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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/94617] Simple if condition not optimized
Date: Thu, 16 Apr 2020 13:11:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94617

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #6)
> (In reply to David Seifert from comment #5)
> > just benchmarked the code on an oldish Ivybridge. GCC with vanilla_band=
pass
> > is 2.1x slower than GCC with funky_bandpass, and GCC with funky_bandpas=
s is
> > 12% slower than Clang with vanilla_bandpass.
>=20
> 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 branch=
es
> if there are (if well, then branches are significantly faster, if they are
> very badly predictable, then conditional moves are faster).
>=20
> As for turning it into
> (unsigned)x - a < (unsigned)b - a
> reassoc has
> optimize_range_tests_var_bound
> which right now handles
>    a >=3D 0 && a < b into (unsigned) a < (unsigned) b
>    a >=3D 0 && a <=3D b into (unsigned) a <=3D (unsigned) b
> Now
>    a >=3D b && a < c into (unsigned) a - b < (unsigned) c - b
> isn't equivalent to that, e.g. if b is 5 and c 4.

sure, there are extra constraints that a <=3D b and the unsigned type
has to have 1 bit more precision.  So if we would want to do this
transform then we'd need symbolic range info constraining a and b
appropriately.=