On Mon, Jun 16 2014, Andrew Morton <a...@linux-foundation.org> wrote:
> On Mon, 16 Jun 2014 23:07:22 +0200 Michal Nazarewicz <min...@mina86.com>
> wrote:
>
>> It appears that gcc is better at optimising a double call to min
>> and max rather than open coded min3 and max3. This can be observed
>> here:
>>
>> ...
>>
>> Furthermore, after ___make allmodconfig && make bzImage modules___ this is
>> the
>> comparison of image and modules sizes:
>>
>> # Without this patch applied
>> $ ls -l arch/x86/boot/bzImage **/*.ko |awk '{size += $5} END {print
>> size}'
>> 350715800
>>
>> # With this patch applied
>> $ ls -l arch/x86/boot/bzImage **/*.ko |awk '{size += $5} END {print
>> size}'
>> 349856528
>
> We saved nearly a megabyte by optimising min3(), max3() and clamp()?
>
> I'm counting a grand total of 182 callsites for those macros. So the
> saving is 4700 bytes per invokation? I don't believe it...
You're absolutely right. I must have messed something up here. This
portion of the commit message should be removed.
So I've redone this just on just the kernel image with allmodconfig:
Linux mpn-glaptop 3.13.0-29-generic #53~precise1-Ubuntu SMP Wed Jun 4
22:06:25 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
-rwx------ 1 mpn eng 51224656 Jun 17 14:15 vmlinux.before
-rwx------ 1 mpn eng 51224608 Jun 17 13:57 vmlinux.after
48 bytes reduction. The do_fault_around was a few instruction shorter
and as far as I can tell saved 12 bytes on the stack, i.e.:
$ grep -e rsp -e pop -e push do_fault_around.*
do_fault_around.before.s:push %rbp
do_fault_around.before.s:mov %rsp,%rbp
do_fault_around.before.s:push %r13
do_fault_around.before.s:push %r12
do_fault_around.before.s:push %rbx
do_fault_around.before.s:sub $0x38,%rsp
do_fault_around.before.s:add $0x38,%rsp
do_fault_around.before.s:pop %rbx
do_fault_around.before.s:pop %r12
do_fault_around.before.s:pop %r13
do_fault_around.before.s:pop %rbp
do_fault_around.after.s:push %rbp
do_fault_around.after.s:mov %rsp,%rbp
do_fault_around.after.s:push %r12
do_fault_around.after.s:push %rbx
do_fault_around.after.s:sub $0x30,%rsp
do_fault_around.after.s:add $0x30,%rsp
do_fault_around.after.s:pop %rbx
do_fault_around.after.s:pop %r12
do_fault_around.after.s:pop %rbp
or here side-by-side:
Before After
push %rbp push %rbp
mov %rsp,%rbp mov %rsp,%rbp
push %r13
push %r12 push %r12
push %rbx push %rbx
sub $0x38,%rsp sub $0x30,%rsp
add $0x38,%rsp add $0x30,%rsp
pop %rbx pop %rbx
pop %r12 pop %r12
pop %r13
pop %rbp pop %rbp
There are also fewer branches:
$ grep ^j do_fault_around.*
do_fault_around.before.s:jae ffffffff812079b7
do_fault_around.before.s:jmp ffffffff812079c5
do_fault_around.before.s:jmp ffffffff81207a14
do_fault_around.before.s:ja ffffffff812079f9
do_fault_around.before.s:jb ffffffff81207a10
do_fault_around.before.s:jmp ffffffff81207a63
do_fault_around.before.s:jne ffffffff812079df
do_fault_around.after.s:jmp ffffffff812079fd
do_fault_around.after.s:ja ffffffff812079e2
do_fault_around.after.s:jb ffffffff812079f9
do_fault_around.after.s:jmp ffffffff81207a4c
do_fault_around.after.s:jne ffffffff812079c8
And here's with allyesconfig on a different machine:
$ uname -a; gcc --version; ls -l vmlinux.*
Linux erwin 3.14.7-mn #54 SMP Sun Jun 15 11:25:08 CEST 2014 x86_64 AMD
Phenom(tm) II X3 710 Processor AuthenticAMD GNU/Linux
gcc (GCC) 4.8.3
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
-rwx------ 1 mina86 mina86 230616126 Jun 17 15:39 vmlinux.before*
-rwx------ 1 mina86 mina86 230614861 Jun 17 14:36 vmlinux.after*
1265 bytes reduction.
--
Best regards, _ _
.o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michał “mina86” Nazarewicz (o o)
ooo +--<m...@google.com>--<xmpp:min...@jabber.org>--ooO--(_)--Ooo--
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
