[WebAssembly] Add DataCount section to object files

Summary:
This ensures that object files will continue to validate as
WebAssembly modules in the presence of bulk memory operations. Engines
that don't support bulk memory operations will not recognize the
DataCount section and will report validation errors, but that's ok
because object files aren't supposed to be run directly anyway.

Reviewers: aheejin, dschuff, sbc100

Subscribers: jgravelle-google, hiraditya, sunfish, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60623

llvm-svn: 358315

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
index 63677095918..2994772c519 100644
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -326,6 +326,7 @@ private:
   void writeFunctionSection(ArrayRef<WasmFunction> Functions);
   void writeExportSection(ArrayRef<wasm::WasmExport> Exports);
   void writeElemSection(ArrayRef<uint32_t> TableElems);
+  void writeDataCountSection();
   void writeCodeSection(const MCAssembler &Asm, const MCAsmLayout &Layout,
                         ArrayRef<WasmFunction> Functions);
   void writeDataSection();
@@ -849,6 +850,16 @@ void WasmObjectWriter::writeElemSection(ArrayRef<uint32_t> TableElems) {
   endSection(Section);
 }
 
+void WasmObjectWriter::writeDataCountSection() {
+  if (DataSegments.empty())
+    return;
+
+  SectionBookkeeping Section;
+  startSection(Section, wasm::WASM_SEC_DATACOUNT);
+  encodeULEB128(DataSegments.size(), W.OS);
+  endSection(Section);
+}
+
 void WasmObjectWriter::writeCodeSection(const MCAssembler &Asm,
                                         const MCAsmLayout &Layout,
                                         ArrayRef<WasmFunction> Functions) {
@@ -1600,6 +1611,7 @@ uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
   writeEventSection(Events);
   writeExportSection(Exports);
   writeElemSection(TableElems);
+  writeDataCountSection();
   writeCodeSection(Asm, Layout, Functions);
   writeDataSection();
   for (auto &CustomSection : CustomSections)