[WebAssembly] Implement @llvm.global_ctors and @llvm.global_dtors

Summary:
- lowers @llvm.global_dtors by adding @llvm.global_ctors
  functions which register the destructors with `__cxa_atexit`.
- impements @llvm.global_ctors with wasm start functions and linker metadata

See [here](https://github.com/WebAssembly/tool-conventions/issues/25) for more background.

Subscribers: jfb, dschuff, mgorny, jgravelle-google, aheejin, sunfish

Differential Revision: https://reviews.llvm.org/D41211

llvm-svn: 320774

1 files changed, 14 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 925e67b..24d4baa 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -1359,6 +1359,18 @@ const MCExpr *TargetLoweringObjectFileWasm::lowerRelativeReference(
 void TargetLoweringObjectFileWasm::InitializeWasm() {
   StaticCtorSection =
       getContext().getWasmSection(".init_array", SectionKind::getData());
-  StaticDtorSection =
-      getContext().getWasmSection(".fini_array", SectionKind::getData());
+}
+
+MCSection *TargetLoweringObjectFileWasm::getStaticCtorSection(
+    unsigned Priority, const MCSymbol *KeySym) const {
+  return Priority == UINT16_MAX ?
+         StaticCtorSection :
+         getContext().getWasmSection(".init_array." + utostr(Priority),
+                                     SectionKind::getData());
+}
+
+MCSection *TargetLoweringObjectFileWasm::getStaticDtorSection(
+    unsigned Priority, const MCSymbol *KeySym) const {
+  llvm_unreachable("@llvm.global_dtors should have been lowered already");
+  return nullptr;
 }