[clang] [clang][deps] Skip writing `DIAG_PRAGMA_MAPPINGS` record (PR #70874)

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 created https://github.com/llvm/llvm-project/pull/70874

None

>From c3602bceb01aa93f801670a31bb43903d6a10d9c Mon Sep 17 00:00:00 2001
From: Jan Svoboda <jan_svoboda at apple.com>
Date: Tue, 31 Oct 2023 16:35:38 -0700
Subject: [PATCH] [clang][deps] Skip writing `DIAG_PRAGMA_MAPPINGS` record

---
 clang/include/clang/Lex/HeaderSearchOptions.h                 | 4 ++++
 clang/lib/Serialization/ASTWriter.cpp                         | 4 ++--
 .../Tooling/DependencyScanning/DependencyScanningWorker.cpp   | 2 ++
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/clang/include/clang/Lex/HeaderSearchOptions.h b/clang/include/clang/Lex/HeaderSearchOptions.h
index c7d95006bb779ad..e7204bf718b5f75 100644
--- a/clang/include/clang/Lex/HeaderSearchOptions.h
+++ b/clang/include/clang/Lex/HeaderSearchOptions.h
@@ -219,6 +219,10 @@ class HeaderSearchOptions {
 
   unsigned ModulesValidateDiagnosticOptions : 1;
 
+  /// Whether to entirely skip writing pragma diagnostic mappings.
+  /// Primarily used to speed up deserialization during dependency scanning.
+  unsigned ModulesSkipPragmaDiagnosticMappings : 1;
+
   unsigned ModulesHashContent : 1;
 
   /// Whether we should include all things that could impact the module in the
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 739344b9a128dcf..011a94c5b55520d 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -1259,8 +1259,8 @@ void ASTWriter::writeUnhashedControlBlock(Preprocessor &PP,
 
   Stream.EmitRecord(HEADER_SEARCH_PATHS, Record);
 
-  // Write out the diagnostic/pragma mappings.
-  WritePragmaDiagnosticMappings(Diags, /* isModule = */ WritingModule);
+  if (!HSOpts.ModulesSkipPragmaDiagnosticMappings)
+    WritePragmaDiagnosticMappings(Diags, /* isModule = */ WritingModule);
 
   // Header search entry usage.
   auto HSEntryUsage = PP.getHeaderSearchInfo().computeUserEntryUsage();
diff --git a/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp b/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
index 29df0c3a0afdb5c..45cd247148dd34d 100644
--- a/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
+++ b/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
@@ -252,6 +252,8 @@ class DependencyScanningAction : public tooling::ToolAction {
     // TODO: Implement diagnostic bucketing to reduce the impact of strict
     // context hashing.
     ScanInstance.getHeaderSearchOpts().ModulesStrictContextHash = true;
+    ScanInstance.getHeaderSearchOpts().ModulesSkipPragmaDiagnosticMappings =
+        true;
 
     // Avoid some checks and module map parsing when loading PCM files.
     ScanInstance.getPreprocessorOpts().ModulesCheckRelocated = false;