swift-frontend DebugInfo: support .dwo split debug info

include/swift/AST/IRGenOptions.h

@@ -293,6 +293,13 @@ class IRGenOptions {
   /// The DWARF version of debug info.
   uint8_t DWARFVersion = 4;
 
+  /// Name for the split DWARF file (embedded as DW_AT_dwo_name in skeleton CU).
+  /// Derived as basename of SplitDwarfOutput.
+  std::string SplitDwarfFile;
+
+  /// Output path for the split DWARF file (.dwo).
+  std::string SplitDwarfOutput;
+
   /// The command line string that is to be stored in the debug info.
   std::string DebugFlags;
 

include/swift/Option/Options.td

@@ -1299,6 +1299,10 @@ def dwarf_version : Joined<["-"], "dwarf-version=">,
   HelpText<"DWARF debug info version to produce if requested">,
   MetaVarName<"<version>">;
 
+def split_dwarf_output : Separate<["-"], "split-dwarf-output">,
+  Flags<[FrontendOption]>, MetaVarName<"<path>">,
+  HelpText<"Emit split DWARF debug info to a .dwo file at the given path">;
+
 def prefix_serialized_debugging_options : Flag<["-"], "prefix-serialized-debugging-options">,
   Flags<[FrontendOption]>,
   HelpText<"Apply debug prefix mappings to serialized debug info in Swiftmodule files">;

lib/Frontend/CompilerInvocation.cpp

@@ -3646,6 +3646,12 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
                      A->getAsString(Args), A->getValue());
   }
 
+  if (auto A = Args.getLastArg(OPT_split_dwarf_output)) {
+    Opts.SplitDwarfOutput = A->getValue();
+    Opts.SplitDwarfFile =
+        llvm::sys::path::filename(Opts.SplitDwarfOutput).str();
+  }
+
   if (auto *A =
           Args.getLastArg(OPT_debug_module_path, OPT_debug_module_self_key)) {
     if (A->getOption().matches(OPT_debug_module_self_key)) {

lib/IRGen/IRGen.cpp

@@ -174,6 +174,7 @@ swift::getIRTargetOptions(const IRGenOptions &Opts, ASTContext &Ctx,
   TargetOpts.EmulatedTLS = Clang->getCodeGenOpts().EmulatedTLS;
 
   TargetOpts.MCOptions.AsmVerbose = Opts.VerboseAsm;
+  TargetOpts.MCOptions.SplitDwarfFile = Opts.SplitDwarfFile;
 
   // WebAssembly doesn't support atomics yet, see
   // https://github.com/apple/swift/issues/54533 for more details.
@@ -908,15 +909,35 @@ bool swift::compileAndWriteLLVM(
     EmitPasses.add(createTargetTransformInfoWrapperPass(
         targetMachine->getTargetIRAnalysis()));
 
+    // Open a separate output file for split DWARF (.dwo) if requested.
+    std::unique_ptr<llvm::ToolOutputFile> DwoFile;
+    if (opts.OutputKind == IRGenOutputKind::ObjectFile &&
+        !opts.SplitDwarfOutput.empty()) {
+      std::error_code EC;
+      DwoFile = std::make_unique<llvm::ToolOutputFile>(
+          opts.SplitDwarfOutput, EC, llvm::sys::fs::OF_None);
+      if (EC) {
+        diagnoseSync(diags, diagMutex, SourceLoc(), diag::error_opening_output,
+                     opts.SplitDwarfOutput, EC.message());
+        return true;
+      }
+    }
+
     bool fail = targetMachine->addPassesToEmitFile(
-        EmitPasses, out, nullptr, FileType, !opts.Verify, nullptr, casid);
+        EmitPasses, out, DwoFile ? &DwoFile->os() : nullptr, FileType,
+        !opts.Verify, nullptr, casid);
     if (fail) {
       diagnoseSync(diags, diagMutex, SourceLoc(),
                    diag::error_codegen_init_fail);
       return true;
     }
 
     EmitPasses.run(*module);
+
+    // Keep the DWO file on success (ToolOutputFile deletes on destruction
+    // unless keep() is called).
+    if (DwoFile)
+      DwoFile->keep();
     break;
   }
   }

lib/IRGen/IRGenDebugInfo.cpp

@@ -2973,8 +2973,7 @@ IRGenDebugInfoImpl::IRGenDebugInfoImpl(const IRGenOptions &Opts,
   std::tie(Major, Minor) = version::getSwiftNumericVersion();
   unsigned MajorRuntimeVersion = Major;
 
