linux-kernel - [PATCH v2 6/6] x86: bug.h: use asm_inline in _BUG

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Date:   Sat, 31 Aug 2019 01:15:27 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Nadav Amit <namit@...are.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        ndesaulniers@...gle.com,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: [PATCH v2 6/6] x86: bug.h: use asm_inline in _BUG_FLAGS definitions

This helps preventing a BUG* or WARN* in some static inline from
preventing that (or one of its callers) being inlined, so should allow
gcc to make better informed inlining decisions.

For example, with gcc 9.2, tcp_fastopen_no_cookie() vanishes from
net/ipv4/tcp_fastopen.o. It does not itself have any BUG or WARN, but
it calls dst_metric() which has a WARN_ON_ONCE - and despite that
WARN_ON_ONCE vanishing since the condition is compile-time false,
dst_metric() is apparently sufficiently "large" that when it gets
inlined into tcp_fastopen_no_cookie(), the latter becomes too large
for inlining.

Overall, if one asks size(1), .text decreases a little and .data
increases by about the same amount (x86-64 defconfig)

$ size vmlinux.{before,after}
   text    data     bss     dec     hex filename
19709726        5202600 1630280 26542606        195020e vmlinux.before
19709330        5203068 1630280 26542678        1950256 vmlinux.after

while bloat-o-meter says

add/remove: 10/28 grow/shrink: 103/51 up/down: 3669/-2854 (815)
...
Total: Before=14783683, After=14784498, chg +0.01%

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 arch/x86/include/asm/bug.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index 6804d6642767..facba9bc30ca 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -32,7 +32,7 @@
 
 #define _BUG_FLAGS(ins, flags)						\
 do {									\
-	asm volatile("1:\t" ins "\n"					\
+	asm_inline volatile("1:\t" ins "\n"				\
 		     ".pushsection __bug_table,\"aw\"\n"		\
 		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
 		     "\t"  __BUG_REL(%c0) "\t# bug_entry::file\n"	\
@@ -49,7 +49,7 @@ do {									\
 
 #define _BUG_FLAGS(ins, flags)						\
 do {									\
-	asm volatile("1:\t" ins "\n"					\
+	asm_inline volatile("1:\t" ins "\n"				\
 		     ".pushsection __bug_table,\"aw\"\n"		\
 		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
 		     "\t.word %c0"        "\t# bug_entry::flags\n"	\
-- 
2.20.1