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

["Junxian Zhu" <zhujunxian@oss.cipunited.com>]

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.