[clang] [clang] Store the Input PCH Path Using Absolute Paths in a PCH (PR #178781)

[Qiongsi Wu via cfe-commits]

https://github.com/qiongsiwu updated https://github.com/llvm/llvm-project/pull/178781

>From 51de86d9d12fd469530ddaca4e9b966dff2552a8 Mon Sep 17 00:00:00 2001
From: Qiongsi Wu <qiongsi_wu at apple.com>
Date: Thu, 29 Jan 2026 15:50:25 -0800
Subject: [PATCH] Store absolute paths of the input PCH in a PCH file.

---
 clang/lib/Serialization/ASTWriter.cpp       | 13 +++++++++++--
 clang/test/PCH/pch-input-path-independent.c | 19 +++++++++++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/PCH/pch-input-path-independent.c

diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 3e10bbfedfe65..ded22300954c3 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -1752,7 +1752,10 @@ void ASTWriter::WriteControlBlock(Preprocessor &PP, StringRef isysroot) {
   Record.push_back(PPOpts.UsePredefines);
   // Detailed record is important since it is used for the module cache hash.
   Record.push_back(PPOpts.DetailedRecord);
-  AddString(PPOpts.ImplicitPCHInclude, Record);
+  if (PPOpts.ImplicitPCHInclude.empty())
+    AddString(PPOpts.ImplicitPCHInclude, Record);
+  else
+    AddPath(PPOpts.ImplicitPCHInclude, Record);
   Record.push_back(static_cast<unsigned>(PPOpts.ObjCXXARCStandardLibrary));
   Stream.EmitRecord(PREPROCESSOR_OPTIONS, Record);
 
@@ -6115,7 +6118,13 @@ ASTFileSignature ASTWriter::WriteASTCore(Sema *SemaPtr, StringRef isysroot,
 
         endian::Writer LE(Out, llvm::endianness::little);
         LE.write<uint8_t>(static_cast<uint8_t>(M.Kind));
-        StringRef Name = M.isModule() ? M.ModuleName : M.FileName;
+        SmallString<128> Name;
+        if (M.isModule())
+          Name = M.ModuleName;
+        else {
+          Name = M.FileName;
+          PreparePathForOutput(Name);
+        }
         LE.write<uint16_t>(Name.size());
         Out.write(Name.data(), Name.size());
 
diff --git a/clang/test/PCH/pch-input-path-independent.c b/clang/test/PCH/pch-input-path-independent.c
new file mode 100644
index 0000000000000..885dfcf9c6f67
--- /dev/null
+++ b/clang/test/PCH/pch-input-path-independent.c
@@ -0,0 +1,19 @@
+// Tests that when PCHs are chained, the dependent PCHs produced are identical
+// whether the input PCH is specified through a relative path or an absolute path.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+// RUN: %clang_cc1 -triple x86_64-apple-macos11 -emit-pch h1.h -o %t/h1.h.pch
+// RUN: %clang_cc1 -triple x86_64-apple-macos11 -emit-pch bridging.h \
+// RUN:  -o %t/bridging1.h.pch -include-pch %t/h1.h.pch
+// RUN: %clang_cc1 -triple x86_64-apple-macos11 -emit-pch bridging.h \
+// RUN:  -o %t/bridging2.h.pch -include-pch ./h1.h.pch
+
+// RUN: diff %t/bridging1.h.pch %t/bridging2.h.pch
+
+//--- h1.h
+int bar1() { return 42; }
+
+//--- bridging.h
+int bar() { return bar1(); }