[WebAssembly] Emit clangast in custom section aligned by 4 bytes

Emit __clangast in custom section instead of named data segment
to find it while iterating sections.
This could be avoided if all data segements (the wasm sense) were
represented as their own sections (in the llvm sense).
This can be resolved by https://github.com/WebAssembly/tool-conventions/issues/138

And the on-disk hashtable in clangast needs to be aligned by 4 bytes,
so add paddings in name length field in custom section header.

The length of clangast section name can be represented in 1 byte
by leb128, and possible maximum pads are 3 bytes, so the section
name length won't be invalid in theory.

Fixes https://bugs.llvm.org/show_bug.cgi?id=35928

Differential Revision: https://reviews.llvm.org/D74531

1 files changed, 36 insertions, 19 deletions

diff --git a/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp b/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
index ce45291..f7b83c4 100644
--- a/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
+++ b/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
@@ -270,25 +270,42 @@ public:
     assert(Buffer->IsComplete && "serialization did not complete");
     auto &SerializedAST = Buffer->Data;
     auto Size = SerializedAST.size();
-    auto Int8Ty = llvm::Type::getInt8Ty(*VMContext);
-    auto *Ty = llvm::ArrayType::get(Int8Ty, Size);
-    auto *Data = llvm::ConstantDataArray::getString(
-        *VMContext, StringRef(SerializedAST.data(), Size),
-        /*AddNull=*/false);
-    auto *ASTSym = new llvm::GlobalVariable(
-        *M, Ty, /*constant*/ true, llvm::GlobalVariable::InternalLinkage, Data,
-        "__clang_ast");
-    // The on-disk hashtable needs to be aligned.
-    ASTSym->setAlignment(llvm::Align(8));
-
-    // Mach-O also needs a segment name.
-    if (Triple.isOSBinFormatMachO())
-      ASTSym->setSection("__CLANG,__clangast");
-    // COFF has an eight character length limit.
-    else if (Triple.isOSBinFormatCOFF())
-      ASTSym->setSection("clangast");
-    else
-      ASTSym->setSection("__clangast");
+
+    if (Triple.isOSBinFormatWasm()) {
+      // Emit __clangast in custom section instead of named data segment
+      // to find it while iterating sections.
+      // This could be avoided if all data segements (the wasm sense) were
+      // represented as their own sections (in the llvm sense).
+      // TODO: https://github.com/WebAssembly/tool-conventions/issues/138
+      llvm::NamedMDNode *MD =
+          M->getOrInsertNamedMetadata("wasm.custom_sections");
+      llvm::Metadata *Ops[2] = {
+          llvm::MDString::get(*VMContext, "__clangast"),
+          llvm::MDString::get(*VMContext,
+                              StringRef(SerializedAST.data(), Size))};
+      auto *NameAndContent = llvm::MDTuple::get(*VMContext, Ops);
+      MD->addOperand(NameAndContent);
+    } else {
+      auto Int8Ty = llvm::Type::getInt8Ty(*VMContext);
+      auto *Ty = llvm::ArrayType::get(Int8Ty, Size);
+      auto *Data = llvm::ConstantDataArray::getString(
+          *VMContext, StringRef(SerializedAST.data(), Size),
+          /*AddNull=*/false);
+      auto *ASTSym = new llvm::GlobalVariable(
+          *M, Ty, /*constant*/ true, llvm::GlobalVariable::InternalLinkage,
+          Data, "__clang_ast");
+      // The on-disk hashtable needs to be aligned.
+      ASTSym->setAlignment(llvm::Align(8));
+
+      // Mach-O also needs a segment name.
+      if (Triple.isOSBinFormatMachO())
+        ASTSym->setSection("__CLANG,__clangast");
+      // COFF has an eight character length limit.
+      else if (Triple.isOSBinFormatCOFF())
+        ASTSym->setSection("clangast");
+      else
+        ASTSym->setSection("__clangast");
+    }
 
     LLVM_DEBUG({
       // Print the IR for the PCH container to the debug output.