[Bug tree-optimization/94617] Simple if condition not optimized

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94617

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jakub at gcc dot gnu.org

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to David Seifert from comment #5)
> just benchmarked the code on an oldish Ivybridge. GCC with vanilla_bandpass
> is 2.1x slower than GCC with funky_bandpass, and GCC with funky_bandpass is
> 12% slower than Clang with vanilla_bandpass.

It really depends, conditional moves are really badly implemented in x86 and
sometimes they do improve speed and at other times they slow things down by
huge amounts, which mostly depends on how well the CPU can predict branches if
there are (if well, then branches are significantly faster, if they are very
badly predictable, then conditional moves are faster).

As for turning it into
(unsigned)x - a < (unsigned)b - a
reassoc has
optimize_range_tests_var_bound
which right now handles
   a >= 0 && a < b into (unsigned) a < (unsigned) b
   a >= 0 && a <= b into (unsigned) a <= (unsigned) b
Now
   a >= b && a < c into (unsigned) a - b < (unsigned) c - b
isn't equivalent to that, e.g. if b is 5 and c 4.