[clang] [clang] Make explicitly-built modules independent of the CWD (PR #164840)

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 created https://github.com/llvm/llvm-project/pull/164840

PR https://github.com/llvm/llvm-project/pull/150123 changed how we normalize the modules cache path. Unfortunately, empty path would get normalized to the current working directory. This means that even explicitly-built PCMs that don't rely on the CWD now embed it, leading to surprising behavior. This PR fixes that by normalizing an empty modules cache path to an empty string.

>From afb87a3929cbd70c92da89ffd6160fbf183dc02e Mon Sep 17 00:00:00 2001
From: Jan Svoboda <jan_svoboda at apple.com>
Date: Thu, 23 Oct 2025 09:18:58 -0700
Subject: [PATCH] [clang] Make explicitly-built modules independent of the CWD

---
 clang/lib/Frontend/CompilerInstance.cpp         |  5 +----
 clang/lib/Lex/HeaderSearch.cpp                  |  6 ++++--
 .../DependencyScannerImpl.cpp                   | 11 +++++------
 clang/test/Modules/explicit-build-cwd.c         | 17 +++++++++++++++++
 4 files changed, 27 insertions(+), 12 deletions(-)
 create mode 100644 clang/test/Modules/explicit-build-cwd.c

diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index 374138fe4cf8f..e3bf0eaa3c391 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -546,14 +546,11 @@ void CompilerInstance::createPreprocessor(TranslationUnitKind TUKind) {
 std::string CompilerInstance::getSpecificModuleCachePath(StringRef ModuleHash) {
   assert(FileMgr && "Specific module cache path requires a FileManager");
 
-  if (getHeaderSearchOpts().ModuleCachePath.empty())
-    return "";
-
   // Set up the module path, including the hash for the module-creation options.
   SmallString<256> SpecificModuleCache;
   normalizeModuleCachePath(*FileMgr, getHeaderSearchOpts().ModuleCachePath,
                            SpecificModuleCache);
-  if (!getHeaderSearchOpts().DisableModuleHash)
+  if (!SpecificModuleCache.empty() && !getHeaderSearchOpts().DisableModuleHash)
     llvm::sys::path::append(SpecificModuleCache, ModuleHash);
   return std::string(SpecificModuleCache);
 }
diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
index 238c5e2f2d9a5..65c324c10ca5d 100644
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -2186,6 +2186,8 @@ std::string HeaderSearch::suggestPathToFileForDiagnostics(
 void clang::normalizeModuleCachePath(FileManager &FileMgr, StringRef Path,
                                      SmallVectorImpl<char> &NormalizedPath) {
   NormalizedPath.assign(Path.begin(), Path.end());
-  FileMgr.makeAbsolutePath(NormalizedPath);
-  llvm::sys::path::remove_dots(NormalizedPath);
+  if (!NormalizedPath.empty()) {
+    FileMgr.makeAbsolutePath(NormalizedPath);
+    llvm::sys::path::remove_dots(NormalizedPath);
+  }
 }
diff --git a/clang/lib/Tooling/DependencyScanning/DependencyScannerImpl.cpp b/clang/lib/Tooling/DependencyScanning/DependencyScannerImpl.cpp
index 05d566922a441..42f52d0ff6241 100644
--- a/clang/lib/Tooling/DependencyScanning/DependencyScannerImpl.cpp
+++ b/clang/lib/Tooling/DependencyScanning/DependencyScannerImpl.cpp
@@ -524,13 +524,12 @@ bool initializeScanCompilerInstance(
   // Use the dependency scanning optimized file system if requested to do so.
   if (DepFS) {
     DepFS->resetBypassedPathPrefix();
-    if (!ScanInstance.getHeaderSearchOpts().ModuleCachePath.empty()) {
-      SmallString<256> ModulesCachePath;
-      normalizeModuleCachePath(
-          ScanInstance.getFileManager(),
-          ScanInstance.getHeaderSearchOpts().ModuleCachePath, ModulesCachePath);
+    SmallString<256> ModulesCachePath;
+    normalizeModuleCachePath(ScanInstance.getFileManager(),
+                             ScanInstance.getHeaderSearchOpts().ModuleCachePath,
+                             ModulesCachePath);
+    if (!ModulesCachePath.empty())
       DepFS->setBypassedPathPrefix(ModulesCachePath);
-    }
 
     ScanInstance.setDependencyDirectivesGetter(
         std::make_unique<ScanningDependencyDirectivesGetter>(
diff --git a/clang/test/Modules/explicit-build-cwd.c b/clang/test/Modules/explicit-build-cwd.c
new file mode 100644
index 0000000000000..af8b74361d685
--- /dev/null
+++ b/clang/test/Modules/explicit-build-cwd.c
@@ -0,0 +1,17 @@
+// This test checks that explicitly building the same module from different
+// working directories results in the same PCM contents.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: mkdir %t/one
+// RUN: mkdir %t/two
+
+//--- module.modulemap
+module M { header "M.h" }
+
+//--- M.h
+
+// RUN: cd %t/one && %clang_cc1 -fmodules -emit-module %t/module.modulemap -fmodule-name=M -o %t/M_one.pcm
+// RUN: cd %t/two && %clang_cc1 -fmodules -emit-module %t/module.modulemap -fmodule-name=M -o %t/M_two.pcm
+
+// RUN: diff %t/M_one.pcm %t/M_two.pcm