Support dwarf fission for wasm object files

Initial support for dwarf fission sections (-gsplit-dwarf) on wasm.
The most interesting change is support for writing 2 files (.o and .dwo) in the
wasm object writer. My approach moves object-writing logic into its own function
and calls it twice, swapping out the endian::Writer (W) in between calls.
It also splits the import-preparation step into its own function (and skips it when writing a dwo).

Differential Revision: https://reviews.llvm.org/D85685

Author: dschuff

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4805,7 +4805,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   // Add the split debug info name to the command lines here so we
   // can propagate it to the backend.
   bool SplitDWARF = (DwarfFission != DwarfFissionKind::None) &&
-                    TC.getTriple().isOSBinFormatELF() &&
+                    (TC.getTriple().isOSBinFormatELF() ||
+                     TC.getTriple().isOSBinFormatWasm()) &&
                     (isa<AssembleJobAction>(JA) || isa<CompileJobAction>(JA) ||
                      isa<BackendJobAction>(JA));
   if (SplitDWARF) {

clang/test/Driver/split-debug.c

@@ -10,6 +10,11 @@
 // RUN: %clang -target x86_64-unknown-linux-gnu -gsplit-dwarf=split -c -### %s 2> %t
 // RUN: FileCheck -check-prefix=CHECK-ACTIONS < %t %s
 
+// RUN: %clang -target wasm32-unknown-unknown -gsplit-dwarf -c -### %s 2> %t
+// RUN: FileCheck -check-prefix=CHECK-ACTIONS < %t %s
+// RUN: %clang -target wasm32-unknown-unknown -gsplit-dwarf=split -c -### %s 2> %t
+// RUN: FileCheck -check-prefix=CHECK-ACTIONS < %t %s
+
 // RUN: %clang -target x86_64-unknown-linux-gnu -gsplit-dwarf=single -c -### %s 2> %t
 // RUN: FileCheck -check-prefix=CHECK-ACTIONS-SINGLE-SPLIT < %t %s
 //

llvm/include/llvm/MC/MCWasmObjectWriter.h

@@ -52,6 +52,10 @@ std::unique_ptr<MCObjectWriter>
 createWasmObjectWriter(std::unique_ptr<MCWasmObjectTargetWriter> MOTW,
                        raw_pwrite_stream &OS);
 
+std::unique_ptr<MCObjectWriter>
+createWasmDwoObjectWriter(std::unique_ptr<MCWasmObjectTargetWriter> MOTW,
+                          raw_pwrite_stream &OS, raw_pwrite_stream &DwoOS);
+
 } // namespace llvm
 
 #endif

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

@@ -422,7 +422,10 @@ DIE &DwarfCompileUnit::updateSubprogramScopeDIE(const DISubprogram *SP) {
       // FIXME: duplicated from Target/WebAssembly/WebAssembly.h
       // don't want to depend on target specific headers in this code?
       const unsigned TI_GLOBAL_RELOC = 3;
-      if (FrameBase.Location.WasmLoc.Kind == TI_GLOBAL_RELOC) {
+      // FIXME: when writing dwo, we need to avoid relocations. Probably
+      // the "right" solution is to treat globals the way func and data symbols
+      // are (with entries in .debug_addr).
+      if (FrameBase.Location.WasmLoc.Kind == TI_GLOBAL_RELOC && !isDwoUnit()) {
         // These need to be relocatable.
         assert(FrameBase.Location.WasmLoc.Index == 0);  // Only SP so far.
         auto SPSym = cast<MCSymbolWasm>(

