[Bug libstdc++/47913] [C++0x] improve ratio_add to overflow less often

["marc.glisse at normalesup dot org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47913

--- Comment #9 from Marc Glisse <marc.glisse at normalesup dot org> 2011-03-02 11:50:41 UTC ---
(In reply to comment #8)
> Right. Mine was sort of a general comment: the comments in ratio_less are also
> rather terse ;)

I'll try to expand a bit on them.

> I don't think you should really handcode something, just pick the right
> __builtin_* depending on the actual type of uintmax_t (after all it's just a
> typedef for one of the builtin types). Thus, if we aim for something really
> general here, use just a bit of templates to pick the best match among the
> various available __builtin_*, via make_signed on uintmax_t and then three
> special cases for int, long, long long. Granted, probably on widespread 32-bit
> and 64-bit machines, long long it's indeed a safe bet.

If intmax_t is guaranteed to be one of int/long/long long, then it seems that
it has to be equivalent to long long. I was more afraid that it might be a
bigger type than long long.

(by the way, __builtin_clz takes an unsigned argument)

> Understood. Since 4.6.0 is approaching, do you think it would make sense for
> this release series to modify only a bit the GCC code, to the point that we are
> as good or even slightly better, if possible, than Boost, without requiring too
> much new code? I fear regressions, as you of course can easily understand.
> Ideally, we should add, mano a mano, testcases for each "subclass" of inputs
> which we are able to process without overflowing.

I think there is nothing wrong with the implementation of ratio_add currently
in libstdc++ (shipping it as it is seems fine), but we could indeed easily
avoid all unnecessary denominator overflows (attachment in a minute). The
factorization of the gcd of numerators is a heuristic that sometimes helps but
doesn't really close a category of overflow, so I am more reluctant to add it,
but it is really easy if you think it should be done.