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From: "amonakov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/111143] [missed optimization] unlikely code slows down diffutils x86-64 ASCII processing
Date: Fri, 25 Aug 2023 06:46:42 +0000 [thread overview]
Message-ID: <bug-111143-4-0ioR7T6tkW@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-111143-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111143
Alexander Monakov <amonakov at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |amonakov at gcc dot gnu.org
--- Comment #4 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Paul Eggert from comment #0)
> The "movl $1, %eax" immediately followed by "addq %rax, %rbx" is poorly
> scheduled; the resulting dependency makes the code run quite a bit slower
> than it should. Replacing it with "addq $1, %rbx" and readjusting the
> surrounding code accordingly, as is done in the attached file
> code-mcel-opt.s, causes the benchmark to run 38% faster on my laptop's Intel
> i5-1335U.
This is a mischaracterization. The modified loop has one uop less, because you
are replacing 'mov eax, 1; add rbx, rax' with 'add rbx, 1'.
To evaluate scheduling aspect, keep 'mov eax, 1' while changing 'add rbx, rax'
to 'add rbx, 1'.
There are two separate loop-carried data dependencies, both one cycle per
iteration (addition chains over r12 and rbx).
next prev parent reply other threads:[~2023-08-25 6:46 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-24 20:05 [Bug rtl-optimization/111143] New: " eggert at cs dot ucla.edu
2023-08-24 20:05 ` [Bug rtl-optimization/111143] " eggert at cs dot ucla.edu
2023-08-24 20:06 ` eggert at cs dot ucla.edu
2023-08-24 20:14 ` pinskia at gcc dot gnu.org
2023-08-25 6:46 ` amonakov at gcc dot gnu.org [this message]
2023-08-26 3:33 ` eggert at cs dot ucla.edu
2023-08-26 8:09 ` amonakov at gcc dot gnu.org
2023-08-26 16:43 ` eggert at cs dot ucla.edu
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