Add an option to disable DWARF skeleton CU breadcrumbs for Clang

module imports. This is useful when building redistributable static
archives, since any pointers into the CLang module cache won't be
portable.

When using this option the Clang type fallback path in LLDB will be
less useful since DWARF type information from those modules will not
be available unless another object file compiled without the option
imported the same modules.

rdar://problem/48827784

Author: adrian-prantl

include/swift/AST/IRGenOptions.h

@@ -104,14 +104,17 @@ class IRGenOptions {
   /// Which sanitizer is turned on.
   OptionSet<SanitizerKind> Sanitizers;
 
+  /// Path prefixes that should be rewritten in debug info.
+  PathRemapper DebugPrefixMap;
+
   /// What level of debug info to generate.
   IRGenDebugInfoLevel DebugInfoLevel : 2;
 
   /// What type of debug info to generate.
   IRGenDebugInfoFormat DebugInfoFormat : 2;
 
-  /// Path prefixes that should be rewritten in debug info.
-  PathRemapper DebugPrefixMap;
+  /// Whether to leave DWARF breadcrumbs pointing to imported Clang modules.
+  unsigned DisableClangModuleSkeletonCUs : 1;
 
   /// Whether we're generating IR for the JIT.
   unsigned UseJIT : 1;
@@ -227,7 +230,9 @@ class IRGenOptions {
         Verify(true), OptMode(OptimizationMode::NotSet),
         Sanitizers(OptionSet<SanitizerKind>()),
         DebugInfoLevel(IRGenDebugInfoLevel::None),
-        DebugInfoFormat(IRGenDebugInfoFormat::None), UseJIT(false),
+        DebugInfoFormat(IRGenDebugInfoFormat::None),
+        DisableClangModuleSkeletonCUs(false),
+        UseJIT(false),
         IntegratedREPL(false), DisableLLVMOptzns(false),
         DisableSwiftSpecificLLVMOptzns(false), DisableLLVMSLPVectorizer(false),
         DisableFPElim(true), Playground(false), EmitStackPromotionChecks(false),

include/swift/Option/FrontendOptions.td

@@ -432,6 +432,10 @@ def disable_playground_transform : Flag<["-"], "disable-playground-transform">,
 def pc_macro : Flag<["-"], "pc-macro">,
   HelpText<"Apply the 'program counter simulation' macro">;
 
+def no_clang_module_breadcrumbs : Flag<["-"], "no-clang-module-breadcrumbs">,
+  HelpText<"Don't emit DWARF skeleton CUs for imported Clang modules. "
+           "Use this when building a redistributable static archive.">;
+
 def use_jit : Flag<["-"], "use-jit">,
   HelpText<"Register Objective-C classes as if the JIT were in use">;
 

lib/Frontend/CompilerInvocation.cpp

@@ -1002,6 +1002,9 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
 
   Opts.ModuleName = FrontendOpts.ModuleName;
 
+  if (Args.hasArg(OPT_no_clang_module_breadcrumbs))
+    Opts.DisableClangModuleSkeletonCUs = true;
+
   if (Args.hasArg(OPT_use_jit))
     Opts.UseJIT = true;
 

lib/IRGen/IRGenDebugInfo.cpp

@@ -640,16 +640,19 @@ class IRGenDebugInfoImpl : public IRGenDebugInfo {
     std::string RemappedIncludePath = DebugPrefixMap.remapPath(IncludePath);
 
     // For Clang modules / PCH, create a Skeleton CU pointing to the PCM/PCH.
-    bool CreateSkeletonCU = !ASTFile.empty();
-    bool IsRootModule = !Parent;
-    if (CreateSkeletonCU && IsRootModule) {
-      llvm::DIBuilder DIB(M);
-      DIB.createCompileUnit(IGM.ObjCInterop ? llvm::dwarf::DW_LANG_ObjC
-                                            : llvm::dwarf::DW_LANG_C99,
-                            DIB.createFile(Name, RemappedIncludePath),
-                            TheCU->getProducer(), true, StringRef(), 0, ASTFile,
-                            llvm::DICompileUnit::FullDebug, Signature);
-      DIB.finalize();
+    if (!Opts.DisableClangModuleSkeletonCUs) {
+      bool CreateSkeletonCU = !ASTFile.empty();
+      bool IsRootModule = !Parent;
+      if (CreateSkeletonCU && IsRootModule) {
+        llvm::DIBuilder DIB(M);
+        DIB.createCompileUnit(IGM.ObjCInterop ? llvm::dwarf::DW_LANG_ObjC
+                                              : llvm::dwarf::DW_LANG_C99,
+                              DIB.createFile(Name, RemappedIncludePath),
+                              TheCU->getProducer(), true, StringRef(), 0,
+                              ASTFile, llvm::DICompileUnit::FullDebug,
+                              Signature);
+        DIB.finalize();
+      }
     }
 
     StringRef Sysroot = IGM.Context.SearchPathOpts.SDKPath;

test/DebugInfo/ClangModuleBreadcrumbs.swift

@@ -1,5 +1,9 @@
 // RUN: %target-swift-frontend -emit-ir %s -g -I %S/Inputs \
 // RUN:   -Xcc -DFOO="foo" -Xcc -UBAR -o - | %FileCheck %s
+//
+// RUN: %target-swift-frontend -emit-ir %s -g -I %S/Inputs \
+// RUN:   -Xcc -DFOO="foo" -Xcc -UBAR -o - -no-clang-module-breadcrumbs \
+// RUN:   | %FileCheck %s --check-prefix=NONE
 import ClangModule.SubModule
 import OtherClangModule.SubModule
 
@@ -11,3 +15,6 @@ import OtherClangModule.SubModule
 // CHECK: !DICompileUnit(language: DW_LANG_{{ObjC|C99}}, {{.*}} producer: "{{.*}}Swift
 // CHECK-SAME:           OtherClangModule
 // CHECK-SAME:           dwoId:
+
+// NONE: DICompileUnit({{.*}}
+// NONE-NOT: DICompileUnit({{.*}}ClangModule