Re: [musl] [PATCH] configure: prevent compilers from turning a * b + c into fma(a, b, c)

[Rich Felker <dalias@libc.org>]

On Wed, Aug 28, 2024 at 11:32:03PM +0300, Alexander Monakov wrote:
> 
> On Wed, 28 Aug 2024, Rich Felker wrote:
> 
> > With that said, I don't see how the compiler could ever generate calls
> > to fma(), since we're -ffreestanding, but I think it could generate
> > inline fma instructions on targets where they're available, and this
> > is generally undesirable (see above). For gcc, I think -std=c99
> > already has that covered, but I'm not sure what clang does.
> 
> Modern Clang implements the FP_CONTRACT pragma, with default state "on".
> Of course, the -ffp-contract= command-line switch is also available.

Even with -std=c99? In that case we should add the proposed:

tryflag CFLAGS_C99FSE -ffp-contract=off

I'm not clear on why the -mno-fused-madd would be needed. It should be
a no-op with -ffp-contract=off unless __builtin_fma were called
explicitly or something, no?

Rich