[PATCH] D106959: [PowerPC] swdiv builtins for XL compatibility

[Quinn Pham via Phabricator via cfe-commits]

quinnp added inline comments.


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Comment at: llvm/test/CodeGen/PowerPC/LowerCheckedFPArith.ll:36
+; CHECK-NEXT:  %2 = fdiv fast float %0, %1
+; CHECK-NEXT:  %3 = fcmp une float %2, %2
+; CHECK-NEXT:  br i1 %3, label %swdiv_HWDIV, label %swdiv_MERGE
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efriedma wrote:
> quinnp wrote:
> > efriedma wrote:
> > > A "fast" fdiv never produces NaN, per LangRef.  Using fcmp like this is fragile at best.
> > > 
> > > (Maybe you want "fdiv arcp"?)
> > Thank you, I see what you mean. I have changed it to emit a `fdiv ninf arcp` instead of a `fdiv fast`. I included the `ninf` flag because without it the compiler doesn't produce the software div estimate.
> ninf is also an issue, although maybe less likely to bite in practice.  Consider what happens if someone passes infinity to swdivs: the fdiv reduces to poison, so the branch is undefined behavior.
We've decided to emit an `fdiv` without any fast math flags for these builtins. This will be safe and will emit the software estimate for `-Ofast`.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D106959/new/

https://reviews.llvm.org/D106959