+++ 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 set ups constant work registers
inside of working loops and not outside of them.
At address (1c): the instruction moveq #1,%d1 to set up the work register is
inside the working loop and will be unneeded executed with very iteration.
Second problem:
At address (16) the instruction movel #1,%a0@ uses the literal value #1 and not
the work register that has the same value. The literal move.l #1 has a length
of 6 bytes while using the work register would have 2 bytes only.
Example: C-source
Code:
void * write_32x4(void *destparam, const void *srcparam, size_t size)
{
int value=1;
int *dst = destparam;
size = size / 16;
for (; size; size--) {
*dst++=value;
*dst++=value;
*dst++=value;
*dst++=value;
}
}
Compile option: m68k-linux-gnu-gcc -mcpu=54455 -msoft-float -o example -Os
-fomit-frame-pointer example.c
Code generated by GCC 4.2:
<write_32x4>:
0a: 202f 000c movel %sp@(12),%d0
0e: 206f 0004 moveal %sp@(4),%a0
12: e888 lsrl #4,%d0
14: 601c bras 32
16: 20bc 0000 0001 movel #1,%a0@
1c: 7201 moveq #1,%d1
1e: 2141 0004 movel %d1,%a0@(4)
22: 2141 0008 movel %d1,%a0@(8)
26: 2141 000c movel %d1,%a0@(12)
2a: d1fc 0000 0010 addal #16,%a0
30: 5380 subql #1,%d0
32: 4a80 tstl %d0
34: 66e0 bnes 16
36: 4e75 rts
Generated code length = 46 Byte
Length of Workloop: 9 instructions, 32 byte
For comparison here is code that you would expect:
0a: 202f 000c movel %sp@(12),%d0
0e: 206f 0004 moveal %sp@(4),%a0
12: 7201 moveq #1,%d1
14: e888 lsrl #4,%d0
16: 601c beqs 24
18: 21c0 movel %d1,[EMAIL PROTECTED]
1a: 21c0 movel %d1,[EMAIL PROTECTED]
1c: 21c0 movel %d1,[EMAIL PROTECTED]
1e: 21c0 movel %d1,[EMAIL PROTECTED]
20: 5380 subql #1,%d0
22: 66e0 bnes 18
24: 4e75 rts
Expected code length = 28 Byte
Length of Workloop: 6 instructions, 12 byte
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 : constant work registers are
set up inside work loops and not outside of the loop
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=36136
