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| author | Dave Kleikamp <shaggy@linux.vnet.ibm.com> | 2009-02-02 13:40:55 -0600 |
|---|---|---|
| committer | Dave Kleikamp <shaggy@linux.vnet.ibm.com> | 2009-02-02 13:40:55 -0600 |
| commit | 8db0c5d5ef3ab99fe9e5151872b75f45c4282e3c (patch) | |
| tree | da9759151e00221c58cdd9f4de893c0b08753670 /include/linux/compiler-gcc.h | |
| parent | 1ad53a98c927a9b5b1b57288ac0edec562fbcf8d (diff) | |
| parent | 45c82b5a770be66845687a7d027c8b52946d59af (diff) | |
Merge branch 'master' of /home/shaggy/git/linus-clean/
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 af40f8eb86f0..1514d534deeb 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 |