[Bug driver/21795] New: reordering/aliasing effect on ppc64

["hvr at gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

when compiling the attached gcc4bug.c with

$ gcc -Wall -O2 -fno-strict-aliasing  gcc4bug.c -o gcc4bug;./gcc4bug ;echo $?
39

the assertion in line 39 gets triggered on ppc64, even though
-fno-strict-aliasing is used; looking at the assembler output, the 2 single-byte
stores (together with the assertion-checks before the call to _hexdump() that
got optimized away...) have been moved before the memcpy() 

        /* setting memory... */
#NO_APP
        li 0,-54
        ld 9,.LC3@toc(2)
        stb 0,120(1)     // m_b
        li 0,-2
        nop
        lhz 11,8(9)
        lwz 10,0(9)
        lwz 8,4(9)
        stb 0,121(1)     // m_c
        sth 11,120(1)
        stw 10,112(1)
        stw 8,116(1)
#APP
        /* done setting memory... */


interestingly, when enabling some of the commented out assert's in the .c file,
the code gets reordered to produce the expected result (i.e. all assertions to
hold, and the program to exit with a return code of 0)

(this behaviour wasn't present in gcc-3.4 yet)

$ gcc -v
Using built-in specs.
Target: powerpc64-linux
Configured with: ../src/configure -v
--enable-languages=c,c++,java,f95,objc,ada,treelang --prefix=/usr
--libexecdir=/usr/lib --enable-shared --with-system-zlib --enable-nls
--enable-threads=posix --without-included-gettext --program-suffix=-4.0
--enable-__cxa_atexit --enable-libstdcxx-allocator=mt --enable-clocale=gnu
--enable-libstdcxx-debug --enable-java-gc=boehm --enable-java-awt=gtk
--enable-mpfr --disable-nof --disable-softfloat --disable-werror
--enable-checking=release powerpc64-linux
Thread model: posix
gcc version 4.0.1 20050503 (prerelease) (Debian 4.0.0-2)

-- 
           Summary: reordering/aliasing effect on ppc64
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: driver
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hvr at gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: powerpc64-linux
  GCC host triplet: powerpc64-linux
GCC target triplet: powerpc64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21795