compiler-gcc.h: add more comments to RELOC_HIDE

[linux-2.6-omap-h63xx.git] / include / linux / compiler-gcc.h

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index b2fd7547b58deaa5e90b0fa5b384c0581da19d58..1514d534deebb7f52361d1ef27382ac791ba955f 100644 (file)
--- 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
@@ -29,10 +39,11 @@
   BUILD_BUG_ON_ZERO(__builtin_types_compatible_p(typeof(a), typeof(&a[0])))
 
 /*
- * Force always-inline if the user requests it so via the .config:
+ * Force always-inline if the user requests it so via the .config,
+ * or if gcc is too old:
  */
 #if !defined(CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING) || \
-    !defined(CONFIG_OPTIMIZE_INLINING) && (__GNUC__ >= 4)
+    !defined(CONFIG_OPTIMIZE_INLINING) || (__GNUC__ < 4)
 # define inline                inline          __attribute__((always_inline))
 # define __inline__    __inline__      __attribute__((always_inline))
 # define __inline      __inline        __attribute__((always_inline))
@@ -60,3 +71,8 @@
 #define  noinline                      __attribute__((noinline))
 #define __attribute_const__            __attribute__((__const__))
 #define __maybe_unused                 __attribute__((unused))
+
+#define __gcc_header(x) #x
+#define _gcc_header(x) __gcc_header(linux/compiler-gcc##x.h)
+#define gcc_header(x) _gcc_header(x)
+#include gcc_header(__GNUC__)