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authorIngo Molnar <mingo@elte.hu>2009-01-10 23:56:42 +0100
committerIngo Molnar <mingo@elte.hu>2009-01-10 23:56:42 +0100
commit1de8cd3cb9f61e854e743c7210df43db517d4832 (patch)
tree2b69c5ba5e4094037fa04d0fcb6c4537c222cde8 /include/linux/compiler-gcc.h
parent1eb1b3b65dc3e3ffcc6a60e115c085c0c11c1077 (diff)
parent3d14bdad40315b54470cb7812293d14c8af2bf7d (diff)
Merge branch 'linus' into x86/cleanups
Diffstat (limited to 'include/linux/compiler-gcc.h')
-rw-r--r--include/linux/compiler-gcc.h14
1 files changed, 12 insertions, 2 deletions
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index af40f8eb86f..1514d534dee 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -11,9 +11,19 @@
/* The "volatile" is due to gcc bugs */
#define barrier() __asm__ __volatile__("": : :"memory")
-/* This macro obfuscates arithmetic on a variable address so that gcc
- shouldn't recognize the original var, and make assumptions about it */
/*
+ * This macro obfuscates arithmetic on a variable address so that gcc
+ * shouldn't recognize the original var, and make assumptions about it.
+ *
+ * This is needed because the C standard makes it undefined to do
+ * pointer arithmetic on "objects" outside their boundaries and the
+ * gcc optimizers assume this is the case. In particular they
+ * assume such arithmetic does not wrap.
+ *
+ * A miscompilation has been observed because of this on PPC.
+ * To work around it we hide the relationship of the pointer and the object
+ * using this macro.
+ *
* Versions of the ppc64 compiler before 4.1 had a bug where use of
* RELOC_HIDE could trash r30. The bug can be worked around by changing
* the inline assembly constraint from =g to =r, in this particular