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>

1 files changed, 4 insertions, 4 deletions

diff --git a/include/linux/init.h b/include/linux/init.h
index 5b52853..f0d0e32 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -40,10 +40,10 @@
 
 /* These are for everybody (although not all archs will actually
    discard it in modules) */
-#define __init		__attribute__ ((__section__ (".init.text")))
+#define __init		__attribute__ ((__section__ (".init.text"))) __cold
 #define __initdata	__attribute__ ((__section__ (".init.data")))
 #define __exitdata	__attribute__ ((__section__(".exit.data")))
-#define __exit_call	__attribute_used__ __attribute__ ((__section__ (".exitcall.exit")))
+#define __exit_call	__attribute_used__ __attribute__ ((__section__ (".exitcall.exit"))) __cold
 
 /* modpost check for section mismatches during the kernel build.
  * A section mismatch happens when there are references from a
@@ -59,9 +59,9 @@
 #define __initdata_refok          __attribute__ ((__section__ (".data.init.refok")))
 
 #ifdef MODULE
-#define __exit		__attribute__ ((__section__(".exit.text")))
+#define __exit		__attribute__ ((__section__(".exit.text"))) __cold
 #else
-#define __exit		__attribute_used__ __attribute__ ((__section__(".exit.text")))
+#define __exit		__attribute_used__ __attribute__ ((__section__(".exit.text"))) __cold
 #endif
 
 /* For assembly routines */