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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/66552] Missed optimization when shift amount is result of signed modulus
Date: Wed, 17 Jun 2015 19:33:00 -0000 [thread overview]
Message-ID: <bug-66552-4-PQCf4hRQ89@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-66552-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66552
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Target|x86/generic |x86, powerpc64
--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
On powerpc64, with -O2, GCC emits the following
f:
srawi 9,4,5
addze 9,9
slwi 9,9,5
subf 4,9,4
srw 3,3,4
rldicl 3,3,0,32
blr
while Clang emits what looks like optimal code:
f:
rlwinm 4, 4, 0, 27, 31
srw 3, 3, 4
blr
With the return expression rewritten as suggested (i.e., x >> (n & 31)), GCC
still emits an extra instruction compare to Clang. I believe this aspect is
tracked in bug 66217.
rldicl 4,4,0,59
srw 3,3,4
rldicl 3,3,0,32
blr
next parent reply other threads:[~2015-06-17 19:33 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <bug-66552-4@http.gcc.gnu.org/bugzilla/>
2015-06-17 19:33 ` msebor at gcc dot gnu.org [this message]
2015-06-30 4:34 ` [Bug rtl-optimization/66552] " segher at gcc dot gnu.org
2015-06-30 15:52 ` msebor at gcc dot gnu.org
2020-10-19 9:27 ` cvs-commit at gcc dot gnu.org
2020-10-21 3:15 ` guojiufu at gcc dot gnu.org
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