Hi, When -O -ftree-vrp optimizations are used the volatileness seems to be lost. Even though this test relies upon undefined behaviour according to the C99 spec the result is different with optimizations on or off.
I think that even when both accesses of vus are in the same expression it should still be read twice and no assumption should be made to its value. Maarten Brock Developer of SDCC, Small Device C Compiler, http://sdcc.sourceforge.net Here is the compiler output: Using built-in specs. Target: i486-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.1-4ubuntu9' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-targets=all --disable-werror --with-arch-32=i486 --with-tune=generic --enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu Thread model: posix gcc version 4.4.1 (Ubuntu 4.4.1-4ubuntu9) COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O' '-ftree-vrp' '-o' 'vus' '-mtune=generic' '-march=i486' /usr/lib/gcc/i486-linux-gnu/4.4.1/cc1 -E -quiet -v vus.c -D_FORTIFY_SOURCE=2 -mtune=generic -march=i486 -ftree-vrp -O -fpch-preprocess -fstack-protector -o vus.i ignoring nonexistent directory "/usr/local/include/i486-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/i486-linux-gnu/4.4.1/../../../../i486-linux-gnu/include" ignoring nonexistent directory "/usr/include/i486-linux-gnu" #include "..." search starts here: #include <...> search starts here: /usr/local/include /usr/lib/gcc/i486-linux-gnu/4.4.1/include /usr/lib/gcc/i486-linux-gnu/4.4.1/include-fixed /usr/include End of search list. COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O' '-ftree-vrp' '-o' 'vus' '-mtune=generic' '-march=i486' /usr/lib/gcc/i486-linux-gnu/4.4.1/cc1 -fpreprocessed vus.i -quiet -dumpbase vus.c -mtune=generic -march=i486 -auxbase vus -O -version -ftree-vrp -fstack-protector -o vus.s GNU C (Ubuntu 4.4.1-4ubuntu9) version 4.4.1 (i486-linux-gnu) compiled by GNU C version 4.4.1, GMP version 4.3.1, MPFR version 2.4.1-p2. GGC heuristics: --param ggc-min-expand=61 --param ggc-min-heapsize=59624 Compiler executable checksum: 8f30a5fdc2c06e07cc337ee4c8f69ccf COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O' '-ftree-vrp' '-o' 'vus' '-mtune=generic' '-march=i486' as -V -Qy -o vus.o vus.s GNU assembler version 2.20 (i486-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.20 COMPILER_PATH=/usr/lib/gcc/i486-linux-gnu/4.4.1/:/usr/lib/gcc/i486-linux-gnu/4.4.1/:/usr/lib/gcc/i486-linux-gnu/:/usr/lib/gcc/i486-linux-gnu/4.4.1/:/usr/lib/gcc/i486-linux-gnu/:/usr/lib/gcc/i486-linux-gnu/4.4.1/:/usr/lib/gcc/i486-linux-gnu/ LIBRARY_PATH=/usr/lib/gcc/i486-linux-gnu/4.4.1/:/usr/lib/gcc/i486-linux-gnu/4.4.1/:/usr/lib/gcc/i486-linux-gnu/4.4.1/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/i486-linux-gnu/4.4.1/../../../:/lib/:/usr/lib/:/usr/lib/i486-linux-gnu/ COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O' '-ftree-vrp' '-o' 'vus' '-mtune=generic' '-march=i486' /usr/lib/gcc/i486-linux-gnu/4.4.1/collect2 --build-id --eh-frame-hdr -m elf_i386 --hash-style=both -dynamic-linker /lib/ld-linux.so.2 -o vus -z relro /usr/lib/gcc/i486-linux-gnu/4.4.1/../../../../lib/crt1.o /usr/lib/gcc/i486-linux-gnu/4.4.1/../../../../lib/crti.o /usr/lib/gcc/i486-linux-gnu/4.4.1/crtbegin.o -L/usr/lib/gcc/i486-linux-gnu/4.4.1 -L/usr/lib/gcc/i486-linux-gnu/4.4.1 -L/usr/lib/gcc/i486-linux-gnu/4.4.1/../../../../lib -L/lib/../lib -L/usr/lib/../lib -L/usr/lib/gcc/i486-linux-gnu/4.4.1/../../.. -L/usr/lib/i486-linux-gnu vus.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/i486-linux-gnu/4.4.1/crtend.o /usr/lib/gcc/i486-linux-gnu/4.4.1/../../../../lib/crtn.o And the preprocessed file: # 1 "vus.c" # 1 "<built-in>" # 1 "<command-line>" # 1 "vus.c" # 1 "/usr/include/assert.h" 1 3 4 # 37 "/usr/include/assert.h" 3 4 # 1 "/usr/include/features.h" 1 3 4 # 313 "/usr/include/features.h" 3 4 # 1 "/usr/include/bits/predefs.h" 1 3 4 # 314 "/usr/include/features.h" 2 3 4 # 346 "/usr/include/features.h" 3 4 # 1 "/usr/include/sys/cdefs.h" 1 3 4 # 353 "/usr/include/sys/cdefs.h" 3 4 # 1 "/usr/include/bits/wordsize.h" 1 3 4 # 354 "/usr/include/sys/cdefs.h" 2 3 4 # 347 "/usr/include/features.h" 2 3 4 # 378 "/usr/include/features.h" 3 4 # 1 "/usr/include/gnu/stubs.h" 1 3 4 # 1 "/usr/include/bits/wordsize.h" 1 3 4 # 5 "/usr/include/gnu/stubs.h" 2 3 4 # 1 "/usr/include/gnu/stubs-32.h" 1 3 4 # 8 "/usr/include/gnu/stubs.h" 2 3 4 # 379 "/usr/include/features.h" 2 3 4 # 38 "/usr/include/assert.h" 2 3 4 # 68 "/usr/include/assert.h" 3 4 extern void __assert_fail (__const char *__assertion, __const char *__file, unsigned int __line, __const char *__function) __attribute__ ((__nothrow__)) __attribute__ ((__noreturn__)); extern void __assert_perror_fail (int __errnum, __const char *__file, unsigned int __line, __const char *__function) __attribute__ ((__nothrow__)) __attribute__ ((__noreturn__)); extern void __assert (const char *__assertion, const char *__file, int __line) __attribute__ ((__nothrow__)) __attribute__ ((__noreturn__)); # 2 "vus.c" 2 volatile unsigned short vus = 0xfffe; int main(void) { ((vus * vus < 1) ? (void) (0) : __assert_fail ("vus * vus < 1", "vus.c", 7, __PRETTY_FUNCTION__)); return 1; } -- Summary: volatile lost in optimization Product: gcc Version: 4.4.1 Status: UNCONFIRMED Severity: minor Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: majbrock at dse dot nl GCC build triplet: i486-linux-gnu GCC host triplet: i486-linux-gnu GCC target triplet: i486-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45126
