SILGen: use builtin prepareInitialization instead of zeroInitializer when generating SIL for Builtin.emplace

`zeroInitializer` is blocking optimizations, but it is not needed because the closure of `Builtin.emplace` will initialize the memory anyway.
We only need to tell mandatory passes that this memory should be treated as initialized.

Author: eeckstein

lib/SILGen/SILGenBuiltin.cpp

@@ -2165,11 +2165,13 @@ static ManagedValue emitBuiltinEmplace(SILGenFunction &SGF,
 
   auto buffer = dest->getAddressForInPlaceInitialization(SGF, loc);
 
-  // Zero-initialize the buffer.
-  // Aside from providing a modicum of predictability if the memory isn't
-  // actually initialized, this also serves to communicate to DI that the memory
+  // Mark the buffer as initializedto communicate to DI that the memory
   // is considered initialized from this point.
-  SGF.B.createZeroInitAddr(loc, buffer);
+  auto markInit = getBuiltinValueDecl(Ctx, Ctx.getIdentifier("prepareInitialization"));
+  SGF.B.createBuiltin(loc, markInit->getBaseIdentifier(),
+                       SILType::getEmptyTupleType(Ctx),
+                       SubstitutionMap(),
+                       buffer);
 
   SILValue bufferPtr = SGF.B.createAddressToPointer(loc, buffer,
         SILType::getPrimitiveObjectType(SGF.getASTContext().TheRawPointerType),

test/SILGen/emplace.swift

@@ -18,7 +18,7 @@ struct Loadable {
 // CHECK-LABEL: sil {{.*}} @$s{{.*}}_in_place_loadable
 func emplace_in_place_loadable() -> Loadable {
     // CHECK: [[TMP:%.*]] = alloc_stack $Loadable
-    // CHECK-NEXT: builtin "zeroInitializer"([[TMP]])
+    // CHECK-NEXT: builtin "prepareInitialization"([[TMP]])
     // CHECK-NEXT: [[PTR:%.*]] = address_to_pointer {{.*}} [[TMP]]
     // CHECK-NEXT: apply {{.*}}([[PTR]])
     // CHECK-NEXT: load [take] [[TMP]]
@@ -29,7 +29,7 @@ func emplace_in_place_loadable() -> Loadable {
 // CHECK-LABEL: sil {{.*}} @$s{{.*}}_in_place_ao
 // CHECK: bb0([[OUT:%.*]] : $*AO):
 func emplace_in_place_ao() -> AO {
-    // CHECK: builtin "zeroInitializer"([[OUT]])
+    // CHECK: builtin "prepareInitialization"([[OUT]])
     // CHECK-NEXT: [[PTR:%.*]] = address_to_pointer {{.*}} [[OUT]]
     // CHECK-NEXT: apply [[FN:%.*]]([[PTR]])
     // CHECK-NEXT: destroy_value [[FN]]
@@ -42,7 +42,7 @@ func emplace_in_place_ao() -> AO {
 // CHECK: bb0([[INOUT:%.*]] : $*Loadable):
 func emplace_assign_loadable(_ x: inout Loadable) {
     // CHECK: [[TMP:%.*]] = alloc_stack $Loadable
-    // CHECK-NEXT: builtin "zeroInitializer"([[TMP]])
+    // CHECK-NEXT: builtin "prepareInitialization"([[TMP]])
     // CHECK-NEXT: [[PTR:%.*]] = address_to_pointer {{.*}} [[TMP]]
     // CHECK-NEXT: apply {{.*}}([[PTR]])
     // CHECK-NEXT: [[RESULT:%.*]] = load [take] [[TMP]]
@@ -57,7 +57,7 @@ func emplace_assign_loadable(_ x: inout Loadable) {
 // CHECK: bb0([[INOUT:%.*]] : $*AO):
 func emplace_assign_ao(_ x: inout AO) {
     // CHECK: [[TMP:%.*]] = alloc_stack $AO
-    // CHECK-NEXT: builtin "zeroInitializer"([[TMP]])
+    // CHECK-NEXT: builtin "prepareInitialization"([[TMP]])
     // CHECK-NEXT: [[PTR:%.*]] = address_to_pointer {{.*}} [[TMP]]
     // CHECK-NEXT: apply {{.*}}([[PTR]])
     // CHECK-NEXT: [[WRITE:%.*]] = begin_access [modify] [unknown] [[INOUT]]
@@ -70,7 +70,7 @@ func emplace_assign_ao(_ x: inout AO) {
 // CHECK-LABEL: sil {{.*}} @$s{{.*}}_ignore_loadable
 func emplace_ignore_loadable() {
     // CHECK: [[TMP:%.*]] = alloc_stack $Loadable
-    // CHECK-NEXT: builtin "zeroInitializer"([[TMP]])
+    // CHECK-NEXT: builtin "prepareInitialization"([[TMP]])
     // CHECK-NEXT: [[PTR:%.*]] = address_to_pointer {{.*}} [[TMP]]
     // CHECK-NEXT: apply {{.*}}([[PTR]])
     // CHECK-NEXT: [[RESULT:%.*]] = load [take] [[TMP]]
@@ -83,7 +83,7 @@ func emplace_ignore_loadable() {
 // CHECK-LABEL: sil {{.*}} @$s{{.*}}_ignore_ao
 func emplace_ignore_ao() {
     // CHECK: [[TMP:%.*]] = alloc_stack $AO
-    // CHECK-NEXT: builtin "zeroInitializer"([[TMP]])
+    // CHECK-NEXT: builtin "prepareInitialization"([[TMP]])
     // CHECK-NEXT: [[PTR:%.*]] = address_to_pointer {{.*}} [[TMP]]
     // CHECK-NEXT: apply {{.*}}([[PTR]])
     // CHECK-NEXT: ignored_use [[TMP]]