[clang][modules] Avoid calling expensive `SourceManager::translateFile()` (#86216)

The `ASTWriter` algorithm for computing affecting module maps uses
`SourceManager::translateFile()` to get a `FileID` from a `FileEntry`.
This is slow (O(n)) since the function performs a linear walk over
`SLocEntries` until it finds one with a matching `FileEntry`.

This patch removes this use of `SourceManager::translateFile()` by
tracking `FileID` instead of `FileEntry` in couple of places in
`ModuleMap`, giving `ASTWriter` the desired `FileID` directly. There are
no changes required for clients that still want a `FileEntry` from
`ModuleMap`: the existing APIs internally use `SourceManager` to perform
the reverse `FileID` to `FileEntry` conversion in O(1).

1 files changed, 8 insertions, 2 deletions

diff --git a/clang/lib/Frontend/FrontendAction.cpp b/clang/lib/Frontend/FrontendAction.cpp
index b9fd9b8..b7c9967 100644
--- a/clang/lib/Frontend/FrontendAction.cpp
+++ b/clang/lib/Frontend/FrontendAction.cpp
@@ -535,8 +535,14 @@ static Module *prepareToBuildModule(CompilerInstance &CI,
     if (*OriginalModuleMap != CI.getSourceManager().getFileEntryRefForID(
                                   CI.getSourceManager().getMainFileID())) {
       M->IsInferred = true;
-      CI.getPreprocessor().getHeaderSearchInfo().getModuleMap()
-        .setInferredModuleAllowedBy(M, *OriginalModuleMap);
+      auto FileCharacter =
+          M->IsSystem ? SrcMgr::C_System_ModuleMap : SrcMgr::C_User_ModuleMap;
+      FileID OriginalModuleMapFID = CI.getSourceManager().getOrCreateFileID(
+          *OriginalModuleMap, FileCharacter);
+      CI.getPreprocessor()
+          .getHeaderSearchInfo()
+          .getModuleMap()
+          .setInferredModuleAllowedBy(M, OriginalModuleMapFID);
     }
   }