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Re: RFC: Define __FPMATH_387__ and __FPMATH_SSE__


On 3/2/06, H. J. Lu <hjl@lucon.org> wrote:
> On Thu, Mar 02, 2006 at 04:44:50PM +0100, Jakub Jelinek wrote:
> > On Thu, Mar 02, 2006 at 07:38:47AM -0800, H. J. Lu wrote:
> > > Yes. That is for float and double functions in libm.
> > >
> > > > to touch x387
> > > > flags for XFmode long long operations.
> > >
> > > I assume you meant long double. If the library knows it doesn't long
> > > double, it doesn't need to touch x87 unit control.
> >
> > But if you call one of the standard fenv.h functions, they
> > are supposed to affect not just the calling routine (where you
> > could through preprocessor magic tell what it uses, though e.g.
> > not for inline asm), but for the whole process.  And I don't understand
> > how you can prove that if you call fesetround or similar from
> > -mfpmath=sse compiled source file that the program doesn't anywhere
> > in some other library or in the main binary use x87 unit (or vice versa).
>
> I was talking about glibc. sysdeps/ieee754/flt-32/e_expf.c has
>
>       feholdexcept (&oldenv);
> #ifdef FE_TONEAREST
>       fesetround (FE_TONEAREST);
> #endif
>
> ...
>
>       /* Return result.  */
>       fesetenv (&oldenv);
>
> I don't see why it has to touch x87 unit. The same applies to most,
> if not all, float/double functions in glibc.

Right, but here a __builtin_fesetround and friends would be more appropriate
that helping glibc with preprocessor magic.

Richard.


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