[Attributor] Deduce "nosync" function attribute.

Introduce and deduce "nosync" function attribute to indicate that a function
does not synchronize with another thread in a way that other thread might free memory.

Reviewers: jdoerfert, jfb, nhaehnle, arsenm

Subscribers: wdng, hfinkel, nhaenhle, mehdi_amini, steven_wu,
dexonsmith, arsenm, uenoku, hiraditya, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D62766

llvm-svn: 365830

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index f98aa6270bb..0bac9e7c5f9 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -1493,6 +1493,16 @@ example:
     Annotated functions may still raise an exception, i.a., ``nounwind`` is not implied.
     If an invocation of an annotated function does not return control back
     to a point in the call stack, the behavior is undefined.
+``nosync``
+    This function attribute indicates that the function does not communicate
+    (synchronize) with another thread through memory or other well-defined means.
+    Synchronization is considered possible in the presence of `atomic` accesses
+    that enforce an order, thus not "unordered" and "monotonic", `volatile` accesses,
+    as well as `convergent` function calls. Note that through `convergent` function calls
+    non-memory communication, e.g., cross-lane operations, are possible and are also
+    considered synchronization. However `convergent` does not contradict `nosync`.
+    If an annotated function does ever synchronize with another thread,
+    the behavior is undefined.
 ``nounwind``
     This function attribute indicates that the function never raises an
     exception. If the function does raise an exception, its runtime