[Bug c++/55288] Improve handling/suppression of maybe-uninitialized warnings

["scovich at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55288

--- Comment #2 from Ryan Johnson <scovich at gmail dot com> 2012-11-12 21:11:43 UTC ---
(In reply to comment #1)
> Why don't just initialize the variable? It seems simpler than implementing yet
> another special attribute in GCC.

In the original program, the "variable" is a largish struct, the function is
hot, and the 'valid' execution path is not the most common one. Avoiding
unnecessary initialization there has a measurable impact on performance. 

Note that, in other parts of the code that gcc understands better, the
initialization is unnecessary (no warning) and gets optimized away even if I do
have it in place... much to my chagrin once, after I did a lot of work to
refactor a complex function, only to realize that gcc emitted *exactly* the
same machine code afterward, because it had already noticed and eliminated the
dead stores. 

There's also a philosophical argument to be made... if we agree that all
warnings subject to false positives should be supressible, the current
mechanism for maybe-uninitialized is inadequate, and a variable attribute would
resolve the issue very nicely. There's precedent for this: you *could* use
#ifndef NDEBUG (or even pragma diagnostic) to avoid unused-variable warnings
for helper variables used by multiple assertions scattered over a region of
code, but setting ((unused)) on the offending variable is much easier to read
and maintain, while still allowing other unused variables to be flagged
properly.