public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug ada/46943] New: Unnecessary ZERO_EXTEND
Date: Tue, 14 Dec 2010 17:34:00 -0000 [thread overview]
Message-ID: <bug-46943-4@http.gcc.gnu.org/bugzilla/> (raw)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46943
Summary: Unnecessary ZERO_EXTEND
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: ada
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: jakub@gcc.gnu.org
Target: x86_64-linux
In http://blog.regehr.org/archives/320 Example 4
unsigned long long v;
unsigned short
foo (signed char x, unsigned short y)
{
v = (unsigned long long) y;
return (unsigned short) ((int) y / 3);
}
we emit a redundant zero-extension:
movzwl %si, %eax
movq %rax, v(%rip)
movzwl %si, %eax
imull $43691, %eax, %eax
shrl $17, %eax
ret
The reason why the second movzwl %si, %eax wasn't CSEd with the first one is
because the first one is (set (reg:DI reg1) (zero_extend:DI (reg:HI reg2))
while the second one is (set (reg:SI reg3) (zero_extend:SI (reg:HI reg2))
Wonder if we can't teach CSE to optimize it (say that reg3 is actually
(subreg:SI (reg:DI reg1) 0), or if e.g. one of the zee/see passes (implicit-zee
e.g.) couldn't handle such cases. Combiner can't do anything here, as there is
no data dependency, so try_combine won't see them together.
next reply other threads:[~2010-12-14 17:34 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-12-14 17:34 jakub at gcc dot gnu.org [this message]
2010-12-16 15:10 ` [Bug rtl-optimization/46943] " rguenth at gcc dot gnu.org
2012-02-02 8:52 ` pinskia at gcc dot gnu.org
2023-05-20 1:57 ` pinskia at gcc dot gnu.org
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=bug-46943-4@http.gcc.gnu.org/bugzilla/ \
--to=gcc-bugzilla@gcc.gnu.org \
--cc=gcc-bugs@gcc.gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).