[Bug c/45126] New: volatile lost in optimization

["majbrock at dse dot nl" <gcc-bugzilla at gcc dot gnu dot org>]

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