Add convergent-removing bits to FunctionAttrs pass.

Summary:
Remove the convergent attribute on any functions which provably do not
contain or invoke any convergent functions.

After this change, we'll be able to modify clang to conservatively add
'convergent' to all functions when compiling CUDA.

Reviewers:  jingyue, joker.eph

Subscribers: llvm-commits, tra, jhen, hfinkel, resistor, chandlerc, arsenm

Differential Revision: http://reviews.llvm.org/D17013

llvm-svn: 260319

1 files changed, 63 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
index 60cc874..2445b7f 100644
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -935,6 +935,68 @@ 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.
+static bool removeConvergentAttrs(const CallGraphSCC &SCC,
+                                  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 = [&] (CallGraphNode *CGN) {
+    Function *F = CGN->getFunction();
+    if (!F) return false;
+
+    if (!F->isConvergent()) return true;
+
+    // Can't remove convergent from declarations.
+    if (F->isDeclaration()) return false;
+
+    // Don't remove convergent from optnone functions.
+    if (F->hasFnAttribute(Attribute::OptimizeNone))
+      return false;
+
+    // Can't remove convergent if any of F's callees -- ignoring functions in the
+    // SCC itself -- are convergent.
+    if (llvm::any_of(*CGN, [&](const CallGraphNode::CallRecord &CR) {
+          Function *F = CR.second->getFunction();
+          return SCCNodes.count(F) == 0 && (!F || F->isConvergent());
+        }))
+      return false;
+
+    // CGN doesn't contain calls to intrinsics, so iterate over all of F's
+    // callsites, looking for any calls to convergent intrinsics.  If we find one,
+    // F must remain marked as convergent.
+    auto IsConvergentIntrinsicCall = [](Instruction &I) {
+      CallSite CS(cast<Value>(&I));
+      if (!CS)
+        return false;
+      Function *Callee = CS.getCalledFunction();
+      return Callee && Callee->isIntrinsic() && Callee->isConvergent();
+    };
+    return !llvm::any_of(*F, [=](BasicBlock &BB) {
+      return llvm::any_of(BB, IsConvergentIntrinsicCall);
+    });
+  };
+
+  // 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(SCC, CanRemoveConvergent)) return false;
+
+  // If we got here, all of the SCC's callees are non-convergent, and none of
+  // the optnone functions in the SCC are marked as convergent.  Therefore all
+  // of the SCC's functions can be marked as non-convergent.
+  for (CallGraphNode *CGN : SCC)
+    if (Function *F = CGN->getFunction()) {
+      if (F->isConvergent())
+        DEBUG(dbgs() << "Removing convergent attr from " << F->getName()
+                     << "\n");
+      F->setNotConvergent();
+    }
+  return true;
+}
+
 static bool setDoesNotRecurse(Function &F) {
   if (F.doesNotRecurse())
     return false;
@@ -1011,6 +1073,7 @@ bool PostOrderFunctionAttrs::runOnSCC(CallGraphSCC &SCC) {
   if (!ExternalNode) {
     Changed |= addNoAliasAttrs(SCCNodes);
     Changed |= addNonNullAttrs(SCCNodes, *TLI);
+    Changed |= removeConvergentAttrs(SCC, SCCNodes);
   }
 
   Changed |= addNoRecurseAttrs(SCC);