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| -rw-r--r-- | Documentation/CodingStyle | 34 |
1 files changed, 31 insertions, 3 deletions
diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle index 187e12077e1..ce780ef648f 100644 --- a/Documentation/CodingStyle +++ b/Documentation/CodingStyle @@ -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 language. - 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 greg@kroah.com at OLS 2002: http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/ -- -Last updated on 16 February 2004 by a community effort on LKML. +Last updated on 30 December 2005 by a community effort on LKML. |