[Bug c/36135] New: GCC creates suboptimal ASM : suboptimal Adressing-Modes used

["gunnar at greyhound-data dot com" <gcc-bugzilla at gcc dot gnu dot org>]

+++ This bug was initially created as a clone of Bug #36133 +++

Hello,

The ASM code created by GCC 4.2.1 for 68k/Coldfire platform is not optimal.
Comparing ASM output created by GCC 2.9 with GCC 4.2,
the generated code got partially much worse with GCC 4.


One problem that was visible a lot was that GCC uses suboptimal addressing
modes.

Please see the below example for details.
In line 14 to line 2E this code was created:
14:       2290            movel %a0@,%a1@
16:       2368 0004 0004  movel %a0@(4),%a1@(4)
1c:       2368 0008 0008  movel %a0@(8),%a1@(8)
22:       2368 000c 000c  movel %a0@(12),%a1@(12)
28:       d3fc 0000 0010  addal #16,%a1
2e:       d1fc 0000 0010  addal #16,%a0

Much shorter and more efficient would have been this:
14:       20d9            movel %a1@+,%a0@+
16:       20d9            movel %a1@+,%a0@+
18:       20d9            movel %a1@+,%a0@+
1a:       20d9            movel %a1@+,%a0@+


Example: C-source
Code:
void * copy_32x4a(void *destparam, const void *srcparam, size_t size)
{
        int *dest = destparam;
        const int *src = srcparam;
        int size32;
        size32 = size / 16;
        for (; size32; size32--) {
                *dest++ = *src++;
                *dest++ = *src++;
                *dest++ = *src++;
                *dest++ = *src++;
        }
}

Compile option: m68k-linux-gnu-gcc -mcpu=54455 -msoft-float -o example -Os
-fomit-frame-pointer example.c

Code generated by GCC 4.2:
04:       202f 000c       movel %sp@(12),%d0
08:       226f 0004       moveal %sp@(4),%a1
0c:       206f 0008       moveal %sp@(8),%a0
10:       e888            lsrl #4,%d0
12:       6022            bras 36
14:       2290            movel %a0@,%a1@
16:       2368 0004 0004  movel %a0@(4),%a1@(4)
1c:       2368 0008 0008  movel %a0@(8),%a1@(8)
22:       2368 000c 000c  movel %a0@(12),%a1@(12)
28:       d3fc 0000 0010  addal #16,%a1
2e:       d1fc 0000 0010  addal #16,%a0
34:       5380            subql #1,%d0
36:       4a80            tstl %d0
38:       66da            bnes 14
3a:       4e75            rts    

For comparison here is code that you would expect:
04:       202f 000c       movel %sp@(12),%d0
08:       226f 0004       moveal %sp@(4),%a1
0c:       206f 0008       moveal %sp@(8),%a0
10:       e888            lsrl #4,%d0
12:       6022            beq 20
14:       20d9            movel %a1@+,%a0@+
16:       20d9            movel %a1@+,%a0@+
18:       20d9            movel %a1@+,%a0@+
1a:       20d9            movel %a1@+,%a0@+
1c:       5380            subql #1,%d0
1e:       66da            bnes 14
20:       4e75            rts 

Compiler used:
m68k-linux-gnu-gcc -v
Using built-in specs.
Target: m68k-linux-gnu
Configured with: /scratch/shinwell/cf-fall-linux-lite/src/gcc-4.2/configure
--build=i686-pc-linux-gnu --host=i686-pc-linux-gnu --target=m68k-linux-gnu
--enable-threads --disable-libmudflap --disable-libssp --disable-libgomp
--disable-libstdcxx-pch --with-arch=cf --with-gnu-as --with-gnu-ld
--enable-languages=c,c++ --enable-shared --enable-symvers=gnu
--enable-__cxa_atexit --with-pkgversion=Sourcery G++ Lite 4.2-47
--with-bugurl=https://support.codesourcery.com/GNUToolchain/ --disable-nls
--prefix=/opt/freescale/usr/local/gcc-4.2.47-eglibc-2.5.47/m68k-linux
--with-sysroot=/opt/freescale/usr/local/gcc-4.2.47-eglibc-2.5.47/m68k-linux/m68k-linux-gnu/libc
--with-build-sysroot=/scratch/shinwell/cf-fall-linux-lite/install/m68k-linux-gnu/libc
--enable-poison-system-directories
--with-build-time-tools=/scratch/shinwell/cf-fall-linux-lite/install/m68k-linux-gnu/bin
--with-build-time-tools=/scratch/shinwell/cf-fall-linux-lite/install/m68k-linux-gnu/bin
Thread model: posix
gcc version 4.2.1 (Sourcery G++ Lite 4.2-47)


I hope that this report help you to improve the quality of GCC.

Kind regards

Gunnar von Boehn
--
P.S. I put the noticed issues in individual tickets for easier tracking. I hope
that this is helpful to you.


-- 
           Summary: GCC creates suboptimal ASM : suboptimal Adressing-Modes
                    used
           Product: gcc
           Version: 4.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gunnar at greyhound-data dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: m68k-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36135