[clang] [clang][deps][modules] Allocate input file paths lazily (PR #114457)

[Jan Svoboda via cfe-commits]

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@@ -779,23 +795,16 @@ static StringRef makeAbsoluteAndPreferred(CompilerInstance &CI, StringRef Path,
 void ModuleDepCollector::addFileDep(StringRef Path) {
   if (IsStdModuleP1689Format) {
     // Within P1689 format, we don't want all the paths to be absolute path
-    // since it may violate the tranditional make style dependencies info.
-    FileDeps.push_back(std::string(Path));
+    // since it may violate the traditional make style dependencies info.
+    FileDeps.emplace_back(Path);
     return;
   }
 
   llvm::SmallString<256> Storage;
   Path = makeAbsoluteAndPreferred(ScanInstance, Path, Storage);
-  FileDeps.push_back(std::string(Path));
+  FileDeps.emplace_back(Path);
 }
 
 void ModuleDepCollector::addFileDep(ModuleDeps &MD, StringRef Path) {
-  if (IsStdModuleP1689Format) {
-    MD.FileDeps.insert(Path);
-    return;
-  }
-
-  llvm::SmallString<256> Storage;
-  Path = makeAbsoluteAndPreferred(ScanInstance, Path, Storage);
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jansvoboda11 wrote:

Looking at the `ASTWriter`/`ASTReader` code, I'm pretty sure Clang modules always have an absolute base directory. (Unless the `-fmodule-file-home-is-cwd` or `-fmodule-map-file-home-is-cwd` flags have been specified, in which case reporting relative paths is expected IMO.)

Standard C++ modules will use the `-isysroot` argument as-is as the base directory.

So I think overall the call to `makeAbsoluteAndPreferred()` is okay to remove. For context, making paths absolute was introduced by @benlangmuir in [D130934](https://reviews.llvm.org/D130934).

https://github.com/llvm/llvm-project/pull/114457