[clang] [clang modules] Setting `DebugCompilationDir` when it is safe to ignore current working directory (PR #128446)

[Dave Lee via cfe-commits]

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@@ -492,8 +492,23 @@ static std::string getModuleContextHash(const ModuleDeps &MD,
   auto &FSOpts = const_cast<FileSystemOptions &>(CI.getFileSystemOpts());
   if (CWD && !IgnoreCWD)
     HashBuilder.add(*CWD);
-  else
+  else {
     FSOpts.WorkingDir.clear();
+    auto &CGOpts = const_cast<CodeGenOptions &>(CI.getCodeGenOpts());
+    if (CGOpts.DwarfVersion && CWD) {
+      // 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), and if
+      // CGOpts.DebugCompilationDir is not explicitly set,
+      // 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,
----------------
kastiglione wrote:

When reading this comment, I was tripped up on "if CGOpts.DebugCompilationDir is not explicitly set", thinking that this should mean a `if (!CGOpts.DebugCompilationDir)` somewhere in this function.

But I understand now. What do you think of this wording?

```suggestion
      // 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,
```

https://github.com/llvm/llvm-project/pull/128446