http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30354
--- Comment #16 from Denis Vlasenko <vda.linux at googlemail dot com>
2013-01-18 10:29:12 UTC ---
(In reply to comment #15)
> Honza, did you find time to have a look?
>
> I think this regressed alot in 4.6
Not really - it's just .eh_frame section.
I re-ran the tests with two gcc's I have here and sizes look like this:
text data bss dec hex filename
257731 0 0 257731 3eec3 divmod-4.2.1-Os.o
242787 0 0 242787 3b463 divmod-4.6.3-Os.o
Stock (unpatched) gcc improved, juggles registers better. For example:
int ib_100_x(int x) { return (100 / x) ^ (100 % x); }
0: b8 64 00 00 00 mov $0x64,%eax
5: 99 cltd
6: f7 7c 24 04 idivl 0x4(%esp)
- a: 31 c2 xor %eax,%edx
- c: 89 d0 mov %edx,%eax
- e: c3 ret
+ a: 31 d0 xor %edx,%eax
+ c: c3 ret
I believe my patch would improve things still - it is orthogonal to register
allocation.
BTW, just so that we are all on the same page wrt compiler options:
here's the script I use to compile, disassemble, and extract function sizes
from test program in comment 3. Tweakable by setting $PREFIX and/or $CC:
gencode.sh
==========
#!/bin/sh
#PREFIX="i686-"
test "$PREFIX" || PREFIX=""
test "$CC" || CC="${PREFIX}gcc"
test "$OBJDUMP" || OBJDUMP="${PREFIX}objdump"
test "$NM" || NM="${PREFIX}nm"
CC_VER=`$CC --version | sed -n 's/[^ ]* [^ ]* \([3-9]\.[1-9][^ ]*\).*/\1/p'`
test "$CC_VER" || exit 1
build()
{
opt=$1
bname=divmod-$CC_VER${opt}${nail}
# -ffunction-sections makes disasm easier to understand
# (insn offsets start from 0 within every function).
# -fno-exceptions -fno-asynchronous-unwind-tables: die, .eh_frame, die!
$CC \
-m32 \
-fomit-frame-pointer \
-ffunction-sections \
-fno-exceptions \
-fno-asynchronous-unwind-tables \
${opt} t.c -c -o $bname.o \
&& $OBJDUMP -dr $bname.o >$bname.asm \
&& $NM --size-sort $bname.o | sort -k3 >$bname.nm
}
build -Os
#build -O2 #not interesting
#build -O3 #not interesting
size *.o | tee SIZES
