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authorAndi Kleen <ak@suse.de>2007-07-21 17:10:00 +0200
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-07-21 18:37:08 -0700
commita586df067afe0580bb02b7a6312ca2afe49bba03 (patch)
tree7806fef0876a2fd3da2f4c16919cfc551a65ff72 /include/linux/compiler.h
parentb520b85a963bf7b14b9614579aff14558d7ee264 (diff)
x86: Support __attribute__((__cold__)) in gcc 4.3
gcc 4.3 supports a new __attribute__((__cold__)) to mark functions cold. Any path directly leading to a call of this function will be unlikely. And gcc will try to generate smaller code for the function itself. Please use with care. The code generation advantage isn't large and in most cases it is not worth uglifying code with this. This patch marks some common error functions like panic(), printk() as cold. This will longer term make many unlikely()s unnecessary, although we can keep them for now for older compilers. BUG is not marked cold because there is currently no way to tell gcc to mark a inline function told. Also all __init and __exit functions are marked cold. With a non -Os build this will tell the compiler to generate slightly smaller code for them. I think it currently only uses less alignments for labels, but that might change in the future. One disadvantage over *likely() is that they cannot be easily instrumented to verify them. Another drawback is that only the latest gcc 4.3 snapshots support this. Unfortunately we cannot detect this using the preprocessor. This means older snapshots will fail now. I don't think that's a problem because they are unreleased compilers that nobody should be using. gcc also has a __hot__ attribute, but I don't see any sense in using this in the kernel right now. But someday I hope gcc will be able to use more aggressive optimizing for hot functions even in -Os, if that happens it should be added. Includes compile fix from Thomas Gleixner. Cc: Jan Hubicka <jh@suse.cz> Signed-off-by: Andi Kleen <ak@suse.de> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include/linux/compiler.h')
-rw-r--r--include/linux/compiler.h9
1 files changed, 9 insertions, 0 deletions
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 8287a72bb6a..12a1291855e 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -174,4 +174,13 @@ extern void __chk_io_ptr(const void __iomem *);
# define __attribute_const__ /* unimplemented */
#endif
+/*
+ * Tell gcc if a function is cold. The compiler will assume any path
+ * directly leading to the call is unlikely.
+ */
+
+#ifndef __cold
+#define __cold
+#endif
+
#endif /* __LINUX_COMPILER_H */