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@@ -344,7 +344,7 @@ Remember: if another thread can find your data structure, and you don't
344have a reference count on it, you almost certainly have a bug. 344have a reference count on it, you almost certainly have a bug.
345 345
346 346
347 Chapter 11: Macros, Enums, Inline functions and RTL 347 Chapter 11: Macros, Enums and RTL
348 348
349Names of macros defining constants and labels in enums are capitalized. 349Names of macros defining constants and labels in enums are capitalized.
350 350
@@ -429,7 +429,35 @@ from void pointer to any other pointer type is guaranteed by the C programming
429language. 429language.
430 430
431 431
432 Chapter 14: References 432 Chapter 14: The inline disease
433
434There appears to be a common misperception that gcc has a magic "make me
435faster" speedup option called "inline". While the use of inlines can be
436appropriate (for example as a means of replacing macros, see Chapter 11), it
437very often is not. Abundant use of the inline keyword leads to a much bigger
438kernel, which in turn slows the system as a whole down, due to a bigger
439icache footprint for the CPU and simply because there is less memory
440available for the pagecache. Just think about it; a pagecache miss causes a
441disk seek, which easily takes 5 miliseconds. There are a LOT of cpu cycles
442that can go into these 5 miliseconds.
443
444A reasonable rule of thumb is to not put inline at functions that have more
445than 3 lines of code in them. An exception to this rule are the cases where
446a parameter is known to be a compiletime constant, and as a result of this
447constantness you *know* the compiler will be able to optimize most of your
448function away at compile time. For a good example of this later case, see
449the kmalloc() inline function.
450
451Often people argue that adding inline to functions that are static and used
452only once is always a win since there is no space tradeoff. While this is
453technically correct, gcc is capable of inlining these automatically without
454help, and the maintenance issue of removing the inline when a second user
455appears outweighs the potential value of the hint that tells gcc to do
456something it would have done anyway.
457
458
459
460 Chapter 15: References
433 461
434The C Programming Language, Second Edition 462The C Programming Language, Second Edition
435by Brian W. Kernighan and Dennis M. Ritchie. 463by Brian W. Kernighan and Dennis M. Ritchie.
@@ -453,4 +481,4 @@ Kernel CodingStyle, by greg@kroah.com at OLS 2002:
453http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/ 481http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
454 482
455-- 483--
456Last updated on 16 February 2004 by a community effort on LKML. 484Last updated on 30 December 2005 by a community effort on LKML.