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

[Hans Wennborg]

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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