[clang] [C++20][Modules] Fix relocatable PCH feature. (PR #180023)

[Michael Park via cfe-commits]

https://github.com/mpark updated https://github.com/llvm/llvm-project/pull/180023

>From 3d27d8d0a7a9fd82185a04c6c6edecb4b93f7986 Mon Sep 17 00:00:00 2001
From: Michael Park <mcypark at gmail.com>
Date: Thu, 5 Feb 2026 12:09:20 -0800
Subject: [PATCH 1/2] [C++20][Modules] Fix relocatable PCH feature.

---
 clang/lib/Serialization/ASTWriter.cpp | 6 +++---
 clang/test/PCH/reloc.c                | 5 +++++
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 3e10bbfedfe65..cd8c552694945 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -1526,10 +1526,10 @@ void ASTWriter::WriteControlBlock(Preprocessor &PP, StringRef isysroot) {
 
       // Write out all other paths relative to the base directory if possible.
       BaseDirectory.assign(BaseDir->begin(), BaseDir->end());
-    } else if (!isysroot.empty()) {
-      // Write out paths relative to the sysroot if possible.
-      BaseDirectory = std::string(isysroot);
     }
+  } else if (!isysroot.empty()) {
+    // Write out paths relative to the sysroot if possible.
+    BaseDirectory = std::string(isysroot);
   }
 
   // Module map file
diff --git a/clang/test/PCH/reloc.c b/clang/test/PCH/reloc.c
index 435fde2e19234..019e3c495218b 100644
--- a/clang/test/PCH/reloc.c
+++ b/clang/test/PCH/reloc.c
@@ -3,6 +3,11 @@
 // RUN: %clang -target x86_64-apple-darwin11 -fsyntax-only \
 // RUN:   -include-pch %t -isysroot %S/Inputs/libroot %s -Xclang -verify
 // RUN: not %clang -target x86_64-apple-darwin11 -include-pch %t %s
+// RUN: llvm-bcanalyzer --dump --disable-histogram %t \
+// RUN:   | FileCheck %s
+// CHECK: <ORIGINAL_FILE {{.*}}/> blob data = 'usr{{[/\\]}}include{{[/\\]}}reloc.h'
+// CHECK: <INPUT_FILE {{.*}}/> blob data = 'usr{{[/\\]}}include{{[/\\]}}reloc.h'
+// CHECK: <INPUT_FILE {{.*}}/> blob data = 'usr{{[/\\]}}include{{[/\\]}}reloc2.h'
 // REQUIRES: x86-registered-target
 
 #include <reloc.h>

>From 8de88599743147f35925fc346b2ba829340ee507 Mon Sep 17 00:00:00 2001
From: Michael Park <mcypark at gmail.com>
Date: Thu, 5 Feb 2026 20:52:52 -0800
Subject: [PATCH 2/2] Try cleaning the path to fix Windows breakage.

---
 clang/lib/Serialization/ASTWriter.cpp | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index cd8c552694945..b55440b4a4f39 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -1529,7 +1529,9 @@ void ASTWriter::WriteControlBlock(Preprocessor &PP, StringRef isysroot) {
     }
   } else if (!isysroot.empty()) {
     // Write out paths relative to the sysroot if possible.
-    BaseDirectory = std::string(isysroot);
+    SmallString<128> CleanedSysroot(isysroot);
+    cleanPathForOutput(PP.getFileManager(), CleanedSysroot);
+    BaseDirectory.assign(CleanedSysroot.begin(), CleanedSysroot.end());
   }
 
   // Module map file