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From: "wilco at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/90838] Detect table-based ctz implementation
Date: Fri, 17 Feb 2023 16:32:26 +0000
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X-Bugzilla-Component: tree-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D90838

--- Comment #21 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Gabriel Ravier from comment #19)

> If the original code being branchless makes it faster, wouldn't that imply
> that we should use the table-based implementation when generating code for
> `__builtin_ctz` ?

__builtin_ctz is 3-4 times faster than the table implementation, so this
optimization is always worth it. This is why I believe the current situatio=
n is
not ideal since various targets still set CTZ_DEFINED_VALUE_AT_ZERO to 0 or=
 1.
One option would be to always allow it in Gimple (perhaps add an extra argu=
ment
for the value to return for a zero input), and at expand time check whether=
 the
backend supports the requested value. It it doesn't, emit branches.=