[Bug c/24178] New: GCC 4.0/4.1 generates code that produces unaligned access exceptions

[tsv at solvo dot ru]

gcc version 4.0.1 20050727 (Red Hat 4.0.1-5)
gcc version 4.0.2
GNU C version 4.1.0 20050927 (experimental) (alpha-unknown-linux-gnu)

All tested versions generate the same wrong code for the following testcase
(EV4/EV5 only with -O2 optimization):
#include <stdio.h>
#include <stddef.h>

typedef struct Foo
{
   void *p1;
   long p2;
   void *p3;
   unsigned char p4;
   char p5[3];
} Foo;

#ifdef __DECC
#undef offsetof
#define offsetof(TYPE, MEMBER) ((char *)(&(((TYPE *)0)-> MEMBER )))
#endif


void foo(const char *fn)
{
   Foo *p;

   p = (Foo *)(fn - offsetof(Foo, p5));
   p->p4 = 1;
}

int main(int argc, char **argv)
{
   Foo p;
#if defined(__alpha__) && defined(__linux__)
# include <asm/sysinfo.h>
# include <asm/unistd.h>

   unsigned int buf[2] =
     {
         SSIN_UACPROC, UAC_SIGBUS | UAC_NOPRINT
     };
   syscall(__NR_osf_setsysinfo, SSI_NVPAIRS, buf, 1, 0, 0, 0);
#endif

   printf("Offset - %d\n", offsetof(Foo, p5));

   foo(p.p5);
}

The assembler for the function "foo" is:
        .globl foo
        .ent foo
$foo..ng:
foo:
        .frame $30,0,$26,0
        .prologue 0
        ldah $2,256($31)
        ldl $1,-4($16)
        zapnot $1,247,$1
        bis $1,$2,$2
        stl $2,-4($16)
        ret $31,($26),1
        .end foo

The offset of "p5" member is 25 bytes, but compiler thinks that "p5" is aligned
in "foo" function and issue "ldl" on unaligned address.
DECC compiler generates "ldl $1, -1($16)" instruction.

It might be not very good coding practice, but it taken from Mozilla source.

Thank you.


-- 
           Summary: GCC 4.0/4.1 generates code that produces unaligned
                    access exceptions
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tsv at solvo dot ru
 GCC build triplet: alpha-*-linux-gnu
  GCC host triplet: alpha-*-linux-gnu
GCC target triplet: alpha-*-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24178