bug in longlong.h for aarch64 sub_ddmmss

[Niels Möller]

Paul Zimmermann <Paul.Zimmermann at inria.fr> writes:

>        Dear Niels,
>
>> The excluded case,
>> 
>>   sub_ddmmss(ah, al, bh, /*compile time constant*/0) 
>> 
>> could clearly be optimized, in a different way, but I'd guess it's rare
>> enough in real code to not be worth the effort?

Sorry for being unclear, sub_ddmmss has six arguments, and the case I
wanted to single out was

   sub_ddmmss(rh, rl, ah, al, bh, /*compile time constant*/0)
 
> in MPFR we have 15 places where we call sub_ddmmss, among which 8 have bl=0:

Those seem to have al == 0 (in above notation), which is a different
case.

  sub_ddmmss (rh, rl, ah, 0, bh, bl)

should be the same as

  rh = ah - bh - (bl > 0);
  rl = -bl;

So we have a borrow to propagate except of also bl == 0, and hence some
runtime carry logic is needed.

Regards,
/Niels

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