[Bug target/59371] [4.8/4.9 Regression] Performance regression in GCC 4.8 and later versions.

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59371

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-12-05
     Ever confirmed|0                           |1

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
Btw, the change suggests a workaround - use -fwrapv.  And indeed, the
loop does not terminate for c == 32768, but that's a "bug" in the testcase,
  signed short < 32768
is always true (both promote to int, and i++ wraps at 32767).

That i++ now correctly wraps 'defined' also means that IV analysis is
pessimized as it can not assume that 'i' does not wrap nor can it assume
that the loop terminates.

But that's just the awkward way the testcase is written (without the
C standard in mind ...).

You may also try -funsafe-loop-optimizations that makes us optimistically
assume IVs don't overflow and loops terminate.

"confirmed", but I think this one needs more analysis on _what_ transform
we want to happen and then get assessment on if that is a valid transform
at all.