diff mbox series

[08/95] asm-generic: force inlining of get_order() to work around gcc10 poor decision

Message ID 20201216044245.gdQ1_1QzT%akpm@linux-foundation.org (mailing list archive)
State New, archived
Headers show
Series [01/95] mm: fix a race on nr_swap_pages | expand

Commit Message

Andrew Morton Dec. 16, 2020, 4:42 a.m. UTC 
From: Christophe Leroy <christophe.leroy@csgroup.eu>
Subject: asm-generic: force inlining of get_order() to work around gcc10 poor decision

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

Link: https://lkml.kernel.org/r/96c6172d619c51acc5c1c4884b80785c59af4102.1602949927.git.christophe.leroy@csgroup.eu
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/asm-generic/getorder.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
diff mbox series

Patch

--- a/include/asm-generic/getorder.h~asm-generic-force-inlining-of-get_order-to-work-around-gcc10-poor-decision
+++ a/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)