On Sat, 17 Oct 2020 at 15:55, Christophe Leroy
<christophe.leroy@xxxxxxxxxx> 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?
Reviewed-by: Joel Stanley <joel@xxxxxxxxx>
> Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> ---
> 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
>
