linux-kernel - Re: [PATCH] asm-generic: Force inlining of get_order() to work around gcc10 poor decision

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Date:   Mon, 19 Oct 2020 07:50:41 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Joel Stanley <joel@....id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arch@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] asm-generic: Force inlining of get_order() to work around
 gcc10 poor decision



Le 19/10/2020 à 06:55, Joel Stanley a écrit :
> On Sat, 17 Oct 2020 at 15:55, Christophe Leroy
> <christophe.leroy@...roup.eu> wrote:
>>
>> When building mpc885_ads_defconfig with gcc 10.1,
>> the function get_order() appears 50 times in vmlinux:
>>
>> [linux]# ppc-linux-objdump -x vmlinux | grep get_order | wc -l
>> 50
>>
>> [linux]# size vmlinux
>>     text    data     bss     dec     hex filename
>> 3842620  675624  135160 4653404  47015c vmlinux
>>
>> In the old days, marking a function 'static inline' was forcing
>> GCC to inline, but since commit ac7c3e4ff401 ("compiler: enable
>> CONFIG_OPTIMIZE_INLINING forcibly") GCC may decide to not inline
>> a function.
>>
>> It looks like GCC 10 is taking poor decisions on this.
>>
>> get_order() compiles into the following tiny function,
>> occupying 20 bytes of text.
>>
>> 0000007c <get_order>:
>>    7c:   38 63 ff ff     addi    r3,r3,-1
>>    80:   54 63 a3 3e     rlwinm  r3,r3,20,12,31
>>    84:   7c 63 00 34     cntlzw  r3,r3
>>    88:   20 63 00 20     subfic  r3,r3,32
>>    8c:   4e 80 00 20     blr
>>
>> By forcing get_order() to be __always_inline, the size of text is
>> reduced by 1940 bytes, that is almost twice the space occupied by
>> 50 times get_order()
>>
>> [linux-powerpc]# size vmlinux
>>     text    data     bss     dec     hex filename
>> 3840680  675588  135176 4651444  46f9b4 vmlinux
> 
> I see similar results with GCC 10.2 building for arm32. There are 143
> instances of get_order with aspeed_g5_defconfig.
> 
> Before:
>   9071838 2630138  186468 11888444         b5673c vmlinux
> After:
>   9069886 2630126  186468 11886480         b55f90 vmlinux
> 
> 1952 bytes smaller with your patch applied. Did you raise this with
> anyone from GCC?

Yes I did, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445

For the time being, it's at a standstill.

Christophe

> 
> Reviewed-by: Joel Stanley <joel@....id.au>
> 
> 
> 
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> ---
>>   include/asm-generic/getorder.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/asm-generic/getorder.h b/include/asm-generic/getorder.h
>> index e9f20b813a69..f2979e3a96b6 100644
>> --- a/include/asm-generic/getorder.h
>> +++ b/include/asm-generic/getorder.h
>> @@ -26,7 +26,7 @@
>>    *
>>    * The result is undefined if the size is 0.
>>    */
>> -static inline __attribute_const__ int get_order(unsigned long size)
>> +static __always_inline __attribute_const__ int get_order(unsigned long size)
>>   {
>>          if (__builtin_constant_p(size)) {
>>                  if (!size)
>> --
>> 2.25.0
>>