Re: [02/12] Add fesetexcept: aarch64

[Joseph Myers <joseph at codesourcery dot com>]

On Thu, 11 Aug 2016, Paul Eggert wrote:

> Thanks, that patch series looks good, though I have one minor question and/or
> quibble.
> 
> > +  if (fpsr != fpsr_new)
> > +    _FPU_SETFPSR (fpsr_new);
> 
> I assume that the 'if (fpsr != fpsr_new)' is present purely for performance
> reasons, i.e., that the code would be correct if '_FPU_SETFPSR (fpsr_new)'
> were executed unconditionally. Are practical programs likely to see a
> performance improvement because of the 'if (fpsr != fpsr_new)'? My guess is
> no, because typically programs will use fesetexcept to change settings. If my
> guess is right, I'd omit the 'if (fpsr != fpsr_new)' test, on the grounds of
> simplicity.

I'm deliberately following the fesetexceptflag implementations for each 
architecture, as the most similar C99/C11 function.  If those have this 
optimization, then I'm including it in fesetexcept as well.  Doing 
otherwise would make fesetexcept for AArch64 inconsistent with all the 
other <fenv.h> function implementations for AArch64, which systematically 
use this optimization (either directly, or in the <fenv_private.h> 
functions they call).

Similarly, when I implement fesetmode, that will have this optimization on 
architectures where fesetenv does.

-- 
Joseph S. Myers
joseph@codesourcery.com