[clang] [Modules] Process include files changes with -fmodules-validate-input-files-content and -fno-pch-timestamp options (PR #90319)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Ivan Murashko (ivanmurashko)

<details>
<summary>Changes</summary>

There were two diffs that introduced some options useful when you build modules externally and cannot rely on file modification time as the key for detecting input file changes:
- [D67249](https://reviews.llvm.org/D67249) introduced the `-fmodules-validate-input-files-content` option, which allows the use of file content hash in addition to the modification time.
- [D141632](https://reviews.llvm.org/D141632) propagated the use of `-fno-pch-timestamps` with Clang modules.

There is a problem when the size of the input file (header) is not modified but the content is. In this case, Clang cannot detect the file change when the `-fno-pch-timestamps` option is used. The `-fmodules-validate-input-files-content` option should help, but there is an issue with its application: it's not applied when the modification time is stored as zero that is the case for `-fno-pch-timestamps`.

The issue can be fixed using the same trick that was applied during the processing of `ForceCheckCXX20ModulesInputFiles`. The patch suggests the corresponding solution and includes a LIT test to verify it.

---
Full diff: https://github.com/llvm/llvm-project/pull/90319.diff


2 Files Affected:

- (modified) clang/lib/Serialization/ASTReader.cpp (+8) 
- (added) clang/test/Modules/implicit-module-no-timestamp.cpp (+33) 


``````````diff
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 0ef57a3ea804ef..4609ca3e1428c8 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -2630,6 +2630,14 @@ InputFile ASTReader::getInputFile(ModuleFile &F, unsigned ID, bool Complain) {
       F.StandardCXXModule && FileChange.Kind == Change::None)
     FileChange = HasInputContentChanged(FileChange);
 
+  // When we have StoredTime equal to zero and ValidateASTInputFilesContent,
+  // it is better to check the content of the input files because we cannot rely
+  // on the file modification time, which will be the same (zero) for these
+  // files.
+  if (!StoredTime && ValidateASTInputFilesContent &&
+      FileChange.Kind == Change::None)
+    FileChange = HasInputContentChanged(FileChange);
+
   // For an overridden file, there is nothing to validate.
   if (!Overridden && FileChange.Kind != Change::None) {
     if (Complain && !Diags.isDiagnosticInFlight()) {
diff --git a/clang/test/Modules/implicit-module-no-timestamp.cpp b/clang/test/Modules/implicit-module-no-timestamp.cpp
new file mode 100644
index 00000000000000..457ad3c16e184c
--- /dev/null
+++ b/clang/test/Modules/implicit-module-no-timestamp.cpp
@@ -0,0 +1,33 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: cp a1.h a.h
+// RUN: %clang -fmodules -fmodules-validate-input-files-content -Xclang -fno-pch-timestamp -fimplicit-modules -fmodule-map-file=module.modulemap -fsyntax-only  -fmodules-cache-path=%t test1.cpp
+// RUN: cp a2.h a.h
+// RUN: %clang -fmodules -fmodules-validate-input-files-content -Xclang -fno-pch-timestamp -fimplicit-modules -fmodule-map-file=module.modulemap -fsyntax-only  -fmodules-cache-path=%t test2.cpp
+
+//--- a1.h
+#define FOO
+
+//--- a2.h
+#define BAR
+
+//--- module.modulemap
+module a {
+  header "a.h"
+}
+
+//--- test1.cpp
+#include "a.h"
+
+#ifndef FOO
+#error foo
+#endif
+
+//--- test2.cpp
+#include "a.h"
+
+#ifndef BAR
+#error bar
+#endif

``````````

</details>


https://github.com/llvm/llvm-project/pull/90319