SILOptimizer: Don't devirtualize witness method calls to tuple conformances that might vanish

Author: slavapestov

lib/AST/Type.cpp

@@ -4765,8 +4765,9 @@ case TypeKind::Id:
     if (!anyChanged)
       return *this;
 
-    // If the transform would yield a singleton tuple, and we didn't
-    // start with one, flatten to produce the element type.
+    // Handle vanishing tuples -- If the transform would yield a singleton
+    // tuple, and we didn't start with one, flatten to produce the
+    // element type.
     if (elements.size() == 1 &&
         !elements[0].getType()->is<PackExpansionType>() &&
         !(tuple->getNumElements() == 1 &&

lib/SILOptimizer/Utils/Devirtualize.cpp

@@ -1146,6 +1146,15 @@ static bool canDevirtualizeWitnessMethod(ApplySite applySite, bool isMandatory)
 
   auto *wmi = cast<WitnessMethodInst>(applySite.getCallee());
 
+  // Handle vanishing tuples: don't devirtualize a call to a tuple conformance
+  // if the lookup type can possibly be unwrapped after substitution.
+  if (auto tupleType = dyn_cast<TupleType>(wmi->getLookupType())) {
+    if (tupleType->containsPackExpansionType() &&
+        tupleType->getNumScalarElements() <= 1) {
+      return false;
+    }
+  }
+
   std::tie(f, wt) = applySite.getModule().lookUpFunctionInWitnessTable(
       wmi->getConformance(), wmi->getMember(), SILModule::LinkingMode::LinkAll);
 

test/SILOptimizer/tuple-conformances.swift

@@ -22,9 +22,20 @@ public func transparentCallee<T: P>(t: T.Type) {
   t.protocolMethod()
 }
 
+// Make sure we don't devirtualize the call when we inline transparentCallee()
+// into transparentCaller(), because we might unwrap the tuple conformance
+// later.
+
+// CHECK-LABEL: sil [transparent] @$s4main17transparentCaller5tupleyxxQp_tm_tRvzAA1PRzlF : $@convention(thin) <each T where repeat each T : P> (@thin (repeat each T).Type) -> () {
 @_transparent public func transparentCaller<each T: P>(tuple: (repeat each T).Type) {
   callee(t: tuple)
+
+// CHECK: [[FN:%.*]] = witness_method $(repeat each T), #P.protocolMethod : <Self where Self : P> (Self.Type) -> () -> () : $@convention(witness_method: P) <τ_0_0 where τ_0_0 : P> (@thick τ_0_0.Type) -> ()
+// CHECK: apply [[FN:%.*]]<(repeat each T)>({{.*}})
+
   transparentCallee(t: tuple)
+
+// FIXME: This one is wrong in the AST
   tuple.protocolMethod()
 }
 
@@ -39,10 +50,13 @@ public func caller() {
 // CHECK: [[FN:%.*]] = function_ref @$s4main6callee1tyxm_tAA1PRzlF : $@convention(thin) <τ_0_0 where τ_0_0 : P> (@thick τ_0_0.Type) -> ()
 // CHECK: apply [[FN]]<Int>({{.*}}) : $@convention(thin) <τ_0_0 where τ_0_0 : P> (@thick τ_0_0.Type) -> ()
 
-// FIXME: These should call Int.protocolMethod()!
+// Make sure the witness method call in transparentCallee() was devirtualized to Int.protocolMethod().
 
-// CHECK: [[FN:%.*]] = function_ref @$sBT4mainRvzAA1PRzlE14protocolMethodyyFZ : $@convention(method) <each τ_0_0 where repeat each τ_0_0 : P> (@thin (repeat each τ_0_0).Type) -> ()
-// CHECK: apply [[FN]]<Pack{Int}>(%4) : $@convention(method) <each τ_0_0 where repeat each τ_0_0 : P> (@thin (repeat each τ_0_0).Type) -> ()
+// CHECK: [[FN:%.*]] = function_ref @$sSi4mainE14protocolMethodyyFZ : $@convention(method) (@thin Int.Type) -> () // user: %6
+// CHECK: apply [[FN]]({{.*}}) : $@convention(method) (@thin Int.Type) -> ()
+
+// FIXME: This is the `tuple.protocolMethod()` in transparentCaller(). It should
+// also refer to Int.protocolMethod()!
 
 // CHECK: [[FN:%.*]] = function_ref @$sBT4mainRvzAA1PRzlE14protocolMethodyyFZ : $@convention(method) <each τ_0_0 where repeat each τ_0_0 : P> (@thin (repeat each τ_0_0).Type) -> ()
-// CHECK: apply [[FN]]<Pack{Int}>(%0) : $@convention(method) <each τ_0_0 where repeat each τ_0_0 : P> (@thin (repeat each τ_0_0).Type) -> ()
+// CHECK: apply [[FN]]<Pack{Int}>({{.*}}) : $@convention(method) <each τ_0_0 where repeat each τ_0_0 : P> (@thin (repeat each τ_0_0).Type) -> ()