https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85406
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Status|WAITING |NEW
Blocks| |53947
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think it's too much asked from GCC to prove that if a == 255 the computation
in the else path would result in x. The simplest fix would be to remove the
conditional in the source.
It's true that GCC doesn't evaluate costs of the vectorization properly
as it looks at the if-converted copy when calculating the cost of the
scalar loop. OTOH any estimate on how often the shortcut triggers
compared to the computation might be off and a conservative estimate may
cause us to not vectorize and thus slow down the code.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations
