diff options
author | Ingo Molnar <mingo@elte.hu> | 2009-01-10 23:56:42 +0100 |
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committer | Ingo Molnar <mingo@elte.hu> | 2009-01-10 23:56:42 +0100 |
commit | 1de8cd3cb9f61e854e743c7210df43db517d4832 (patch) | |
tree | 2b69c5ba5e4094037fa04d0fcb6c4537c222cde8 /include/linux/compiler-gcc.h | |
parent | 1eb1b3b65dc3e3ffcc6a60e115c085c0c11c1077 (diff) | |
parent | 3d14bdad40315b54470cb7812293d14c8af2bf7d (diff) |
Merge branch 'linus' into x86/cleanups
Diffstat (limited to 'include/linux/compiler-gcc.h')
-rw-r--r-- | include/linux/compiler-gcc.h | 14 |
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 |