[Bug c/36093] New: __align__ produces incorrect results in certain cases

[zhirsch at vmware dot com]

Hi,

The following code produces incorrect results under -O1 (or higher), but works
fine under -O0 for gcc 4.1.2 and 4.2.3. gcc 3.4.6 produces the correct result
for all optimization levels.

# 1 "t.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "t.c"
extern int printf(const char *format, ...);

typedef struct Bar {
      char c[129];
} Bar __attribute__((__aligned__(128)));

typedef struct Foo {
      Bar bar[4];
} Foo;

Foo foo[4];

int main()
{
   int i, j;
   Foo *foop = &foo[0];

   for (i=0; i < 4; i++) {
      Bar *bar = &foop->bar[i];
      for (j=0; j < 129; j++) {
         bar->c[j] = 'a' + i;
      }
   }

   printf("foo[0].bar[3].c[128] = %c\n", foo[0].bar[3].c[128]);
   return 0;
}

The expected behavior when the resulting program is run is:

 foo[0].bar[3].c[128] = d

The actual behavior is:

 foo[0].bar[3].c[128] = 

Output from gcc -v:

Using built-in specs.
Target: i686-linux
Configured with: ../gcc-4.1.2/configure
--prefix=/build/toolchain/lin32/gcc-4.1.2-2 --disable-nls --enable-shared
--enable-threads=posix --enable-languages=c,c++ --with-gnu-as --with-gnu-ld
--target=i686-linux
--with-as=/build/toolchain/lin32/binutils-2.17.50.0.15/bin/i686-linux-as
--with-ld=/build/toolchain/lin32/binutils-2.17.50.0.15/bin/i686-linux-ld
--enable-__cxa_atexit --with-sysroot=/build/toolchain/lin32/glibc-2.2.5-44
--disable-tls --enable-clocale=gnu
Thread model: posix
gcc version 4.1.2


and

Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--with-gxx-include-dir=/usr/include/c++/4.2 --program-suffix=-4.2
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.2.3 (Ubuntu 4.2.3-2ubuntu7)

Thanks for taking a look.


-- 
           Summary: __align__ produces incorrect results in certain cases
           Product: gcc
           Version: 4.2.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: zhirsch at vmware dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36093