http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48789
Summary: missed ARM optimization: use LDMIA
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: edwinto...@gmail.com
Host: x86_64-linux-gnu
Target: arm-elf
Build: x86_64-linux-gnu
The attached testcase compiles to larger and slower code than the
hand-optimized version, although the C code follows closely the structure of
hand-optimized assembly.
To reproduce the missed optimization:
arm-elf-gcc reverse.c -O3 -mcpu=arm946e-s -msoft-float
The reverse_bytes_order_c2 has too many ldr/str instructions, it should use
ldmia/stmia as seen in the hand-optimized version (test.S
reverse_bytes_order2).
Note: without -msoft-float it generates faster code by using VFP instructions,
but my CPU doesn't support them, so I have to turn off floating point
generation.
Attachments:
reverse.c: the testcase
test.S: the hand-optimized version of the reverse_bytes_order_c2, called
reverse_bytes_order2 here (code from CHDK's lib/armutil/)
bench.c: a simple benchmark runner to compare gcc's version with the hand
optimized one
This happens both with 4.6 and 4.5:
$ arm-elf-gcc -v
Using built-in specs.
COLLECT_GCC=../build-dir/arm/toolchain/bin/arm-elf-gcc
COLLECT_LTO_WRAPPER=/home/edwin/chdk/build-dir/arm/toolchain/libexec/gcc/arm-elf/4.6.0/lto-wrapper
Target: arm-elf
Configured with: ../gcc-4.6.0/configure --target=arm-elf
--prefix=/home/edwin/chdk/build-dir/arm/toolchain --enable-interwork
--enable-multilib --enable-languages=c --with-newlib
--with-gmp-include=/home/edwin/chdk/build-dir/build/gmp
--with-gmp-lib=/home/edwin/chdk/build-dir/build/gmp/.libs --without-headers
--disable-libssp --disable-nls --disable-zlib --disable-libc --disable-libm
--disable-intl --disable-hardfloat --disable-threads --with-gnu-as
--with-gnu-ld
Thread model: single
gcc version 4.6.0 (GCC)
$ /opt/cfarm/release/4.5.0/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/opt/cfarm/release/4.5.0/bin/gcc
COLLECT_LTO_WRAPPER=/home/guerby/opt/release/4.5.0/bin/../libexec/gcc/armv7l-unknown-linux-gnueabi/4.5.0/lto-wrapper
Target: armv7l-unknown-linux-gnueabi
Configured with: ../gcc-4.5.0/configure --prefix=/opt/cfarm/release/4.5.0
--enable-languages=c,ada --enable-__cxa_atexit --disable-nls
--enable-threads=posix --disable-multilib --with-gmp=/opt/cfarm/gmp-4.2.4
--with-mpfr=/opt/cfarm/mpfr-2.4.2 --with-mpc=/opt/cfarm/mpc-0.8
--with-cpu=cortex-a8 --with-fpu=neon --with-float=softfp --disable-werror
Thread model: posix
gcc version 4.5.0 (GCC)
Some benchmarks (run on gcc33, which would support armv7, but my CPU won't, so
I can only use armv5te):
base: 0.340810 (hand-optimized assembly)
3: 0.840712 (alternate version)
c: 0.379164 (C code, compiled with -O3)
c2: 0.395410 (C code, unrolled 8 times as the hand assembly, compiled with -O3)
(note: run benchmark as ./a.out; ./a.out; ./a.out. I think there is some
frequency scaling causing the first run to be slower)
To run benchmark:
/opt/cfarm/release/4.5.0/bin/gcc bench.c reverse.c test.S -O3 -mcpu=arm946e-s
-msoft-float
