[sil-combine] Fix a couple of oversights in the propagation of concrete types of existentials

- Do not try to make a lookup type of a witness_method more concrete if it not an opened existential.
- Replace witness_method instruction by a new one with a more concrete type only in the specific apply instruction instead of doing it everywhere. This allows for more optimization opportunities if the same witness_method is used by multiple apply instructions.

Author: swiftix

lib/SILOptimizer/SILCombiner/SILCombinerApplyVisitors.cpp

@@ -871,12 +871,17 @@ SILCombiner::propagateConcreteTypeOfInitExistential(FullApplySite AI,
       return nullptr;
   }
 
-  // Obtain the protocol whose which should be used by the conformance.
+  // The lookup type is not a opened existential type, 
+  // thus it cannot be made more concrete.
+  if (!WMI->getLookupType()->isOpenedExistential())
+    return nullptr;
+
+  // Obtain the protocol which should be used by the conformance.
   auto *PD = WMI->getLookupProtocol();
 
   // Propagate the concrete type into a callee-operand, which is a
   // witness_method instruction.
-  auto PropagateIntoOperand = [this, &WMI](CanType ConcreteType,
+  auto PropagateIntoOperand = [this, &WMI, &AI](CanType ConcreteType,
                                            ProtocolConformanceRef Conformance) {
     // Keep around the dependence on the open instruction unless we've
     // actually eliminated the use.
@@ -885,8 +890,15 @@ SILCombiner::propagateConcreteTypeOfInitExistential(FullApplySite AI,
                                                 Conformance, WMI->getMember(),
                                                 WMI->getType(),
                                                 WMI->isVolatile());
-    replaceInstUsesWith(*WMI, NewWMI);
-    eraseInstFromFunction(*WMI);
+    // Replace only uses of the witness_method in the apply that is going to
+    // be changed.
+    MutableArrayRef<Operand> Operands = AI.getInstruction()->getAllOperands();
+    for (auto &Op : Operands) {
+      if (Op.get() == WMI)
+         Op.set(NewWMI); 
+    }
+    if (WMI->use_empty())
+      eraseInstFromFunction(*WMI);
   };
 
   // Try to perform the propagation.

test/SILOptimizer/sil_combine.sil

@@ -3038,6 +3038,31 @@ bb0(%0 : $BBB):
   return %11 : $()
 }
 
+// Check that both applies can be devirtualized by means of propagating the concrete
+// type of the existential into witness_method and apply instructions.
+// CHECK-LABEL: sil @silcombine_devirt_both_applies_of_witness_method
+// CHECK-NOT: open_existential_ref
+// CHECK-NOT: witness_method
+// CHECK: [[FR1:%.*]] = function_ref @_TTWC4nix23XXXS_3PPPS_FS1_3foofT_T_
+// CHECK: apply [[FR1]](%0)
+// CHECK: [[FR2:%.*]] = function_ref @_TTWC4nix23XXXS_3PPPS_FS1_3foofT_T_
+// CHECK: apply [[FR2]](%0)
+// CHECK: return
+sil @silcombine_devirt_both_applies_of_witness_method : $@convention(thin) (@owned XXX) -> () {
+bb0(%0 : $XXX):
+  strong_retain %0 : $XXX
+  %3 = init_existential_ref %0 : $XXX : $XXX, $PPP
+  %5 = open_existential_ref %3 : $PPP to $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP
+  %6 = witness_method $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP, #PPP.foo!1, %5 : $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
+  %7 = apply %6<@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP>(%5) : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
+  %8 = apply %6<@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP>(%5) : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
+  strong_release %3 : $PPP
+  %9 = tuple ()
+  strong_release %0 : $XXX
+  %11 = tuple ()
+  return %11 : $()
+}
+
 sil @_TTWC4nix23XXXS_3PPPS_FS1_3foofT_T_ : $@convention(witness_method) (@guaranteed XXX) -> ()
 
 sil_witness_table XXX: PPP module nix2 {