Re: [patch 00/2] improve .text size on gcc 4.0 and newer compilers

Arjan van de Ven
2006-01-02T22:56:35Z

> > Are these typical targets for non-inline?

these are very simple 1 line things, and are the cases where inline is
just fine. The problem cases are the ones with a whole lot more than
that though, say 3 or more real code lines with things like loops or
udelays or ... There's 50+ line functions marked "inline". Those are the
"bad guys" not so much the simple 1 liners

>
> according to the latest flamewars, maybe it would be better
> to just turn the #defines into static functions instead on static inlines...
> guess even better would be to just get CodingStyle fixed ASAP ;)

I proposed the following chunks:

Adds a bit of text to Documentation/Codingstyle to state that
inlining everything "just because" is a bad idea

Signed-off-by: Arjan van de Ven <arjan@infradead.org>

diff -purN linux-2.6.15-rc6/Documentation/CodingStyle linux-2.6.15-rc6-deinline/Documentation/CodingStyle
--- linux-2.6.15-rc6/Documentation/CodingStyle 2005-10-28 02:02:08.000000000 +0200
+++ linux-2.6.15-rc6-deinline/Documentation/CodingStyle 2005-12-30 13:31:13.000000000 +0100
@@ -344,7 +344,7 @@ Remember: if another thread can find you
have a reference count on it, you almost certainly have a bug.

- Chapter 11: Macros, Enums, Inline functions and RTL
+ Chapter 11: Macros, Enums and RTL

Names of macros defining constants and labels in enums are capitalized.

@@ -429,7 +429,35 @@ from void pointer to any other pointer t
language.

- Chapter 14: References
+ Chapter 14: The inline disease
+
+There appears to be a common misperception that gcc has a magic "make me
+faster" ricing option called "inline". While the use of inlines can be
+appropriate (for example as a means of replacing macros, see Chapter 11), it
+very often is not. Abundant use of the inline keyword leads to a much bigger
+kernel, which in turn slows the system as a whole down, due to a bigger
+icache footprint for the CPU and simply because there is less memory
+available for the pagecache. Just think about it; a pagecache miss causes a
+disk seek, which easily takes 5 miliseconds. There are a LOT of cpu cycles
+that can go into these 5 miliseconds.
+
+A reasonable rule of thumb is to not put inline at functions that have more
+than 3 lines of code in them. An exception to this rule are the cases where
+a parameter is known to be a compiletime constant, and as a result of this
+constantness you *know* the compiler will be able to optimize most of your
+function away at compile time. For a good example of this later case, see
+the kmalloc() inline function.
+
+Often people argue that adding inline to functions that are static and used
+only once is always a win since there is no space tradeoff. While this is
+technically correct, gcc is capable of inlining these automatically without
+help, and the maintenance issue of removing the inline when a second user
+appears outweighs the potential value of the hint that tells gcc to do
+something it would have done anyway.
+
+
+
+ Chapter 15: References

The C Programming Language, Second Edition
by Brian W. Kernighan and Dennis M. Ritchie.
@@ -450,4 +478,4 @@ WG14 is the international standardizatio
language C, URL: http://std.dkuug.dk/JTC1/SC22/WG14/

--
-Last updated on 16 February 2004 by a community effort on LKML.
+Last updated on 30 December 2005 by a community effort on LKML.