[wasm] Add metadata registration for WebAssembly

This patch adds the metadata registration for the wasm targets, and also
adds build support for it.

stdlib/public/runtime/SwiftRT-ELF-WASM.cpp

@@ -0,0 +1,98 @@
+//===--- SwiftRT-ELF-WASM.cpp ---------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "ImageInspectionCommon.h"
+#include "swift/shims/MetadataSections.h"
+
+#include <cstddef>
+#include <new>
+
+#if defined(__ELF__)
+extern "C" const char __dso_handle[];
+#elif defined(__wasm__)
+// NOTE: Multi images in a single process is not yet
+// stabilized in WebAssembly toolchain outside of Emscripten.
+static constexpr const void *__dso_handle = nullptr;
+#endif
+
+// Create empty sections to ensure that the start/stop symbols are synthesized
+// by the linker.  Otherwise, we may end up with undefined symbol references as
+// the linker table section was never constructed.
+#if defined(__ELF__)
+# define DECLARE_EMPTY_METADATA_SECTION(name) __asm__("\t.section " #name ",\"a\"\n");
+#elif defined(__wasm__)
+# define DECLARE_EMPTY_METADATA_SECTION(name) __asm__("\t.section " #name ",\"\",@\n");
+#endif
+
+#define DECLARE_SWIFT_SECTION(name)                                                          \
+  DECLARE_EMPTY_METADATA_SECTION(name)                                                                \
+  __attribute__((__visibility__("hidden"),__aligned__(1))) extern const char __start_##name; \
+  __attribute__((__visibility__("hidden"),__aligned__(1))) extern const char __stop_##name;
+
+extern "C" {
+DECLARE_SWIFT_SECTION(swift5_protocols)
+DECLARE_SWIFT_SECTION(swift5_protocol_conformances)
+DECLARE_SWIFT_SECTION(swift5_type_metadata)
+
+DECLARE_SWIFT_SECTION(swift5_typeref)
+DECLARE_SWIFT_SECTION(swift5_reflstr)
+DECLARE_SWIFT_SECTION(swift5_fieldmd)
+DECLARE_SWIFT_SECTION(swift5_assocty)
+DECLARE_SWIFT_SECTION(swift5_replace)
+DECLARE_SWIFT_SECTION(swift5_replac2)
+DECLARE_SWIFT_SECTION(swift5_builtin)
+DECLARE_SWIFT_SECTION(swift5_capture)
+DECLARE_SWIFT_SECTION(swift5_mpenum)
+DECLARE_SWIFT_SECTION(swift5_accessible_functions)
+DECLARE_SWIFT_SECTION(swift5_runtime_attributes)
+}
+
+#undef DECLARE_SWIFT_SECTION
+
+namespace {
+static swift::MetadataSections sections{};
+}
+
+__attribute__((__constructor__))
+static void swift_image_constructor() {
+#define SWIFT_SECTION_RANGE(name)                                              \
+  { reinterpret_cast<uintptr_t>(&__start_##name),                              \
+    static_cast<uintptr_t>(&__stop_##name - &__start_##name) }
+
+  ::new (&sections) swift::MetadataSections {
+      swift::CurrentSectionMetadataVersion,
+      { __dso_handle },
+
+      nullptr,
+      nullptr,
+
+      SWIFT_SECTION_RANGE(swift5_protocols),
+      SWIFT_SECTION_RANGE(swift5_protocol_conformances),
+      SWIFT_SECTION_RANGE(swift5_type_metadata),
+
+      SWIFT_SECTION_RANGE(swift5_typeref),
+      SWIFT_SECTION_RANGE(swift5_reflstr),
+      SWIFT_SECTION_RANGE(swift5_fieldmd),
+      SWIFT_SECTION_RANGE(swift5_assocty),
+      SWIFT_SECTION_RANGE(swift5_replace),
+      SWIFT_SECTION_RANGE(swift5_replac2),
+      SWIFT_SECTION_RANGE(swift5_builtin),
+      SWIFT_SECTION_RANGE(swift5_capture),
+      SWIFT_SECTION_RANGE(swift5_mpenum),
+      SWIFT_SECTION_RANGE(swift5_accessible_functions),
+      SWIFT_SECTION_RANGE(swift5_runtime_attributes),
+  };
+
+#undef SWIFT_SECTION_RANGE
+
+  swift_addNewDSOImage(&sections);
+}