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| author | Ingo Molnar <mingo@elte.hu> | 2009-01-11 03:41:39 +0100 |
|---|---|---|
| committer | Ingo Molnar <mingo@elte.hu> | 2009-01-11 03:41:39 +0100 |
| commit | abede81c4fb2e3b85d8760f25e3da39d2c69a134 (patch) | |
| tree | 26c893ec108d837eb9171d678c55a1cea7b22af4 /include/linux/compiler-gcc.h | |
| parent | c9d557c19f94df42db78d4a5de4d25feee694bad (diff) | |
| parent | c59765042f53a79a7a65585042ff463b69cb248c (diff) | |
Merge commit 'v2.6.29-rc1' into core/urgent
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 |