bug in longlong.h for aarch64 sub_ddmmss
nisse at lysator.liu.se
Wed Jun 17 12:56:15 UTC 2020
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
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.
Niels Möller. PGP-encrypted email is preferred. Keyid 368C6677.
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