[rc-id] Make RCIdentity strip off single-pred arguments.

This was already done in a few different places in the compiler. There is no
reason not to have it in RCIdentity directly.

rdar://24156136

Author: gottesmm

lib/SILOptimizer/Analysis/RCIdentityAnalysis.cpp

@@ -244,6 +244,18 @@ findDominatingNonPayloadedEdge(SILBasicBlock *IncomingEdgeBB,
   return false;
 }
 
+static SILValue allIncomingValuesEqual(
+    llvm::SmallVectorImpl<std::pair<SILBasicBlock *,
+                                    SILValue >> &IncomingValues) {
+  SILValue First = stripRCIdentityPreservingInsts(IncomingValues[0].second);
+  if (std::all_of(std::next(IncomingValues.begin()), IncomingValues.end(),
+                     [&First](std::pair<SILBasicBlock *, SILValue> P) -> bool {
+                       return stripRCIdentityPreservingInsts(P.second) == First;
+                     }))
+    return First;
+  return SILValue();
+}
+
 /// Return the underlying SILValue after stripping off SILArguments that cannot
 /// affect RC identity.
 ///
@@ -301,11 +313,16 @@ SILValue RCIdentityFunctionInfo::stripRCIdentityPreservingArgs(SILValue V,
 
   unsigned IVListSize = IncomingValues.size();
 
-  // If we only have one incoming value, just return the identity root of that
-  // incoming value. There can be no loop problems.
-  if (IVListSize == 1) {
-    return IncomingValues[0].second;
-  }
+  assert(IVListSize != 1 && "Should have been handled in "
+         "stripRCIdentityPreservingInsts");
+
+  // Ok, we have multiple predecessors. See if all of them are the same
+  // value. If so, just return that value.
+  //
+  // This returns a SILValue to save a little bit of compile time since we
+  // already compute that value here.
+  if (SILValue V = allIncomingValuesEqual(IncomingValues))
+    return V;
 
   // Ok, we have multiple predecessors. First find the first non-payloaded enum.
   llvm::SmallVector<SILBasicBlock *, 8> NoPayloadEnumBBs;

test/SILOptimizer/rcidentity.sil

@@ -140,3 +140,34 @@ bb4(%6 : $Builtin.NativeObject):
 bb5:
   return undef : $()
 }
+
+// All of the SSA values in the given function besides %6 can be resolved to
+// (%0, %1). Because %6 has multiple SSA values and there is nothing tricky we
+// can do here, just bail.
+//
+// CHECK-LABEL: @test_multiple_pred_same_rcid@
+// CHECK: RESULT #0: 0 = 0
+// CHECK: RESULT #1: 1 = 0
+// CHECK: RESULT #2: 2 = 0
+// CHECK: RESULT #3: 3 = 0
+// CHECK: RESULT #4: 4 = 0
+// CHECK: RESULT #5: 5 = 0
+sil @test_multiple_pred_same_rcid : $@convention(thin) (Builtin.NativeObject) -> () {
+bb0(%0 : $Builtin.NativeObject):
+  br bb1(%0 : $Builtin.NativeObject, %0 : $Builtin.NativeObject)
+
+bb1(%2 : $Builtin.NativeObject, %3 : $Builtin.NativeObject):
+  cond_br undef, bb2(%3 : $Builtin.NativeObject), bb3(%2 : $Builtin.NativeObject)
+
+bb2(%4 : $Builtin.NativeObject):
+  br bb4(%4 : $Builtin.NativeObject)
+
+bb3(%5 : $Builtin.NativeObject):
+  br bb4(%5 : $Builtin.NativeObject)
+
+bb4(%6 : $Builtin.NativeObject):
+  br bb5
+
+bb5:
+  return undef : $()
+}