From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id 8DB2A385781F; Fri, 17 Feb 2023 13:03:48 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 8DB2A385781F
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1676639028;
	bh=bTCSsCA3JsgmcdDSpgKnxdVNwz7io6Xv91F7RTIa3JI=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=LFVh64EPzaLtI+IR2D2n3hOZeRnCFichL0b8wm9kSpdIYux5AZMY9xyXtRg0jSM4K
	 XU+jLFZLrLuGSwAu97GkO4UCyLjoaJvqyBChHIH1v94oKXy65tAJvR7KOWAppPM6QF
	 JUAPSCaULLYwRosL6BMuHo88hlxesmM1xiXzCKfo=
From: "jakub 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 13:03:48 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 9.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: jakub at gcc dot gnu.org
X-Bugzilla-Status: RESOLVED
X-Bugzilla-Resolution: FIXED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: wilco at gcc dot gnu.org
X-Bugzilla-Target-Milestone: 10.0
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: 
Message-ID: <bug-90838-4-z9OkwLzljo@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-90838-4@http.gcc.gnu.org/bugzilla/>
References: <bug-90838-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D90838

--- Comment #16 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Wilco from comment #15)
> It would make more sense to move x86 backends to CTZ_DEFINED_VALUE_AT_ZERO
> =3D=3D 2 so that you always get the same result even when you don't have =
tzcnt.
> A conditional move would be possible, so it adds an extra 2 instructions =
at
> worst (ie. still significantly faster than doing the table lookup, multip=
ly
> etc). And it could be optimized when you know CLZ/CTZ input is non-zero.

Conditional moves are a lottery on x86, in many cases very bad idea.  And w=
hen
people actually use __builtin_clz*, they state that they don't care about t=
he 0
value, so emitting terribly performing code for it just in case would be wr=
ong.
If forwprop emits the conditional in separate blocks for the CTZ_DVAZ!=3D2 =
case,
on targets where conditional moves are beneficial for it it can also emit t=
hem,
or emit the jump which say on x86 will be most likely faster than cmov.=