SILGen: Emit a closure literal in a function conversion as the converted type.

Closure literals are sometimes type-checked as one type then immediately converted to another
type in the AST. One particular case of this is when a closure body never throws, but the closure
is used as an argument to a function that takes a parameter that `throws`. Emitting this naively,
by emitting the closure as its original type, then converting to throws, can be expensive for
async closures, since that takes a reabstraction thunk. Even for non-async functions, we still want
to get the benefit of reabstraction optimization for the closure literal through the conversion too.
So if the function conversion just add `throws`, emit the closure as throwing, and pass down the
context abstraction pattern when emitting the closure as well.

lib/SILGen/SILGenExpr.cpp

@@ -1714,6 +1714,26 @@ static ManagedValue convertFunctionRepresentation(SILGenFunction &SGF,
   llvm_unreachable("bad representation");
 }
 
+// Ideally our prolog/epilog emission would be able to handle all possible
+// reabstractions and conversions. Until then, this returns true if a closure
+// literal of type `literalType` can be directly emitted by SILGen as
+// `convertedType`.
+static bool canPeepholeLiteralClosureConversion(Type literalType,
+                                                Type convertedType) {
+  auto literalFnType = literalType->getAs<FunctionType>();
+  auto convertedFnType = convertedType->getAs<FunctionType>();
+  
+  if (!literalFnType || !convertedFnType)
+    return false;
+  
+  // Does the conversion only add `throws`?
+  if (literalFnType->isEqual(convertedFnType->getWithoutThrowing())) {
+    return true;
+  }
+  
+  return false;
+}
+
 RValue RValueEmitter::visitFunctionConversionExpr(FunctionConversionExpr *e,
                                                   SGFContext C)
 {
@@ -1748,6 +1768,38 @@ RValue RValueEmitter::visitFunctionConversionExpr(FunctionConversionExpr *e,
     return RValue(SGF, e, result);
   }
   
+  // If the function being converted is a closure literal, then the only use
+  // of the closure should be as the destination type of the conversion. Rather
+  // than emit the closure as is and convert it, see if we can emit the closure
+  // directly as the desired type.
+  //
+  // TODO: Move this up when we can emit closures directly with C calling
+  // convention.
+  auto subExpr = e->getSubExpr()->getSemanticsProvidingExpr();
+  if (isa<AbstractClosureExpr>(subExpr)
+      && canPeepholeLiteralClosureConversion(subExpr->getType(),
+                                             e->getType())) {
+    // If we're emitting into a context with a preferred abstraction pattern
+    // already, carry that along.
+    auto origType = C.getAbstractionPattern();
+    // If not, use the conversion type as the desired abstraction pattern.
+    if (!origType) {
+      origType = AbstractionPattern(e->getType()->getCanonicalType());
+    }
+    
+    auto substType = subExpr->getType()->getCanonicalType();
+    
+    auto conversion = Conversion::getSubstToOrig(*origType, substType,
+                                      SGF.getLoweredType(*origType, substType));
+    ConvertingInitialization convertingInit(conversion, SGFContext());
+    auto closure = SGF.emitRValue(subExpr,
+                                  SGFContext(&convertingInit))
+      .getAsSingleValue(SGF, e);
+    closure = SGF.emitSubstToOrigValue(e, closure, *origType, substType);
+
+    return RValue(SGF, e, closure);
+  }
+  
   // Handle a reference to a "thin" native Swift function that only changes
   // representation and refers to an inherently thin function reference.
   if (destRepTy->getRepresentation() == FunctionTypeRepresentation::Thin) {

