[PATCH] D116015: [PowerPC] Add generic fnmsub intrinsic

[Qiu Chaofan via Phabricator via llvm-commits]

qiucf added a comment.

In D116015#3203067 <https://reviews.llvm.org/D116015#3203067>, @nemanjai wrote:

> Converting more generic code to target-specific intrinsics is sometimes necessary to ensure the generic IR doesn't get transformed in a way that is disadvantageous. I believe that the description of this review claims that to be the case for these negated FMA's. The obvious disadvantage of producing target-specific intrinsics is that the optimizer knows nothing about them so no advantageous transformations can happen either (i.e. hiding the semantics from the optimizer is sometimes a good thing and sometimes a bad thing).
>
> The description of this patch includes no test case that shows the optimizer performing an undesirable transformation. So the motivation for making the front end produce more opaque code is not at all clear.

Here's a pretty simple case: `vector float foo(vector float a, vector float b, vector float c, vector float d) { return __builtin_vsx_xvnmsubasp(c, d, a*b); }`

It current produces `xvnegsp+xvmulsp+xvnmaddasp`, after this patch it produces `xvmulsp+xvnmsubasp`. In some complicated cases, we can see much more unexpected instructions generated.


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  https://reviews.llvm.org/D116015/new/

https://reviews.llvm.org/D116015