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

[Yuta Saito via Phabricator via llvm-commits]

kateinoigakukun updated this revision to Diff 249827.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D74531/new/

https://reviews.llvm.org/D74531

Files:
  llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
  llvm/lib/MC/WasmObjectWriter.cpp
  llvm/test/MC/WebAssembly/clangast.ll


Index: llvm/test/MC/WebAssembly/clangast.ll
===================================================================
--- /dev/null
+++ llvm/test/MC/WebAssembly/clangast.ll
@@ -0,0 +1,38 @@
+; RUN: llc -filetype=obj %s -o - | llvm-readobj -S | FileCheck %s
+
+target triple = "wasm32-unknown-unknown"
+
+ at dummy = hidden global [6 x i8] c"hello\00", align 1
+ at __clang_ast = internal constant [4 x i8] c"CPCH", section "__clangast", align 4
+!0 = !{ !"anchor", !"\00" }
+!wasm.custom_sections = !{ !0 }
+
+; The size of __clang_ast should be 27.
+; ┌───────────────────────────────┬───────────────────────────────┬─────────┐
+; │         content type          │       contet bytes (hex)      │   size  │
+; ├───────────────────────────────┼───────────────────────────────┼─────────┤
+; │ section id                    │ 00                            │  1 byte │
+; │ size of content (LEB128)      │ 95 80 80 80 00                │  5 byte │
+; │ size of section name (LEB128) │ 8a                            │  1 byte │
+; │ padding to align              │ 80 80 80 80 80 00             │  6 byte │
+; │ section name                  │ 5f 5f 63 6c 61 6e 67 61 73 74 │ 10 byte │
+; │ content of __clang_ast        │ 43 50 43 48                   │  4 byte │
+; ├───────────────────────────────┼───────────────────────────────┼─────────┤
+; │             sum               │                -              │ 27 byte │
+; └─────────────────────────────────────────────────────────────────────────┘
+
+; The content of __clang_ast should be aligned by 4,
+; so the size of section name is padded to round up.
+
+; CHECK:  Section {
+; CHECK:    Type: CUSTOM (0x0)
+; CHECK:    Size: 4
+; CHECK:    Offset: [[#OFFSET:]]
+; CHECK:    Name: __clangast
+; CHECK:  }
+; CHECK:  Section {
+; CHECK:    Type: CUSTOM (0x0)
+; CHECK:    Size: 1
+; CHECK:    Offset: [[#OFFSET + 27]]
+; CHECK:    Name: anchor
+; CHECK:  }
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
@@ -1712,6 +1712,12 @@
 
   StringRef Name = GO->getSection();
 
+  // Clang precompiled header data isn't needed at runtime; use custom section
+  // TODO: This special case may be generalized as wasm binary format
+  // https://github.com/WebAssembly/tool-conventions/issues/138
+  if (Name == "__clangast")
+    Kind = SectionKind::getMetadata();
+
   StringRef Group = "";
   if (const Comdat *C = getWasmComdat(GO)) {
     Group = C->getName();


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