[PATCH] D132987: [PPC] Add RM as call preserved register

[Phoebe Wang via Phabricator via llvm-commits]

pengfei added a comment.

In D132987#3821120 <https://reviews.llvm.org/D132987#3821120>, @Carrot wrote:

> If RM is not preserved by a function call, then in the following example
>
>   void foo(double a, double b, double *p) {
>     *p++ = a + b;
>     bar();
>     *p = a + b;
>   }
>
> The expression a+b can not be CSEd, right?

The answer is yes and no. It depends on whether the user cares about the rounding mode or not. If user has strict expectation to it, they should use option like `-frounding-math`. It definitely be CSEd if user uses it in fast FP mode. For the default mode, it still should be CSEd according to C language standard.

It makes no sense to just take care of calling conversion given a lot of middle end optimization don't respect rounding mode. That's why we need to introduce strcit FP.


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https://reviews.llvm.org/D132987