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From: "francesco.zappa.nardelli at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/54149] New: write introduction incorrect wrt the C11 memory model Date: Wed, 01 Aug 2012 08:47:00 -0000 [thread overview] Message-ID: <bug-54149-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54149 Bug #: 54149 Summary: write introduction incorrect wrt the C11 memory model Classification: Unclassified Product: gcc Version: 4.8.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned@gcc.gnu.org ReportedBy: francesco.zappa.nardelli@gmail.com [ possibly related to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52558 ] Consider this program: int g_13 = 1; void main () { int l_245; for (l_245 = 0; l_245 <= 1; l_245 += 1) for (; g_13 <= 0; g_13 = 1); } If I compile it with gcc --param allow-store-data-races=0 -S -O1 (I am running gcc (GCC) 4.8.0 20120627 (experimental) on Linux - x86_64 -- the same applies to -O2 and -O3 and compilation with g++ -std=c++11 -O1) I get the following optimised assembler (I just hand removed some noise): main: movl g_13(%rip), %eax testl %eax, %eax movl $1, %edx cmovle %edx, %eax movl %eax, g_13(%rip) ret g_13: .long 1 which always performs a write to g_13 and executes with this memory trace (I am tracing only the accesses to the global - potentially shared - variables): g_13 1 4 Init g_13 1 4 Load g_13 1 4 Store However the reference trace for the C11 program above never performs a write to g_13: g_13 1 4 Init g_13 1 4 Load g_13 1 4 Load This looks related to the problem I reported in http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52558 , as the optimiser introduces a memory write which should not be there (and in turn performs an optimisation not correct wrt the C11 or the C++11 memory model). The same discriminating context of bug 52558 applies. For reference: $ gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/home/yquem/moscova/zappa/source/gcc-svn-bin/libexec/gcc/x86_64-unknown-linux-gnu/4.8.0/lto-wrapper Target: x86_64-unknown-linux-gnu Configured with: ../gcc-svn/configure --prefix=/home/yquem/moscova/zappa/source/gcc-svn-bin/ Thread model: posix gcc version 4.8.0 20120627 (experimental) (GCC)
next reply other threads:[~2012-08-01 8:47 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2012-08-01 8:47 francesco.zappa.nardelli at gmail dot com [this message] 2012-08-23 16:35 ` [Bug c/54149] " francesco.zappa.nardelli at gmail dot com 2012-09-07 19:45 ` aldyh at gcc dot gnu.org 2012-09-11 12:28 ` [Bug middle-end/54149] " aldyh at gcc dot gnu.org 2012-09-11 12:30 ` aldyh at gcc dot gnu.org
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