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

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Date:   Sat, 17 Oct 2020 15:55:21 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arch@...r.kernel.org
Subject: [PATCH] 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

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