linux-kernel - [PATCH v2 4/6] compiler-gcc.h: add asm

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Message-Id: <20190830231527.22304-5-linux@rasmusvillemoes.dk>
Date:   Sat, 31 Aug 2019 01:15:25 +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 4/6] compiler-gcc.h: add asm_inline definition

This adds an asm_inline macro which expands to "asm inline" [1] when gcc
is new enough (>= 9.1), and just asm for older gccs and other
compilers.

Using asm inline("foo") instead of asm("foo") overrules gcc's
heuristic estimate of the size of the code represented by the asm()
statement, and makes gcc use the minimum possible size instead. That
can in turn affect gcc's inlining decisions.

I wasn't sure whether to make this a function-like macro or not - this
way, it can be combined with volatile as

  asm_inline volatile()

but perhaps we'd prefer to spell that

  asm_inline_volatile()

anyway.

[1] Technically, asm __inline, since both inline and __inline__
are macros that attach various attributes, making gcc barf if one
literally does "asm inline()". However, the third spelling __inline is
available for referring to the bare keyword.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 include/linux/compiler-gcc.h   | 4 ++++
 include/linux/compiler_types.h | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index d7ee4c6bad48..544b87b41b58 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -172,3 +172,7 @@
 #endif
 
 #define __no_fgcse __attribute__((optimize("-fno-gcse")))
+
+#if GCC_VERSION >= 90100
+#define asm_inline asm __inline
+#endif
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index ee49be6d6088..ba8d81b716c7 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -198,6 +198,10 @@ struct ftrace_likely_data {
 #define asm_volatile_goto(x...) asm goto(x)
 #endif
 
+#ifndef asm_inline
+#define asm_inline asm
+#endif
+
 #ifndef __no_fgcse
 # define __no_fgcse
 #endif
-- 
2.20.1