[attrs] Handle convergent CallSites.

Summary:
Previously we had a notion of convergent functions but not of convergent
calls.  This is insufficient to correctly analyze calls where the target
is unknown, e.g. indirect calls.

Now a call is convergent if it targets a known-convergent function, or
if it's explicitly marked as convergent.  As usual, we can remove
convergent where we can prove that no convergent operations are
performed in the call.

Reviewers: chandlerc, jingyue

Subscribers: hfinkel, jhen, tra, llvm-commits

Differential Revision: http://reviews.llvm.org/D17317

llvm-svn: 261544

1 files changed, 25 insertions, 37 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
index 2eec4381a90..c321afb8aae 100644
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -903,49 +903,37 @@ static bool addNonNullAttrs(const SCCNodeSet &SCCNodes,
   return MadeChange;
 }
 
-/// Removes convergent attributes where we can prove that none of the SCC's
-/// callees are themselves convergent.  Returns true if successful at removing
-/// the attribute.
+/// Remove the convergent attribute from all functions in the SCC if every
+/// callsite within the SCC is not convergent (except for calls to functions
+/// within the SCC).  Returns true if changes were made.
 static bool removeConvergentAttrs(const SCCNodeSet &SCCNodes) {
-  // Determines whether a function can be made non-convergent, ignoring all
-  // other functions in SCC.  (A function can *actually* be made non-convergent
-  // only if all functions in its SCC can be made convergent.)
-  auto CanRemoveConvergent = [&](Function *F) {
-    if (!F->isConvergent())
-      return true;
-
-    // Can't remove convergent from declarations.
-    if (F->isDeclaration())
-      return false;
-
-    for (Instruction &I : instructions(*F))
-      if (auto CS = CallSite(&I)) {
-        // Can't remove convergent if any of F's callees -- ignoring functions
-        // in the SCC itself -- are convergent. This needs to consider both
-        // function calls and intrinsic calls. We also assume indirect calls
-        // might call a convergent function.
-        // FIXME: We should revisit this when we put convergent onto calls
-        // instead of functions so that indirect calls which should be
-        // convergent are required to be marked as such.
-        Function *Callee = CS.getCalledFunction();
-        if (!Callee || (SCCNodes.count(Callee) == 0 && Callee->isConvergent()))
-          return false;
-      }
-
-    return true;
-  };
+  // No point checking if none of SCCNodes is convergent.
+  if (!llvm::any_of(SCCNodes, [](Function *F) { return F->isConvergent(); }))
+    return false;
 
-  // We can remove the convergent attr from functions in the SCC if they all
-  // can be made non-convergent (because they call only non-convergent
-  // functions, other than each other).
-  if (!llvm::all_of(SCCNodes, CanRemoveConvergent))
+  // Can't remove convergent from function declarations.
+  if (llvm::any_of(SCCNodes, [](Function *F) { return F->isDeclaration(); }))
     return false;
 
-  // If we got here, all of the SCC's callees are non-convergent. Therefore all
-  // of the SCC's functions can be marked as non-convergent.
+  // Can't remove convergent if any of our functions has a convergent call to a
+  // function not in the SCC.
+  for (Function *F : SCCNodes)
+    for (Instruction &I : instructions(*F)) {
+      CallSite CS(&I);
+      // Bail if is CS a convergent call to a function not in the SCC.
+      if (CS && CS.isConvergent() &&
+          SCCNodes.count(CS.getCalledFunction()) == 0)
+        return false;
+    }
+
+  // If we got here, all of the calls the SCC makes to functions not in the SCC
+  // are non-convergent. Therefore all of the SCC's functions can also be made
+  // non-convergent.  We'll remove the attr from the callsites in
+  // InstCombineCalls.
   for (Function *F : SCCNodes) {
     if (F->isConvergent())
-      DEBUG(dbgs() << "Removing convergent attr from " << F->getName() << "\n");
+      DEBUG(dbgs() << "Removing convergent attr from fn " << F->getName()
+                   << "\n");
     F->setNotConvergent();
   }
   return true;