test/AutoDiff/SILOptimizer/activity_analysis.swift

@@ -552,13 +552,12 @@ func testTryApply(_ x: Float) -> Float {
 // CHECK: bb0:
 // CHECK: [ACTIVE] %0 = argument of bb0 : $Float
 // CHECK: [NONE]   // function_ref closure #1 in testTryApply(_:)
-// CHECK: [NONE]   %3 = thin_to_thick_function %2 : $@convention(thin) () -> () to $@noescape @callee_guaranteed () -> ()
-// CHECK: [NONE]   %4 = convert_function %3 : $@noescape @callee_guaranteed () -> () to $@noescape @callee_guaranteed () -> @error Error
+// CHECK: [NONE]   %3 = thin_to_thick_function %2 : $@convention(thin) () -> @error Error to $@noescape @callee_guaranteed () -> @error Error
 // CHECK: [NONE]   // function_ref rethrowing(_:)
 // CHECK: bb1:
-// CHECK: [NONE] %7 = argument of bb1 : $()
+// CHECK: [NONE] %6 = argument of bb1 : $()
 // CHECK: bb2:
-// CHECK: [NONE] %9 = argument of bb2 : $Error
+// CHECK: [NONE] %8 = argument of bb2 : $Error
 
 //===----------------------------------------------------------------------===//
 // Coroutine differentiation (`begin_apply`)

test/DebugInfo/patternvars.swift

@@ -35,7 +35,7 @@ public func mangle(s: [UnicodeScalar]) -> [UnicodeScalar] {
 // Do we care to expose these via lldb?
 
 // CHECK: define {{.*}}@"$s11patternvars6mangle1sSayAA13UnicodeScalarVGAF_tFA2EXEfU_"
-// CHECK: %[[VAL:[0-9]+]] = call swiftcc i32 @"$s11patternvars13UnicodeScalarV5values6UInt32Vvg"(i32 %0)
+// CHECK: %[[VAL:[0-9]+]] = call swiftcc i32 @"$s11patternvars13UnicodeScalarV5values6UInt32Vvg"(
 // CHECK:       {{[0-9]+}}:
 // CHECK-NOT:   call void @llvm.dbg.value
 // CHECK-NOT:   call void asm sideeffect "", "r"

test/IRGen/big_types_corner_cases.swift

@@ -203,7 +203,7 @@ public func testGetFunc() {
 // CHECK-LABEL: define{{( dllexport)?}}{{( protected)?}} hidden swiftcc void @"$s22big_types_corner_cases7TestBigC4testyyF"(%T22big_types_corner_cases7TestBigC* swiftself %0)
 // CHECK: [[CALL1:%.*]] = call {{.*}} @__swift_instantiateConcreteTypeFromMangledName({{.*}} @"$sSayy22big_types_corner_cases9BigStructVcSgGMD"
 // CHECK: [[CALL2:%.*]] = call i8** @"$sSayy22big_types_corner_cases9BigStructVcSgGSayxGSlsWl
-// CHECK:  call swiftcc void @"$sSlsE10firstIndex5where0B0QzSgSb7ElementQzKXE_tKF"(%swift.opaque* noalias nocapture sret({{.*}}) %{{[0-9]+}}, i8* bitcast ({{.*}}* @"$s22big_types_corner_cases9BigStruct{{.*}}_TRTA{{(\.ptrauth)?}}" to i8*), %swift.opaque* %{{[0-9]+}}, %swift.type* %{{[0-9]+}}, i8** [[CALL2]]
+// CHECK:  call swiftcc void @"$sSlsE10firstIndex5where0B0QzSgSb7ElementQzKXE_tKF"(%swift.opaque* noalias nocapture sret({{.*}}) %{{[0-9]+}}, i8* bitcast ({{.*}}* @"$s22big_types_corner_cases7TestBig{{.*}}" to i8*), %swift.opaque* null, %swift.type* %{{[0-9]+}}, i8** [[CALL2]]
 class TestBig {
     typealias Handler = (BigStruct) -> Void
 

test/Profiler/coverage_closure_returns_never.swift

@@ -2,7 +2,8 @@
 
 // CHECK-LABEL: closure #1 (Swift.Never) -> Swift.Never in coverage_closure_returns_never.closure_with_fatal_error(Swift.Array<Swift.Never>) -> ()
 // CHECK: builtin "int_instrprof_increment"
-// CHECK-NEXT: debug_value {{.*}} : $Never
+// CHECK-NEXT: [[LOAD:%.*]] = load {{.*}} : $*Never
+// CHECK-NEXT: debug_value [[LOAD]] : $Never
 // CHECK-NEXT: unreachable
 
