[Bug middle-end/323] optimized code gives strange floating point results

["manu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=323

--- Comment #216 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Vincent Lefèvre from comment #215)
> According to https://gcc.gnu.org/bugzilla/page.cgi?id=fields.html#bug_status
> the possible status are UNCONFIRMED, CONFIRMED and IN_PROGRESS. I think that
> the correct one is CONFIRMED.

Those are not the main policies. They are here:
https://gcc.gnu.org/bugs/management.html

fields.html needs to be updated some day to match the actual fields used by GCC
(there is no CONFIRMED status, there is NEW) and the above policies.

This is one of those bugs that it is so broad, controversial and noisy that
almost no active developer is going to look at it. Bugs don't get fixed because
they are NEW. There are 6979 NEW bug reports right now and many of them will
never get fixed (1300 of them are more than 10 years old).

My humble suggestion for those interested in floating-point issues in GCC would
be to create self-contained specific bugs with minimised reproducible
testcases, a clear analysis of what GCC is doing wrong, what GCC should be
doing instead, and suggestions on how it could be fixed. If the bug just says
some variation of "optimized code gives strange floating point results", it
will end up here and probably nobody will ever look at it.

For Rich's specific bug report, the relevant discussion is in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=323#c127 and the expected fix is
known: Implement -fexcess-precision=standard for C++ as it was done for C.
Perhaps it would be useful to create a new PR that blocks this one that
analyses what needs to be done towards that specific goal, collects testcases,
etc. The main issue is not that this PR is not in the developers' radar. All
GCC developers working on the C/C++ FEs and optimizers are aware of the
infamous PR323 and of the solution suggested in comment 127.

The issue is simply that no one working on the C++ FE has the time or
motivation to implement -fexcess-precision=standard. If you are interested in
that, just study this email:
https://gcc.gnu.org/ml/gcc-patches/2008-11/msg00105.html and do for C++ the
same steps that Joseph did C.

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