Add speculatable function attribute

This attribute tells the optimizer that the function may be speculated.

Patch by Tom Stellard

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301680 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 11 insertions, 0 deletions

diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index b0a31589cc4f..bf4973ca9aed 100644
--- a/docs/LangRef.rst
+++ b/docs/LangRef.rst
@@ -1535,6 +1535,17 @@ example:
 ``sanitize_thread``
     This attribute indicates that ThreadSanitizer checks
     (dynamic thread safety analysis) are enabled for this function.
+``speculatable``
+    This function attribute indicates that the function does not have any
+    effects besides calculating its result and does not have undefined behavior.
+    Note that ``speculatable`` is not enough to conclude that along any
+    particular exection path the number of calls to this function will not be
+    externally observable. This attribute is only valid on functions
+    and declarations, not on individual call sites. If a function is
+    incorrectly marked as speculatable and really does exhibit
+    undefined behavior, the undefined behavior may be observed even
+    if the call site is dead code.
+
 ``ssp``
     This attribute indicates that the function should emit a stack
     smashing protector. It is in the form of a "canary" --- a random value