The following function: static struct { int w; } s;
void wait (void) { int t; loop: t = *((volatile int *) &s.w); if (t > 0) goto loop; } is compiled incorrectly with "gcc -v -O3 v.c -o v.s -Wall -S". The generated assembly does not respetc the semantics of volatile: wait: movl s, %eax pushl %ebp movl %esp, %ebp testl %eax, %eax jg .L6 popl %ebp ret .L3: .L6: jmp .L6 The attached testscase shows the problem. It also includes a similar function which does get compiled correctly, the only difference is that it introduces a temporary variable. Detailed compiled output: /home/ricardo/mine/install/bin/gcc -v -O3 v.c -o v.s -Wall -S Using built-in specs. Target: i686-pc-linux-gnu Configured with: ../configure --prefix=/home/ricardo/mine/install/ --enable-ckecking=all --enable-languages=c,cil --disable-bootstrap : (reconfigured) ../configure --prefix=/home/ricardo/mine/install/ --enable-languages=c,cil --disable-bootstrap Thread model: posix gcc version 4.2.0 20060826 (experimental) /home/ricardo/mine/install/bin/../libexec/gcc/i686-pc-linux-gnu/4.2.0/cc1 -quiet -v -iprefix /home/ricardo/mine/install/bin/../lib/gcc/i686-pc-linux-gnu/4.2.0/ v.c -quiet -dumpbase v.c -mtune=generic -auxbase-strip v.s -O3 -Wall -version -o v.s ignoring nonexistent directory "/home/ricardo/mine/install/bin/../lib/gcc/i686-pc-linux-gnu/4.2.0/../../../../i686-pc-linux-gnu/include" ignoring duplicate directory "/home/ricardo/mine/install//lib/gcc/i686-pc-linux-gnu/4.2.0/include" ignoring nonexistent directory "/home/ricardo/mine/install//lib/gcc/i686-pc-linux-gnu/4.2.0/../../../../i686-pc-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /home/ricardo/mine/install/bin/../lib/gcc/i686-pc-linux-gnu/4.2.0/include /usr/local/include /home/ricardo/mine/install//include /usr/include End of search list. GNU C version 4.2.0 20060826 (experimental) (i686-pc-linux-gnu) compiled by GNU C version 4.1.2 20061020 (prerelease) (Debian 4.1.1-17). GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096 Compiler executable checksum: 39ae1c561861def994882f2081f65beb -- Summary: Volatile semantics ignored in some cases Product: gcc Version: 4.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: ricardo dot fernandez at st dot com GCC build triplet: i686-pc-linux-gnu GCC host triplet: i686-pc-linux-gnu GCC target triplet: i686-pc-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29753