WebAssembly: add start/end objects for metadata

The WebAssembly linker, unlike ELF linkers, doesn't define
__start and __stop symbols for sections. So we have to make our own.

If you put an object first in the command line, its symbols are placed first;
same for last in command line. So we make two files.

This is the same method that Swift used to use on ELF platforms.

run ./buildstartend.sh; these two objects must be linked at the start
and the end. see the updated linkPlease.sh file.

Author: zhuowei

buildstartend.sh

@@ -0,0 +1,4 @@
+#!/bin/bash
+set -e
+/home/zhuowei/wasi-sdk/bin/clang++ -c swift_start.cpp
+/home/zhuowei/wasi-sdk/bin/clang++ -c swift_end.cpp

linkPlease.sh

@@ -2,6 +2,8 @@
 # Copy to ../build/Ninja-RelWithDebInfoAssert/swift-linux-x86_64/bin
 exec /home/zhuowei/wasi-sdk/bin/wasm-ld --error-limit=0 -o hello.wasm \
 	/home/zhuowei/wasi-sdk/share/sysroot/lib/wasm32-wasi/crt1.o \
+	/home/zhuowei/swift-source/swift/swift_start.o \
+	../lib/swift_static/wasm/wasm32/swiftrt.o \
 	hello.o \
 	-L../lib/swift_static/wasm/wasm32 \
 	-L../lib/swift/wasm/wasm32 \
@@ -10,4 +12,6 @@ exec /home/zhuowei/wasi-sdk/bin/wasm-ld --error-limit=0 -o hello.wasm \
 	-lswiftCore \
 	-lc -lc++ -lc++abi -lswiftImageInspectionShared \
 	-licuuc -licudata \
-	/home/zhuowei/wasi-sdk/lib/clang/8.0.0/lib/wasi/libclang_rt.builtins-wasm32.a /home/zhuowei/Documents/FakePthread/*.o --verbose
+	/home/zhuowei/wasi-sdk/lib/clang/8.0.0/lib/wasi/libclang_rt.builtins-wasm32.a /home/zhuowei/Documents/FakePthread/*.o \
+	/home/zhuowei/swift-source/swift/swift_end.o \
+	--verbose --no-gc-sections

stdlib/public/runtime/SwiftRT-ELF.cpp

@@ -43,9 +43,16 @@ static swift::MetadataSections sections{};
 
 __attribute__((__constructor__))
 static void swift_image_constructor() {
+#ifndef __wasm__
 #define SWIFT_SECTION_RANGE(name)                                              \
   { reinterpret_cast<uintptr_t>(&__start_##name),                              \
     static_cast<uintptr_t>(&__stop_##name - &__start_##name) }
+#else
+// WebAssembly hack: ok this should really go in its own file
+#define SWIFT_SECTION_RANGE(name)                                              \
+  { reinterpret_cast<uintptr_t>(&__start_##name) + sizeof(void*),                              \
+    static_cast<uintptr_t>(&__stop_##name - &__start_##name) }
+#endif
 
   sections = {
       swift::CurrentSectionMetadataVersion,

swift_end.cpp

@@ -0,0 +1,29 @@
+//===--- SwiftRT-ELF.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 <cstddef>
+
+// We synthesize the start/stop symbols ourselves.
+#define DECLARE_SWIFT_SECTION(name)                                                          \
+  __attribute__((__section__(#name),__visibility__("hidden"),__aligned__(1))) const void* __stop_##name = (void*)0xfacefeed; \
+
+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)
+}

swift_start.cpp

@@ -0,0 +1,29 @@
+//===--- SwiftRT-ELF.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 <cstddef>
+
+// We synthesize the start/stop symbols ourselves.
+#define DECLARE_SWIFT_SECTION(name)                                                          \
+  __attribute__((__section__(#name),__visibility__("hidden"),__aligned__(1))) const void* __start_##name = (void*)0xdeadbeef; \
+
+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)
+}