On 6/18/2007 12:16 PM, tbp wrote:
Of course there are cases with every optimization enabled by
-ffast-math that
can break existing programs. Just that we know of one case beforehand
shouldn't
prevent us from enabling -mrecip at -ffast-math (provided -mno-recip
still works,
regardless if provided before or after -ffast-math). [We'll at least
get some more
testing coverage this way]
Argh! Please do not make -ffast-math even more of a pain to work with
than it is already.
You have to enable it, on the whole compilation unit, to get anywhere
near decent performance; there's no escape: either you do not turn it
on and everything slows to a crawl, or you pay for not being able to
inline from another unit.
Until now, the contract was: you have to deal with (and contain) NaN
and infinities. Fair enough, even if tricky that remained manageable.
But if i can't expect a mere division by 0, or sqrt of 0 (quite common
with FTZ/DAZ on) to give me respectively an infinite and 0 and instead
get a NaN (which i can't filter, you remember?) because of the NR
round, that's pure madness.
So please, for the love of everything's sacred, leave such stunts out
of -ffast-math.
I understand your problems, but let me state that your objections are
totally subjective. *You* need a specific behaviour from -ffast-math
(eg: keep NaN/Inf), but that's not what *I* need. So, we have different
goals.
Both you and me can still get what they need from GCC by using more
fine-grained options (you probably want -fno-finite-math or something
like that, and I want -mrecip).
Both our goals are legitimate. But that's not the point. The point is
what -ffast-math semantically means (the simplistic list of suboptions
activated by it is of couse unsufficiente because it doesn't explain how
to behave in face of new options, like -mrecip). My proposal is:
"-ffast-math activates all the mathematical-related optimizations that
improves code speed while destroying floating point accuracy."
If you have a different proposal, please provide your own statement.
Saying "-ffast-math provides all the optimizations which are not a pain
to work with" obviously isn't acceptable.
--
Giovanni Bajo
