[PATCH 2/2] MIPS: Hard-float rounding instructions support

[Xi Ruoyao]

On Tue, 2023-12-26 at 10:37 +0800, Junxian Zhu wrote:
> 在 2023/12/25 18:51, Xi Ruoyao 写道:
> > On Mon, 2023-12-25 at 18:35 +0800, Junxian Zhu wrote:
> > 
> > /* snip */
> > 
> > > +/*
> > > + * ceil(x)
> > > + * Return x rounded toward -inf to integral value
> > > + * Method:
> > > + *	Bit twiddling.
> > > + */
> > > +
> > > +#if ((__mips_fpr == 64) && (__mips_hard_float == 1) && ((__mips == 32 && __mips_isa_rev > 1) || __mips == 64))
> > > +#include <sys/regdef.h>
> > > +#include <sysdep.h>
> > > +#include <libm-alias-double.h>
> > > +
> > > +ENTRY(__ceil)
> > > +	.set push
> > > +	.set noreorder
> > > +	.set noat
> > > +# $f0=ret, $f12=double, a0=int64/int32_h, a1=int32_l, a2=sign, a3=exp
> > > +#if __mips == 64
> > > +	dmfc1   a0, $f12 # assign int64
> > > +#else
> > > +	mfhc1   a0, $f12 # assign int64
> > > +#endif
> > > +	cfc1    t0, $f26
> > > +	ceil.l.d    $f0, $f12
> > No, C23 does not allow this function to raise an INEXACT exception, but
> > ceil.l.d will do so.
> > 
> > Such optimizations should be performed in GCC which can be controlled by
> > the programmer with -std=c23 and/or -f[no-]fp-int-builtin-inexact, not
> > in Glibc where we cannot know if the programmer wants to deviate from
> > C23.
> 
> The cfc1 instruction will backup float point exception status before 
> running ceil.l.d, and the following ctc1 will restore float point 
> exception status to avoid INEXACT exception raised by ceil.l.d. It's the 
> same way like what have been done in s_ceil.S for i386.

Still incorrect because when the Enable field of FCSR contains INEXACT a
SIGFPE will be immediately delivered and there is no way to recover.  A
demonstration:

#define _GNU_SOURCE
#include <stdio.h>
#include <fenv.h>

int main()
{
  printf("%d\n", feenableexcept(FE_INEXACT));

  double data = 114.514;
  long control;
  asm("cfc1\t%1,$f26\n\t"
      "ceil.l.d\t%0,%0\n\t"
      "cvt.d.l\t%0,%0\n\t"
      "ctc1\t%1,$f26": "+f"(data), "=r"(control));
  printf("%.15f\n", data);
  return 0;
}

On i386 the fnstenv instruction also masks out all the FP exceptions so
this is not a problem.  See commit 26b0bf96000a.

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University