Commit a771f2b8 authored by Arjan van de Ven's avatar Arjan van de Ven Committed by Linus Torvalds
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[PATCH] Add a section about inlining to Documentation/CodingStyle

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

Signed-off-by: default avatarArjan van de Ven <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent e0804b17
......@@ -344,7 +344,7 @@ Remember: if another thread can find your data structure, and you don't
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 type is guaranteed by the C programming
Chapter 14: References
Chapter 14: The inline disease
There appears to be a common misperception that gcc has a magic "make me
faster" speedup 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.
......@@ -453,4 +481,4 @@ Kernel CodingStyle, by at OLS 2002:
Last updated on 16 February 2004 by a community effort on LKML.
Last updated on 30 December 2005 by a community effort on LKML.
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