WebAssembly: pad __clangast section to 4 bytes

Like Patcheng's https://reviews.llvm.org/D42233 but for Custom Sections instead

padding for the padding gods.

Author: zhuowei

lib/MC/WasmObjectWriter.cpp

@@ -359,7 +359,15 @@ 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 {
+    // pad section start to nearest 4 bytes for Clang PCH
+    uint64_t MinLength = Section.PayloadOffset + 5ULL /* min ULEB128 length */ + Name.size();
+    uint64_t RoundedUpLength = (MinLength + 3ULL) & ~3ULL;
+    encodeULEB128(Name.size(), W.OS, 5 + (RoundedUpLength - MinLength));
+    W.OS << Name;
+  }
 
   // The position where the custom section starts.
   Section.ContentsOffset = W.OS.tell();
@@ -1052,6 +1060,14 @@ void WasmObjectWriter::writeCustomSections(const MCAssembler &Asm,
     auto *Sec = CustomSection.Section;
     startCustomSection(Section, CustomSection.Name);
 
+    if (CustomSection.Name == "__clangast") {
+      // pad to nearest 4 bytes
+      uint64_t RoundedUp = (Section.ContentsOffset + 3ULL) & ~3ULL;
+      for (uint64_t Count = 0; Count < RoundedUp - Section.ContentsOffset; Count++) {
+        W.OS << char(0);
+      }
+    }
+
     Sec->setSectionOffset(W.OS.tell() - Section.ContentsOffset);
     Asm.writeSectionData(W.OS, Sec, Layout);
 
@@ -1342,6 +1358,8 @@ uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
       LLVM_DEBUG(dbgs() << "  -> function index: " << Index << "\n");
 
     } else if (WS.isData()) {
+      if (WS.getName() == "__clang_ast")
+        continue;
       if (WS.isTemporary() && !WS.getSize())
         continue;