[PATCH] D17314: Update langref to indicate that calls may be convergent.

[Justin Lebar via llvm-commits]

jlebar created this revision.
jlebar added reviewers: resistor, jingyue, joker.eph.
jlebar added subscribers: hfinkel, chandlerc, arsenm, jhen, tra, llvm-commits.

As previously written, only functions could be convergent.  But calls
need to have a notion of convergence as well.

To see why this is important, consider an indirect call.  We may or may
not want to disable optimizations around it and behave as though we're
calling a convergent function -- it depends on the semantics of the
language we're compiling.  Thus the need for this attr on the call.

http://reviews.llvm.org/D17314

Files:
  docs/LangRef.rst

Index: docs/LangRef.rst
===================================================================
--- docs/LangRef.rst
+++ docs/LangRef.rst
@@ -1240,15 +1240,24 @@
 ``convergent``
     In some parallel execution models, there exist operations that cannot be
     made control-dependent on any additional values.  We call such operations
-    ``convergent``, and mark them with this function attribute.
+    ``convergent``, and mark them with this attribute.
 
+    The ``convergent`` attribute may appear on functions or call/invoke
+    instructions.  When it appears on a function, it indicates that calls to
+    this function should not be made control-dependent on additional values.
     For example, the intrinsic ``llvm.cuda.syncthreads`` is ``convergent``, so
     calls to this intrinsic cannot be made control-dependent on additional
-    values.  Other functions may also be marked as convergent; this prevents
-    the same optimization on those functions.
+    values.
 
-    The optimizer may remove the ``convergent`` attribute when it can prove
-    that the function does not execute any convergent operations.
+    When the appears on a call/invoke, the ``convergent`` attribute indicates
+    that we should treat the call as though we're calling a convergent
+    function.  This is particularly useful on indirect calls; without this we
+    may treat such calls as though the target is non-convergent.
+
+    The optimizer may remove the ``convergent`` attribute on functions when it
+    can prove that the function does not execute any convergent operations.
+    Similarly, the optimizer may remove ``convergent`` on calls/invokes when it
+    can prove that the call/invoke does not run a convergent function.
 ``inaccessiblememonly``
     This attribute indicates that the function may only access memory that
     is not accessible by the module being compiled. This is a weaker form


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