Something like this?
#include <stdio.h>
#include <fenv.h>
int main(void)
{
/* Compute 1.0/3.0 in default FPU mode */
volatile float somefloat = 1.0;
somefloat /= 3.0;
/* Output it in float and hex formats */
printf("%.9g\thex: %x\n", somefloat, *(int *)&somefloat);
fenv_t a, b;
fegetenv(&a); /* Save fpu env */
fesetround(FE_TOWARDZERO); /* Set round mode */
fesetenv(&a); /* Restore fpu env (round mode must also be restored) */
fegetenv(&b); /* Get current fpu env */
/* Compute 1.0/3.0 after changing and restoring fpu env */
volatile float somefloat2 = 1.0;
somefloat2 /= 3.0;
/* Output it in float and hex formats */
printf("%.9g\thex: %x\n", somefloat2, *(int *)&somefloat2);
/* Output FPU control words */
printf ("FPU control words: %x, %x\n", a.__control_word, b.__control_word);
return 0;
}
Output for incorrect work of fesetenv:
0.333333343 hex: 3eaaaaab
0.333333313 hex: 3eaaaaaa
FPU control words: 37f, f7f
For correct work output :
0.333333343 hex: 3eaaaaab
0.333333343 hex: 3eaaaaab
FPU control words: 37f, 37f
чт, 7 мар. 2019 г. в 21:50, Liu Hao <lh_mo...@126.com>:
>
> 在 2019/2/25 22:41, Liu Hao 写道:
> > 在 2019/2/25 19:27, Zidane Sama 写道:
> >> After this commit:
> >>
> >> [e98d80] (2.0 kB) by Kai Tietz
> >> Add a fix for working fesetenv in libquadmath-library
> >>
> >> fesetenv was broken and do not sets new fpu state because it's now
> >> overwrites new env state by fnstenv.
> >>
> >> And this patch must fix it by store only mxcsr and load its afterwards:
> >>
> >> diff --git a/mingw-w64-crt/misc/fesetenv.c b/mingw-w64-crt/misc/fesetenv.c
> >> index f998017b..07035c66 100644
> >> --- a/mingw-w64-crt/misc/fesetenv.c
> >> +++ b/mingw-w64-crt/misc/fesetenv.c
> >> @@ -66,8 +66,7 @@ int fesetenv (const fenv_t * envp)
> >> {
> >> fenv_t env = *envp;
> >> int _mxcsr;
> >> - __asm__ ("fnstenv %0\n"
> >> - "stmxcsr %1" : "=m" (*&env), "=m" (*&_mxcsr));
> >> + __asm__ ("stmxcsr %0" : "=m" (*&_mxcsr));
> >> /*_mxcsr = ((int)envp->__unused0 << 16) | (int)envp->__unused1;
> >> *//* mxcsr low and high */
> >> env.__unused0 = 0xffff;
> >> env.__unused1 = 0xffff;
> >>
> >>
> >>
> >
> > This seems a good catch.
> >
> > However I think the `STMXCSR` instruction should follow
> > `__mingw_has_sse()`, although CPUs without SSE are rare. I propose
> > another patch to address this issue.
> >
> >
> >
>
> Ping? Do you have any testcases related to this issue? Please notice
> that without any response we will not act on this.
>
>
> --
> Best regards,
> LH_Mouse
>
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