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: unassig...@gcc.gnu.org
ReportedBy: u...@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)
