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Re: [Patch ,gcc/MIPS] add an build-time/runtime option to disable madd.fmt
Matthew Fortune <Matthew.Fortune@imgtec.com> writes:
> Sandra Loosemore <sandra@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.
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.
Thanks,
Richard