[llvm-branch-commits] [clang] 53d474b - [clang][modules] Read PCM validation timestamp earlier (#177062)

[Cullen Rhodes via llvm-branch-commits]

Author: Jan Svoboda
Date: 2026-01-26T09:30:07Z
New Revision: 53d474b4fa47fc7a5809401fe2a3d5e6f1d81e81

URL: https://github.com/llvm/llvm-project/commit/53d474b4fa47fc7a5809401fe2a3d5e6f1d81e81
DIFF: https://github.com/llvm/llvm-project/commit/53d474b4fa47fc7a5809401fe2a3d5e6f1d81e81.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could read an out-of-date PCM file and then read the validation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.

(cherry picked from commit ada79f4c2691ab6546d379a144377162fd4f5191)

Added: 
    

Modified: 
    clang/lib/Serialization/ModuleManager.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/Serialization/ModuleManager.cpp b/clang/lib/Serialization/ModuleManager.cpp
index 236fe20fdad7c..dad6840947ece 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
   if (std::unique_ptr<llvm::MemoryBuffer> Buffer = lookupBuffer(FileName)) {