:  divsd
> > +; CHECK-NEXT:  jmp
> > +entry:
> > +  br label %while.body
> > +
> > +while.body:
> > +  %x.0 = phi double [ undef, %entry ], [ %div, %while.body ]
> > +  %call = call { double, double } @g(double %x.0)
> > +  %xv0 = extractvalue { double, double } %call, 0
> > +  %xv1 = extractvalue { double, double } %call, 1
> > +  %div = fdiv double %xv0, %xv1
> > +  br label %while.body
> > +}
> > +
> > +declare { double, double } @g(double)
> > +
> >  ; FIXME: If the backend understands 'arcp', then this attribute is
> unnecessary.
> >  attributes #0 = { "unsafe-fp-math"="true" }
> >
> >
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