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

clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp

@@ -270,25 +270,42 @@ class PCHContainerGenerator : public ASTConsumer {
     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.

clang/test/PCH/pch-wasm.c

@@ -0,0 +1,7 @@
+// REQUIRES: webassembly-registered-target
+// RUN: %clang_cc1 -triple wasm32-unknown-unknown-wasm -emit-pch -fmodule-format=obj %S/pchpch1.h -o - | llvm-objdump --section-headers - | FileCheck %s
+
+// Ensure that clangast section should be emitted in a section for wasm object file
+
+// CHECK: file format wasm
+// CHECK: __clangast   {{[0-9a-f]+}} {{[0-9a-f]+}}

llvm/lib/MC/WasmObjectWriter.cpp

@@ -292,6 +292,8 @@ class WasmObjectWriter : public MCObjectWriter {
     W->OS << Str;
   }
 
+  void writeStringWithAlignment(const StringRef Str, unsigned Alignment);
+
   void writeI32(int32_t val) {
     char Buffer[4];
     support::endian::write32le(Buffer, val);
@@ -362,6 +364,28 @@ void WasmObjectWriter::startSection(SectionBookkeeping &Section,
   Section.Index = SectionCount++;
 }
 
+// Write a string with extra paddings for trailing alignment
+// TODO: support alignment at asm and llvm level?
+void WasmObjectWriter::writeStringWithAlignment(const StringRef Str,
+                                                unsigned Alignment) {
+
+  // Calculate the encoded size of str length and add pads based on it and
+  // alignment.
+  raw_null_ostream NullOS;
+  uint64_t StrSizeLength = encodeULEB128(Str.size(), NullOS);
+  uint64_t Offset = W->OS.tell() + StrSizeLength + Str.size();
+  uint64_t Paddings = offsetToAlignment(Offset, Align(Alignment));
+  Offset += Paddings;
+
+  // LEB128 greater than 5 bytes is invalid
+  assert((StrSizeLength + Paddings) <= 5 && "too long string to align");
+
+  encodeSLEB128(Str.size(), W->OS, StrSizeLength + Paddings);
+  W->OS << Str;
+
+  assert(W->OS.tell() == Offset && "invalid padding");
+}
+
 void WasmObjectWriter::startCustomSection(SectionBookkeeping &Section,
                                           StringRef Name) {
   LLVM_DEBUG(dbgs() << "startCustomSection " << Name << "\n");
@@ -371,7 +395,12 @@ void WasmObjectWriter::startCustomSection(SectionBookkeeping &Section,
   Section.PayloadOffset = W->OS.tell();
 
   // Custom sections in wasm also have a string identifier.
-  writeString(Name);
+  if (Name != "__clangast") {
+    writeString(Name);
+  } else {
+    // The on-disk hashtable in clangast needs to be aligned by 4 bytes.
+    writeStringWithAlignment(Name, 4);
+  }
 
   // The position where the custom section starts.
   Section.ContentsOffset = W->OS.tell();

llvm/test/MC/WebAssembly/custom-section-alignment.ll

@@ -0,0 +1,10 @@
+; RUN: llc -filetype=obj %s -o - | od -t x1 -v | FileCheck %s
+
+target triple = "wasm32-unknown-unknown"
+
+!0 = !{ !"before", !"\de\ad\be\ef" }
+!1 = !{ !"__clangast", !"\fe\ed\fa\ce" }
+!wasm.custom_sections = !{ !0, !1 }
+
+; Ensure that __clangast content is aligned by 4 bytes
+; CHECK: {{(([0-9a-f]{2} ){4})*}}fe ed fa ce