> 在 2016年12月22日，23:31，Richard Sandiford <rdsandif...@googlemail.com> 写道：
>
> Matthew Fortune <matthew.fort...@imgtec.com> writes:
>> Sandra Loosemore <san...@codesourcery.com> writes:
>>> On 12/21/2016 11:54 AM, Yunqiang Su wrote:
>>>> By this patch, I add a build-time option ` --with-unfused-madd4=yes/no',
>>>> and runtime option -m(no-)unfused-madd4,
>>>> to disable generate madd.fmt instructions.
>>>
>>> Your patch also needs a documentation change so that the new
>>> command-line option is listed in the GCC manual with other MIPS target
>>> options.
>>
>> Any opinions on option names to control this? Is it best to target the
>> specific
>> feature that is non-compliant on loongson or apply a general -mfix-loongson
>> type option?
>>
>> I'm not sure I have a strong opinion either way but there do seem to be
>> multiple possible variants.
>
> Wasn't sure from this thread whether Loongson simply had a fused
> implementation (without intermediate rounding) or whether the
> instructions gave numerically incorrect results for some inputs.
I test to define ISA_HAS_FUSED_MADD4 true and
define ISA_HAS_UNFUSED_MADD4 false, and try to build a test case.
With ISA_HAS_FUSED_MADD4, the result is about 1e-17,
and with ISA_HAS_UNFUSED_MADD4, the result is about 1e-17,
both of the are incorrect (the expect value is 0).
The test case is
#include <stdio.h>
double a = 0.6;
double b = 0.4;
double c = 0.6;
double d = 0.4;
int main(void)
{
double x = a * b - c * d;
printf("%le\n", x);
return 0;
}
> It sounds from a later thread like it's generating incorrect results,
> is that right? If so, then FWIW I agree an -mfix option would be more
> consistent. E.g. one of the -mfix-vr4120 errata was an incorrect
> integer division result and one of the -mfix-sb1 errata was an incorrect
> single-precision float division result. The latter case could have been
> handled by an option to disable DIV.S and DIV.PS, but the -mfix option
> gave more control.
>
> If instead the problem is that the instructions are fused then that's
> also what the original MIPS 4 parts did, so maybe an option to control
> fusedness would make sense.
The result to thread it fused or unfused, is different, while neither of them
is correct.
>
> Thanks,
> Richard
