[Bug target/30354] -Os doesn't optimize a/CONST even if it saves size.

["vda.linux at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>]

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