extending fpmuls

[Frederic Riss]

Hi

On 25 October 2011 10:00, Jakub Jelinek <jakub@redhat.com> wrote:
> On Mon, Oct 24, 2011 at 11:12:29PM -0400, David Miller wrote:
>> While working on some test cases I noticed that the 'fsmuld'
>> instruction on sparc was not being matched by the combiner for
>> things like:
>>
>> double fsmuld (float a, float b)
>> {
>>   return a * b;
>> }
>>
>> Combine does try to match:
>>
>>       (set x (float_extend:DF (mul:SF y z)))
>
> That is correct.  float a * float b is supposed to e.g. result in Inf
> if the product overflows float (but would still fit into double).

Some time ago, I tried to read the C standard to see what ports were
authorized to do with regard to that kind of issues.  I (most
certainly wrongly) came to the conclusion that it was conformant to
generate an insn doing  (set x (mul:DF (float_extend:DF y)
(float_extend:DF y))) for a pattern of (set x (float_extend:DF (mul:SF
y z))). The parts that lead me to believe this were eg. the paragaph
on FP contracted expressions (6.5.8) and the sentence on floating
point expression precision (6.3.1.8.2) that seem to allow a port to
compute results in higher precisions than requested.

Could you explain why the overflow is required here with respect to
the standard?

Thanks,
Fred