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

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

在 2023/12/26 16:29, Xi Ruoyao 写道:
> 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.

I can use "ctc1 $0, $28" to disable all float point exception to ensure 
no FP exceptions occur at here. But it will introduce additional 
consumption.