r182604 - Add caveat to __builtin_readcyclecounter documentation.

[Tim Northover]

Author: tnorthover
Date: Thu May 23 14:14:12 2013
New Revision: 182604

URL: http://llvm.org/viewvc/llvm-project?rev=182604&view=rev
Log:
Add caveat to __builtin_readcyclecounter documentation.

The ARM cycle-counter can be restricted by the operating system; it's
worth warning potential users of this issue.

Modified:
    cfe/trunk/docs/LanguageExtensions.rst

Modified: cfe/trunk/docs/LanguageExtensions.rst
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/LanguageExtensions.rst?rev=182604&r1=182603&r2=182604&view=diff
==============================================================================
--- cfe/trunk/docs/LanguageExtensions.rst (original)
+++ cfe/trunk/docs/LanguageExtensions.rst Thu May 23 14:14:12 2013
@@ -1410,7 +1410,9 @@ should only be used for timing small int
 target, the return value is always zero.  This builtin takes no arguments and
 produces an unsigned long long result.
 
-Query for this feature with ``__has_builtin(__builtin_readcyclecounter)``.
+Query for this feature with ``__has_builtin(__builtin_readcyclecounter)``. Note
+that even if present, its use may depend on run-time privilege or other OS
+controlled state.
 
 .. _langext-__builtin_shufflevector: