[all-commits] [llvm/llvm-project] 14db06: [InstCombine] Fix a cycle when folding fneg(select...

[ssijaric-nv via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 14db06946839729befd6bd3ced8142547f5fd139
      https://github.com/llvm/llvm-project/commit/14db06946839729befd6bd3ced8142547f5fd139
  Author: ssijaric-nv <ssijaric at nvidia.com>
  Date:   2024-10-25 (Fri, 25 Oct 2024)

  Changed paths:
    M llvm/lib/IR/ConstantFold.cpp
    M llvm/test/Transforms/InstCombine/fneg.ll
    M llvm/test/Transforms/InstSimplify/fp-nan.ll

  Log Message:
  -----------
  [InstCombine] Fix a cycle when folding fneg(select) with scalable vector types (#112465)

The two folding operations are causing a cycle for the following case
with
scalable vector types:

define <vscale x 2 x double> @test_fneg_select_abs(<vscale x 2 x i1>
%cond, <vscale x 2 x double> %b) {
%1 = select <vscale x 2 x i1> %cond, <vscale x 2 x double>
zeroinitializer, <vscale x 2 x double> %b
  %2 = fneg fast <vscale x 2 x double> %1
  ret <vscale x 2 x double> %2
}

1) fold fneg:  -(Cond ? C : Y) -> Cond ? -C : -Y

2) fold select: (Cond ? -X : -Y) -> -(Cond ? X : Y)

1) results in the following since '<vscale x 2 x double>
zeroinitializer' passes
the check for the immediate constant:

%.neg = fneg fast <vscale x 2 x double> zeroinitializer
%b.neg = fneg fast <vscale x 2 x double> %b
%1 = select fast <vscale x 2 x i1> %cond, <vscale x 2 x double> %.neg,
<vscale x 2 x double> %b.neg

and so we end up going back and forth between 1) and 2).

Attempt to fold scalable vector constants, so that we end up with a
splat instead:

define <vscale x 2 x double> @test_fneg_select_abs(<vscale x 2 x i1>
%cond, <vscale x 2 x double> %b) {
  %b.neg = fneg fast <vscale x 2 x double> %b
%1 = select fast <vscale x 2 x i1> %cond, <vscale x 2 x double>
shufflevector (<vscale x 2 x double> insertelement (<vscale x 2 x
double> poison, double -0.000000e+00, i64 0), <vscale x 2 x double>
poison, <vscale x 2 x i32> zeroinitializer), <vscale x 2 x double>
%b.neg
  ret <vscale x 2 x double> %1
}



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