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

[Xi Ruoyao <xry111@xry111.site>]

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.

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