 func closure_with_fatal_error(_ arr: [Never]) {

test/SILGen/closure_literal_reabstraction_async.swift

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend -emit-silgen %s -disable-availability-checking | %FileCheck %s
+// REQUIRES: concurrency
+
+@_silgen_name("takeThrowingAsyncClosure")
+func takeThrowingAsyncClosure<T>(_: () async throws -> T) 
+
+// CHECK-LABEL: sil {{.*}} @{{.*}}34passNonthrowingAsyncClosureLiteral
+func passNonthrowingAsyncClosureLiteral() {
+    // Check that the literal closure was emitted directly with an error return,
+    // without a reabstraction thunk to convert from nonthrowing.
+    // CHECK: [[INVOKE_FN:%.*]] = function_ref
+    // CHECK: [[CLOSURE:%.*]] = thin_to_thick_function [[INVOKE_FN]]
+    // CHECK: [[CALLEE:%.*]] = function_ref @takeThrowingAsyncClosure
+    // CHECK: apply [[CALLEE]]<Int>([[CLOSURE]])
+    takeThrowingAsyncClosure { return 42 }
+}

test/SILGen/rethrows.swift

@@ -70,11 +70,10 @@ func test2() {
 }
 
 // CHECK-LABEL: sil hidden @$s8rethrows5test3yyF : $@convention(thin) () -> () {
-// CHECK:       [[CLOSURE:%.*]] = function_ref @$s8rethrows5test3yyFSiyXEfU_ : $@convention(thin) () -> Int
+// CHECK:       [[CLOSURE:%.*]] = function_ref @$s8rethrows5test3yyFSiyXEfU_ :
 // CHECK:       [[T0:%.*]] = thin_to_thick_function [[CLOSURE]]
-// CHECK:       [[T1:%.*]] = convert_function [[T0]] : $@noescape @callee_guaranteed () -> Int to $@noescape @callee_guaranteed () -> (Int, @error Error)
-// CHECK:       [[RETHROWER:%.*]] = function_ref @$s8rethrows9rethroweryS2iyKXEKF : $@convention(thin) (@noescape @callee_guaranteed () -> (Int, @error Error)) -> (Int, @error Error)
-// CHECK:       try_apply [[RETHROWER]]([[T1]]) : $@convention(thin) (@noescape @callee_guaranteed () -> (Int, @error Error)) -> (Int, @error Error), normal [[NORMAL:bb1]], error [[ERROR:bb2]]
+// CHECK:       [[RETHROWER:%.*]] = function_ref @$s8rethrows9rethroweryS2iyKXEKF :
+// CHECK:       try_apply [[RETHROWER]]([[T0]])
 // CHECK:     [[NORMAL]]({{%.*}} : $Int):
 // CHECK-NEXT:  [[RESULT:%.*]] = tuple ()
 // CHECK-NEXT:  return [[RESULT]]

test/SILGen/type_lowering_subst_function_type_conditional_conformance.swift

@@ -64,7 +64,8 @@ struct S4<Base> where Base : P1, Base.Element: P1 {
 // CHECK-LABEL: {{^}}sil {{.*}} @${{.*}}2S4{{.*}}3foo{{.*}}F :
 // CHECK: @callee_guaranteed @substituted <τ_0_0, τ_0_1 where τ_0_0 : P1, τ_0_0 == τ_0_1, τ_0_0.Element : P1> (@in_guaranteed S3<τ_0_0>) -> () for <Base, Base>
   func foo(index: S3<Base>?) {
-    _ = index.map({ _ = $0 })
+    let f: (S3<Base>) -> () = { _ = $0 }
+    _ = index.map(f)
   }
 }