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

[msieweke at broadcom dot com]

--- Comment #2 from msieweke at broadcom dot com  2009-04-16 15:06 ---
As mentioned in the original report, the bug doesn't exist in GCC 4.x.x.  It
has since been tested with GCC 3.4.4, where the bug is fixed.
GCC 3.2.x - broken
GCC 3.3.x - broken
GCC 3.4.x - fixed
GCC 4.x.x - fixed
For a number of reasons, we're stuck using GCC 3.2.1 with only minor updates.
I tried porting parts of the MIPS code generator from 3.4.4 into 3.2.1, but
this is a larger project than it appears.  I don't expect anyone to spend much
time fixing this, but I would appreciate a hint about how I might approach
a fix.


-- 


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

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

[msieweke at broadcom dot com]

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 );
}
---
 f:
   0:   88a2lwl v0,0(a1)
   4:   98a20003lwr v0,3(a1)
   8:   a882swl v0,0(a0)
   c:   03e8jr  ra
  10:   b8820003swr 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