llvm/lib/MC/MCAsmBackend.cpp

@@ -54,10 +54,17 @@ std::unique_ptr<MCObjectWriter>
 MCAsmBackend::createDwoObjectWriter(raw_pwrite_stream &OS,
                                     raw_pwrite_stream &DwoOS) const {
   auto TW = createObjectTargetWriter();
-  if (TW->getFormat() != Triple::ELF)
-    report_fatal_error("dwo only supported with ELF");
-  return createELFDwoObjectWriter(cast<MCELFObjectTargetWriter>(std::move(TW)),
-                                  OS, DwoOS, Endian == support::little);
+  switch (TW->getFormat()) {
+  case Triple::ELF:
+    return createELFDwoObjectWriter(
+        cast<MCELFObjectTargetWriter>(std::move(TW)), OS, DwoOS,
+        Endian == support::little);
+  case Triple::Wasm:
+    return createWasmDwoObjectWriter(
+        cast<MCWasmObjectTargetWriter>(std::move(TW)), OS, DwoOS);
+  default:
+    report_fatal_error("dwo only supported with ELF and Wasm");
+  }
 }
 
 Optional<MCFixupKind> MCAsmBackend::getFixupKind(StringRef Name) const {

llvm/lib/MC/MCObjectFileInfo.cpp

@@ -796,6 +796,10 @@ void MCObjectFileInfo::initWasmMCObjectFileInfo(const Triple &T) {
   DwarfFrameSection = Ctx->getWasmSection(".debug_frame", SectionKind::getMetadata());
   DwarfPubNamesSection = Ctx->getWasmSection(".debug_pubnames", SectionKind::getMetadata());
   DwarfPubTypesSection = Ctx->getWasmSection(".debug_pubtypes", SectionKind::getMetadata());
+  DwarfGnuPubNamesSection =
+      Ctx->getWasmSection(".debug_gnu_pubnames", SectionKind::getMetadata());
+  DwarfGnuPubTypesSection =
+      Ctx->getWasmSection(".debug_gnu_pubtypes", SectionKind::getMetadata());
 
   DwarfDebugNamesSection =
       Ctx->getWasmSection(".debug_names", SectionKind::getMetadata());
@@ -808,6 +812,37 @@ void MCObjectFileInfo::initWasmMCObjectFileInfo(const Triple &T) {
   DwarfLoclistsSection =
       Ctx->getWasmSection(".debug_loclists", SectionKind::getMetadata());
 
+  // Fission Sections
+  DwarfInfoDWOSection =
+      Ctx->getWasmSection(".debug_info.dwo", SectionKind::getMetadata());
+  DwarfTypesDWOSection =
+      Ctx->getWasmSection(".debug_types.dwo", SectionKind::getMetadata());
+  DwarfAbbrevDWOSection =
+      Ctx->getWasmSection(".debug_abbrev.dwo", SectionKind::getMetadata());
+  DwarfStrDWOSection =
+      Ctx->getWasmSection(".debug_str.dwo", SectionKind::getMetadata());
+  DwarfLineDWOSection =
+      Ctx->getWasmSection(".debug_line.dwo", SectionKind::getMetadata());
+  DwarfLocDWOSection =
+      Ctx->getWasmSection(".debug_loc.dwo", SectionKind::getMetadata());
+  DwarfStrOffDWOSection =
+      Ctx->getWasmSection(".debug_str_offsets.dwo", SectionKind::getMetadata());
+  DwarfRnglistsDWOSection =
+      Ctx->getWasmSection(".debug_rnglists.dwo", SectionKind::getMetadata());
+  DwarfMacinfoDWOSection =
+      Ctx->getWasmSection(".debug_macinfo.dwo", SectionKind::getMetadata());
+  DwarfMacroDWOSection =
+      Ctx->getWasmSection(".debug_macro.dwo", SectionKind::getMetadata());
+
+  DwarfLoclistsDWOSection =
+      Ctx->getWasmSection(".debug_loclists.dwo", SectionKind::getMetadata());
+
+  // DWP Sections
+  DwarfCUIndexSection =
+      Ctx->getWasmSection(".debug_cu_index", SectionKind::getMetadata(), 0);
+  DwarfTUIndexSection =
+      Ctx->getWasmSection(".debug_tu_index", SectionKind::getMetadata(), 0);
+
   // Wasm use data section for LSDA.
   // TODO Consider putting each function's exception table in a separate
   // section, as in -function-sections, to facilitate lld's --gc-section.