[clang] ada79f4 - [clang][modules] Read PCM validation timestamp earlier (#177062)

[via cfe-commits]

Author: Jan Svoboda
Date: 2026-01-22T07:15:26-08:00
New Revision: ada79f4c2691ab6546d379a144377162fd4f5191

URL: https://github.com/llvm/llvm-project/commit/ada79f4c2691ab6546d379a144377162fd4f5191
DIFF: https://github.com/llvm/llvm-project/commit/ada79f4c2691ab6546d379a144377162fd4f5191.diff

LOG: [clang][modules] Read PCM validation timestamp earlier (#177062)

When building a module, the PCM file is always written first and then
the validation timestamp gets created. Clang needs to first read the
validation timestamp and only then read the PCM file. Otherwise, it
could read an out-of-date PCM file and then read the validation
timestamp for its new up-to-date version. This would erroneously skip
validation with `-fmodules-validate-once-per-build-session`. I'm not
concerned about multiple Clang instances seeing different filesystem
contents from each other within a single build session, since that would
break the assumption `-fmodules-validate-once-per-build-session` relies
on.

Added: 
    

Modified: 
    clang/lib/Serialization/ModuleManager.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/Serialization/ModuleManager.cpp b/clang/lib/Serialization/ModuleManager.cpp
index 353121c9b5184..16c3a1e04f649 100644
--- a/clang/lib/Serialization/ModuleManager.cpp
+++ b/clang/lib/Serialization/ModuleManager.cpp
@@ -105,6 +105,10 @@ ModuleManager::addModule(StringRef FileName, ModuleKind Type,
                          std::string &ErrorStr) {
   Module = nullptr;
 
+  uint64_t InputFilesValidationTimestamp = 0;
+  if (Type == MK_ImplicitModule)
+    InputFilesValidationTimestamp = ModCache.getModuleTimestamp(FileName);
+
   // Look for the file entry. This only fails if the expected size or
   // modification time 
diff er.
   OptionalFileEntryRef Entry;
@@ -172,11 +176,7 @@ ModuleManager::addModule(StringRef FileName, ModuleKind Type,
   NewModule->Index = Chain.size();
   NewModule->FileName = FileName.str();
   NewModule->ImportLoc = ImportLoc;
-  NewModule->InputFilesValidationTimestamp = 0;
-
-  if (NewModule->Kind == MK_ImplicitModule)
-    NewModule->InputFilesValidationTimestamp =
-        ModCache.getModuleTimestamp(NewModule->FileName);
+  NewModule->InputFilesValidationTimestamp = InputFilesValidationTimestamp;
 
   // Load the contents of the module
   std::unique_ptr<llvm::MemoryBuffer> NewFileBuffer = nullptr;