[PATCH] D80801: [DAGCombiner] allow more folding of fadd + fmul into fma

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D80801#2102014 <https://reviews.llvm.org/D80801#2102014>, @bryanpkc wrote:

> Sorry for the late question, but I don't understand why this kind of folding is not considered reassociation. I thought reassociation was not allowed even when `-ffp-contract=fast`.


General reassociation of FP ops is not allowed without "reassoc" fast-math-flags or the legacy global setting - see isContractableFMUL().

But this transform is a special-case. We are only pulling the trailing addition in with an existing multiply. That's the kind of transform -ffp-contract=fast ("contract" FMF) is intended to enable. So with that setting, the compiler can choose whether we end up with fma(A, B, fma(C, D, E)) or fma(C, D, fma(A, B, E)). AFAIK, LLVM matches gcc behavior here.

I think "contract" alone is also enough to split an fma back into fmul + fadd as was suggested earlier in the review, but I'm not sure if there's precedence for that.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D80801/new/

https://reviews.llvm.org/D80801