Simplify metatype instructions.

Tuple, Struct, and Enum MetatypesTypes have a single value. In this
case, replace metatype instructions with arguments of the same
MetatypeType to enable downstream CSE and SILCombines.

Author: atrick

lib/SILOptimizer/Analysis/SimplifyInstruction.cpp

@@ -65,6 +65,7 @@ namespace {
     SILValue visitThinFunctionToPointerInst(ThinFunctionToPointerInst *TFTPI);
     SILValue visitPointerToThinFunctionInst(PointerToThinFunctionInst *PTTFI);
     SILValue visitBeginAccessInst(BeginAccessInst *BAI);
+    SILValue visitMetatypeInst(MetatypeInst *MTI);
 
     SILValue simplifyOverflowBuiltin(BuiltinInst *BI);
   };
@@ -453,6 +454,24 @@ SILValue InstSimplifier::visitBeginAccessInst(BeginAccessInst *BAI) {
   return SILValue();
 }
 
+SILValue InstSimplifier::visitMetatypeInst(MetatypeInst *MI) {
+  auto metaType = MI->getType().castTo<MetatypeType>();
+  auto instanceType = metaType.getInstanceType();
+  // Tuple, Struct, and Enum MetatypeTypes have a single value.
+  // If this metatype is already passed as an argument reuse it to enable
+  // downstream CSE/SILCombine optimizations.
+  // Note: redundant metatype instructions are already handled by CSE.
+  if (isa<TupleType>(instanceType)
+      || instanceType.getStructOrBoundGenericStruct()
+      || instanceType.getEnumOrBoundGenericEnum()) {
+    for (SILArgument *argument : MI->getFunction()->getArguments()) {
+      if (argument->getType().getASTType() == metaType)
+        return argument;
+    }
+  }
+  return SILValue();
+}
+
 static SILValue simplifyBuiltin(BuiltinInst *BI) {
 
   switch (BI->getBuiltinInfo().ID) {

test/SILOptimizer/sil_simplify_instrs.sil

@@ -352,3 +352,22 @@ bb0(%0 : $A):
 // CHECK-NEXT: %{{.*}} = tuple ()
 // CHECK-NEXT: return %{{.*}} : $()
 // CHECK-LABEL: } // end sil function 'simplify_accesssil'
+
+// Test replacement of metatype instructions with metatype arguments.
+protocol SomeP {}
+
+public enum SpecialEnum : SomeP {}
+
+// CHECK-LABEL: sil @testSimplifyMetatype : $@convention(thin) (@thick SpecialEnum.Type) -> Bool {
+// CHECK:   init_existential_metatype %0 : $@thick SpecialEnum.Type, $@thick Any.Type
+// CHECK:   init_existential_metatype %0 : $@thick SpecialEnum.Type, $@thick Any.Type
+// CHECK-LABEL: }  // end sil function 'testSimplifyMetatype'
+sil @testSimplifyMetatype : $@convention(thin) (@thick SpecialEnum.Type) -> Bool {
+bb0(%0 : $@thick SpecialEnum.Type):
+  %1 = init_existential_metatype %0 : $@thick SpecialEnum.Type, $@thick Any.Type
+  %2 = metatype $@thick SpecialEnum.Type
+  %3 = init_existential_metatype %2 : $@thick SpecialEnum.Type, $@thick Any.Type
+  %4 = builtin "is_same_metatype"(%1 : $@thick Any.Type, %3 : $@thick Any.Type) : $Builtin.Int1
+  %5 = struct $Bool (%4 : $Builtin.Int1)
+  return %5 : $Bool
+}