[clang][ScanDeps] Clear compilation directory if needed (#150129)

During explicit module build, clear the compilation directory flags when
CWD optimization is on. Now clang CodeGen will not add compilation
directories to DIFile when the corresponding flags are not passed and
make debug info not dependent on CWD.

Author: cachemeifyoucan

clang/lib/Tooling/DependencyScanning/ModuleDepCollector.cpp

@@ -144,30 +144,8 @@ static void optimizeDiagnosticOpts(DiagnosticOptions &Opts,
 
 static void optimizeCWD(CowCompilerInvocation &BuildInvocation, StringRef CWD) {
   BuildInvocation.getMutFileSystemOpts().WorkingDir.clear();
-  if (BuildInvocation.getCodeGenOpts().DwarfVersion) {
-    // It is necessary to explicitly set the DebugCompilationDir
-    // to a common directory (e.g. root) if IgnoreCWD is true.
-    // When IgnoreCWD is true, the module's content should not
-    // depend on the current working directory. However, if dwarf
-    // information is needed (when CGOpts.DwarfVersion is
-    // non-zero), then CGOpts.DebugCompilationDir must be
-    // populated, because otherwise the current working directory
-    // will be automatically embedded in the dwarf information in
-    // the pcm, contradicting the assumption that it is safe to
-    // ignore the CWD. Thus in such cases,
-    // CGOpts.DebugCompilationDir is explicitly set to a common
-    // directory.
-    // FIXME: It is still excessive to create a copy of
-    // CodeGenOpts for each module. Since we do not modify the
-    // CodeGenOpts otherwise per module, the following code
-    // ends up generating identical CodeGenOpts for each module
-    // with DebugCompilationDir pointing to the root directory.
-    // We can optimize this away by creating a _single_ copy of
-    // CodeGenOpts whose DebugCompilationDir points to the root
-    // directory and reuse it across modules.
-    BuildInvocation.getMutCodeGenOpts().DebugCompilationDir =
-        llvm::sys::path::root_path(CWD);
-  }
+  BuildInvocation.getMutCodeGenOpts().DebugCompilationDir.clear();
+  BuildInvocation.getMutCodeGenOpts().CoverageCompilationDir.clear();
 }
 
 static std::vector<std::string> splitString(std::string S, char Separator) {

clang/test/ClangScanDeps/modules-debug-dir.c

@@ -1,12 +1,16 @@
-// REQUIRES: shell
-
 // RUN: rm -rf %t
 // RUN: split-file %s %t
 // RUN: sed -e "s|DIR|%/t|g" %t/cdb.json.in > %t/cdb.json
 // RUN: clang-scan-deps -compilation-database %t/cdb.json -format \
 // RUN:   experimental-full -optimize-args=all > %t/result.json
 // RUN: cat %t/result.json | sed 's:\\\\\?:/:g' | FileCheck %s
 
+// RUN: %deps-to-rsp %t/result.json --module-name=mod > %t/mod.rsp
+// RUN: %clang @%t/mod.rsp -o %t/mod.pcm
+// RUN: llvm-dwarfdump --debug-info %t/mod.pcm | FileCheck %s --check-prefix=DWARF
+// DWARF: DW_TAG_compile_unit
+// DWARF-NOT: DW_AT_comp_dir
+
 //--- cdb.json.in
 [{
   "directory": "DIR",
@@ -28,5 +32,5 @@ module mod {
 // directory when current working directory optimization is in effect.
 // CHECK:  "modules": [
 // CHECK: "command-line": [
-// CHECK: "-fdebug-compilation-dir={{\/|.*:(\\)?}}",
+// CHECK-NOT: -fdebug-compilation-dir
 // CHECK:  "translation-units": [