[Bug c/39755] New: inline memcpy() incorrectly optimized on MIPS target

["msieweke at broadcom dot com" <gcc-bugzilla@gcc.gnu.org>]

In one case the C compiler can optimize away an inline memcpy() on a
MIPS target.  The problem was duplicated with GCC 3.2.1 and 3.3.3.
The problem does not affect x86 targets or GCC 4.x (tested on 4.2.4).
The MIPS cross-compiler runs under RHEL.  Tested MIPS cross-compilers
under Cygwin and Mingw with the same results.

This is the test code and the resulting output.  The "if" statement and
16-byte memcpy() are optimized away, leaving only the 4-byte memcpy.

Compile with:  mipsisa32-elf-gcc -O2 -c test.c
---------------------------
#include <string.h>
typedef struct {
    char C[16];
    int  E;
} st;

void f( st *S, char* c );
void f( st *S, char* c )
{
    if ( S->E == 6 )
        memcpy( &S->C, c, 16 );
    else
        memcpy( &S->C, c, 4 );
}
---------------------------
00000000 <f>:
   0:   88a20000        lwl     v0,0(a1)
   4:   98a20003        lwr     v0,3(a1)
   8:   a8820000        swl     v0,0(a0)
   c:   03e00008        jr      ra
  10:   b8820003        swr     v0,3(a0)
---------------------------

Compiler info:
$ mipsisa32-elf-gcc -v                  
Reading specs from
/tools/ecos/3.2.1_opt/Linux/opt/ecos/gnutools/mipsisa32-elf/bin/../lib/gcc-lib/mipsisa32-elf/3.2.1/specs
Configured with: /home/msieweke/ecos20/gnutools-src/gcc-3.2.1/configure
--target=mipsisa32-elf --prefix=/home/msieweke/gnutools_321
--enable-languages=c,c++ --enable-gofast --with-gnu-as --with-gnu-ld
--with-newlib
--with-gxx-include-dir=/home/msieweke/gnutools_321/mipsisa32-elf/include -v
Thread model: single
gcc version 3.2.1


-- 
           Summary: inline memcpy() incorrectly optimized on MIPS target
           Product: gcc
           Version: 3.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: msieweke at broadcom dot com
 GCC build triplet: i386-gnu-linux
  GCC host triplet: i386-gnu-linux
GCC target triplet: mipsisa32-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39755