-  // No split DWARF on Darwin.
-  StringRef SplitName = StringRef();
+  StringRef SplitName = StringRef(Opts.SplitDwarfFile);
   // Note that File + Dir need not result in a valid path.
   // The directory part of the main file is the current working directory.
   std::string RemappedFile = DebugPrefixMap.remapPath(SourcePath);

test/DebugInfo/split-dwarf-ir.swift

@@ -0,0 +1,6 @@
+// RUN: %target-swift-frontend -primary-file %s -emit-ir -g \
+// RUN:   -split-dwarf-output "test.dwo" -o - | %FileCheck %s
+
+// CHECK: !DICompileUnit({{.*}}splitDebugFilename: "test.dwo"
+
+func hello() -> Int { return 42 }

test/DebugInfo/split-dwarf-wasm-multi.swift

@@ -0,0 +1,59 @@
+// RUN: %empty-directory(%t)
+// RUN: %{python} %utils/split_file.py -o %t %s
+
+// Compile two files with split DWARF via the driver.
+// RUN: %empty-directory(%t/out)
+// RUN: cd %t/out && %target-swiftc_driver -c %t/main.swift %t/other.swift \
+// RUN:   -g -enable-split-dwarf -parse-stdlib -module-name TestMod
+
+// Verify each object has only a skeleton CU.
+// RUN: %llvm-dwarfdump -v --debug-info %t/out/main.o | %FileCheck --check-prefix=MAIN-OBJ %s
+// RUN: %llvm-dwarfdump -v --debug-info %t/out/other.o | %FileCheck --check-prefix=OTHER-OBJ %s
+
+// Verify each .dwo has the full debug info.
+// RUN: %llvm-dwarfdump -v --debug-info %t/out/main.dwo | %FileCheck --check-prefix=MAIN-DWO %s
+// RUN: %llvm-dwarfdump -v --debug-info %t/out/other.dwo | %FileCheck --check-prefix=OTHER-DWO %s
+
+// Merge into .dwp and verify both CUs are present.
+// RUN: llvm-dwp %t/out/main.dwo %t/out/other.dwo -o %t/out/TestMod.dwp
+// RUN: %llvm-dwarfdump -v --debug-info %t/out/TestMod.dwp | %FileCheck --check-prefix=MERGED %s
+
+// REQUIRES: CPU=wasm32
+
+// main.o: skeleton only, no subprogram
+// MAIN-OBJ: DW_TAG_compile_unit
+// MAIN-OBJ:   DW_AT_GNU_dwo_name{{.*}}"main.dwo"
+// MAIN-OBJ:   DW_AT_GNU_dwo_id
+// MAIN-OBJ-NOT: DW_TAG_subprogram
+
+// other.o: skeleton only, no subprogram
+// OTHER-OBJ: DW_TAG_compile_unit
+// OTHER-OBJ:   DW_AT_GNU_dwo_name{{.*}}"other.dwo"
+// OTHER-OBJ:   DW_AT_GNU_dwo_id
+// OTHER-OBJ-NOT: DW_TAG_subprogram
+
+// main.dwo: full debug info with foo()
+// MAIN-DWO: DW_TAG_compile_unit
+// MAIN-DWO: DW_TAG_subprogram
+// MAIN-DWO:   DW_AT_name{{.*}}"foo"
+
+// other.dwo: full debug info with bar()
+// OTHER-DWO: DW_TAG_compile_unit
+// OTHER-DWO: DW_TAG_subprogram
+// OTHER-DWO:   DW_AT_name{{.*}}"bar"
+
+// Merged .dwp: both CUs present (order matches input order to llvm-dwp)
+// MERGED: DW_TAG_compile_unit
+// MERGED:   DW_AT_name{{.*}}main.swift
+// MERGED: DW_TAG_subprogram
+// MERGED:   DW_AT_name{{.*}}"foo"
+// MERGED: DW_TAG_compile_unit
+// MERGED:   DW_AT_name{{.*}}other.swift
+// MERGED: DW_TAG_subprogram
+// MERGED:   DW_AT_name{{.*}}"bar"
+
+// BEGIN main.swift
+public func foo() {}
+
+// BEGIN other.swift
+public func bar() {}

test/DebugInfo/split-dwarf-wasm.swift

@@ -0,0 +1,23 @@
+// RUN: %empty-directory(%t)
+// RUN: cd %t && %target-swiftc_driver -c %s \
+// RUN:   -g -enable-split-dwarf -parse-stdlib -module-name test
+// RUN: %llvm-dwarfdump -v --debug-info %t/split-dwarf-wasm.o | %FileCheck --check-prefix=OBJ %s
+// RUN: %llvm-dwarfdump -v %t/split-dwarf-wasm.dwo | %FileCheck --check-prefix=DWO %s
+// RUN: llvm-objdump -h %t/split-dwarf-wasm.dwo | %FileCheck --check-prefix=DWO-SECTIONS %s
+
+// REQUIRES: CPU=wasm32
+
+/// The object should contain only a skeleton CU (no full debug info).
+// OBJ: DW_TAG_compile_unit
+// OBJ:   DW_AT_GNU_dwo_name{{.*}}"split-dwarf-wasm.dwo"
+// OBJ:   DW_AT_GNU_dwo_id
+// OBJ-NOT: DW_TAG_subprogram
+
+/// The DWO should contain the full debug info including subprograms.
+// DWO: DW_TAG_compile_unit
+// DWO: DW_TAG_subprogram
+
+// DWO-SECTIONS: .debug_info.dwo
+// DWO-SECTIONS: .debug_abbrev.dwo
+
+public func hello() {}