public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "us15 at os dot inf.tu-dresden.de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/47698] New: CMOV accessing volatile memory with read side effect
Date: Fri, 11 Feb 2011 15:40:00 -0000 [thread overview]
Message-ID: <bug-47698-4@http.gcc.gnu.org/bugzilla/> (raw)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47698
Summary: CMOV accessing volatile memory with read side effect
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: us15@os.inf.tu-dresden.de
In the attached testcase, the variable "mmio" is a memory-mapped device
register with read side effects. It must only be accessed if "cond" is true.
When compiling the testcase:
gcc -Os -c testcase.cc -o testcase.o
the following output is generated:
0000000000000000 <foo()>:
0: 31 c0 xor %eax,%eax
2: 48 83 3d 00 00 00 00 cmpq $0x0,0x0(%rip) # a <foo()+0xa>
9: 00
a: 48 0f 45 05 00 00 00 cmovne 0x0(%rip),%rax # 12 <foo()+0x12>
11: 00
12: c3 retq
Due to the use of CMOV, gcc is reading the memory location regardless of "cond"
being true, even though "mmio" is casted to volatile.
Reading "mmio" through a volatile pointer, as in
return *static_cast<volatile unsigned long *>(&mmio);
does not get rid of the CMOV either.
What is the correct way to ensure mmio is read if and only if cond is true?
My gcc version is: gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-slackware-linux/4.6.0/lto-wrapper
Target: x86_64-slackware-linux
Configured with: ../gcc-svn/configure --prefix=/usr --libdir=/usr/lib64
--mandir=/usr/man --infodir=/usr/info --enable-shared --enable-bootstrap
--enable-languages=c,c++,lto --enable-threads=posix --enable-checking=release
--with-system-zlib --with-python-dir=/lib64/python2.6/site-packages
--disable-libunwind-exceptions --enable-__cxa_atexit --enable-libssp
--enable-lto --with-gnu-ld --verbose --disable-multilib
--target=x86_64-slackware-linux --build=x86_64-slackware-linux
--host=x86_64-slackware-linux
Thread model: posix
gcc version 4.6.0 20110211 (experimental) (GCC)
next reply other threads:[~2011-02-11 15:40 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-02-11 15:40 us15 at os dot inf.tu-dresden.de [this message]
2011-02-11 15:43 ` [Bug rtl-optimization/47698] " us15 at os dot inf.tu-dresden.de
2011-02-11 16:04 ` us15 at os dot inf.tu-dresden.de
2011-02-11 16:36 ` rguenth at gcc dot gnu.org
2011-02-11 17:07 ` matz at gcc dot gnu.org
2011-02-11 17:57 ` hjl.tools at gmail dot com
2011-11-07 8:42 ` kyukhin at gcc dot gnu.org
2011-11-08 13:46 ` ebotcazou at gcc dot gnu.org
2012-01-21 20:27 ` pinskia at gcc dot gnu.org
2021-07-29 22:34 ` 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-47698-4@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).