[PATCH] D74531: [WebAssembly] Emit PCH __clang_ast in custom section

[Yuta Saito via Phabricator via llvm-commits]

kateinoigakukun created this revision.
kateinoigakukun added a project: LLVM.
Herald added subscribers: llvm-commits, sunfish, aheejin, hiraditya, jgravelle-google, sbc100, dschuff.
kateinoigakukun added reviewers: dschuff, uweigand.

Resolve https://bugs.llvm.org/show_bug.cgi?id=35928 to promote Swift for WebAssembly

`__clang_ast` section should be aligned by 4 bytes, so I added padding after the size of custom section name.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D74531

Files:
  llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
  llvm/lib/MC/WasmObjectWriter.cpp


Index: llvm/lib/MC/WasmObjectWriter.cpp
===================================================================
--- llvm/lib/MC/WasmObjectWriter.cpp
+++ llvm/lib/MC/WasmObjectWriter.cpp
@@ -373,7 +373,16 @@
   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();
@@ -1099,6 +1108,10 @@
   if (Sym.isSection())
     return false;
 
+  // Clang's precompiled headers are in a separate custom section
+  if (Sym.getName() == "__clang_ast")
+    return false;
+
   return true;
 }
 
Index: llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
===================================================================
--- llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -1696,6 +1696,10 @@
   if (K.isText())
     return SectionKind::getText();
 
+  // Clang precompiled header data isn't needed at runtime; use custom section
+  if (Name == "__clangast")
+    return SectionKind::getMetadata();
+
   // Otherwise, ignore whatever section type the generic impl detected and use
   // a plain data section.
   return SectionKind::getData();


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