compiler-types.h: add asm_inline definition

This adds an asm_inline macro which expands to "asm inline" [1] when
the compiler supports it. This is currently gcc 9.1+, gcc 8.3
and (once released) gcc 7.5 [2]. It expands to just "asm" for 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.

The Kconfig logic is taken from an RFC patch by Masahiro Yamada [3].

[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.

[2] https://lore.kernel.org/lkml/20190907001411.GG9749@gate.crashing.org/

[3] https://lore.kernel.org/lkml/1544695154-15250-1-git-send-email-yamada.masahiro@socionext.com/

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

include/linux/compiler_types.h

@@ -198,6 +198,12 @@ struct ftrace_likely_data {
 #define asm_volatile_goto(x...) asm goto(x)
 #endif
 
+#ifdef CONFIG_CC_HAS_ASM_INLINE
+#define asm_inline asm __inline
+#else
+#define asm_inline asm
+#endif
+
 #ifndef __no_fgcse
 # define __no_fgcse
 #endif

init/Kconfig

@@ -30,6 +30,9 @@ config CC_CAN_LINK
 config CC_HAS_ASM_GOTO
 	def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))
 
+config CC_HAS_ASM_INLINE
+	def_bool $(success,echo 'void foo(void) { asm inline (""); }' | $(CC) -x c - -c -o /dev/null)
+
 config CC_HAS_WARN_MAYBE_UNINITIALIZED
 	def_bool $(cc-option,-Wmaybe-uninitialized)
 	help