Quick support for calling extern c functions with Builtin vector types. (#82225)

This gives us a means to use llvm's intrinsics that implement more niche
SIMD instructions from the standard library, where we cannot use the C
intrinsics headers from clang (because they're in the cpp module).

Author: stephentyrone

lib/AST/ClangTypeConverter.cpp

@@ -813,6 +813,15 @@ ClangTypeConverter::visitBuiltinFloatType(BuiltinFloatType *type) {
   llvm_unreachable("cannot translate floating-point format to C");
 }
 
+clang::QualType
+ClangTypeConverter::visitBuiltinVectorType(BuiltinVectorType *type) {
+  auto &clangCtx = ClangASTContext;
+  auto eltTy = visit(type->getElementType());
+  return clangCtx.getVectorType(
+    eltTy, type->getNumElements(), clang::VectorKind::Generic
+  );
+}
+
 clang::QualType ClangTypeConverter::visitArchetypeType(ArchetypeType *type) {
   // We see these in the case where we invoke an @objc function
   // through a protocol.

lib/AST/ClangTypeConverter.h

@@ -157,6 +157,7 @@ class ClangTypeConverter :
   clang::QualType visitBuiltinRawPointerType(BuiltinRawPointerType *type);
   clang::QualType visitBuiltinIntegerType(BuiltinIntegerType *type);
   clang::QualType visitBuiltinFloatType(BuiltinFloatType *type);
+  clang::QualType visitBuiltinVectorType(BuiltinVectorType *type);
   clang::QualType visitArchetypeType(ArchetypeType *type);
   clang::QualType visitDependentMemberType(DependentMemberType *type);
   template <bool templateArgument = false>

test/Prototypes/builtin-vector-via-extern-c.swift

@@ -0,0 +1,29 @@
+//===----------------------------------------------------------*- swift -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+// REQUIRES: swift_feature_Extern
+// REQUIRES: swift_feature_BuiltinModule
+// RUN: %target-swift-frontend -primary-file %s -enable-experimental-feature BuiltinModule -enable-experimental-feature Extern -emit-ir | %FileCheck %s --check-prefix=CHECK
+
+import Builtin
+
+@_extern(c, "llvm.uadd.sat.v16i8") @usableFromInline
+func _uaddSat(_ a: Builtin.Vec16xInt8, _ b: Builtin.Vec16xInt8) -> Builtin.Vec16xInt8
+
+@_transparent
+public func saturatingAdd(_ a: SIMD16<UInt8>, _ b: SIMD16<UInt8>) -> SIMD16<UInt8> {
+  // Hack around init from Builtin type being stdlib-internal using unsafeBitCast.
+  unsafeBitCast(_uaddSat(a._storage._value, b._storage._value), to: SIMD16<UInt8>.self)
+}
+// CHECK: saturatingAdd{{.*}} {
+// CHECK: entry:
+// CHECK: call <16 x i8> @llvm.uadd.sat.v16i8
+