[PATCH] D4583: don't transform splats of vector FP (PR20358)

[Sanjay Patel via llvm-commits]

spatel added a comment.

I just connected the dots between this case and other recent speculative execution bugs/patches:
https://llvm.org/bugs/show_bug.cgi?id=24343
https://llvm.org/bugs/show_bug.cgi?id=24818
https://llvm.org/bugs/show_bug.cgi?id=25572
http://reviews.llvm.org/D12882
http://reviews.llvm.org/D13297
http://reviews.llvm.org/D14630

This splat is just a special (and probably extremely rare) case of the general problem:

  #define DENORM ((float)(1.0e-39))
  float foo(float x, float y) {
     if (x > DENORM) return x * y;
     else return y;
  }

$ ./clang -O2 denorm.c -S -o - -emit-llvm

  define float @foo(float %x, float %y) #0 {
    %cmp = fcmp ogt float %x, 0x37D5C73000000000
    %mul = fmul float %x, %y    <--- crazy expensive op that we wanted to avoid just got executed
    %retval.0 = select i1 %cmp, float %mul, float %y
    ret float %retval.0
  }

Unless there is objection, I'll abandon this patch. If we really want to solve this, we'd have to add some global flag (-fno-speculative-fp-math?) or instruction-level annotation to include FP ops under !isSafeToSpeculativelyExecute().


http://reviews.llvm.org/D4583