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

[Jan Svoboda via cfe-commits]

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

>From 450c5adeb3ddc5bf7e01fcbe379c37b08d4de9d1 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             | 8 +++++++-
 clang/lib/Serialization/ASTWriter.cpp                     | 4 ++--
 .../DependencyScanning/DependencyScanningWorker.cpp       | 2 ++
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/clang/include/clang/Lex/HeaderSearchOptions.h b/clang/include/clang/Lex/HeaderSearchOptions.h
index 114af14dec7f5a8..fa2d0b502d72c19 100644
--- a/clang/include/clang/Lex/HeaderSearchOptions.h
+++ b/clang/include/clang/Lex/HeaderSearchOptions.h
@@ -247,6 +247,11 @@ class HeaderSearchOptions {
   LLVM_PREFERRED_TYPE(bool)
   unsigned ModulesSkipHeaderSearchPaths : 1;
 
+  /// Whether to entirely skip writing pragma diagnostic mappings.
+  /// Primarily used to speed up deserialization during dependency scanning.
+  LLVM_PREFERRED_TYPE(bool)
+  unsigned ModulesSkipPragmaDiagnosticMappings : 1;
+
   LLVM_PREFERRED_TYPE(bool)
   unsigned ModulesHashContent : 1;
 
@@ -270,7 +275,8 @@ class HeaderSearchOptions {
         ForceCheckCXX20ModulesInputFiles(false), UseDebugInfo(false),
         ModulesValidateDiagnosticOptions(true),
         ModulesSkipDiagnosticOptions(false),
-        ModulesSkipHeaderSearchPaths(false), ModulesHashContent(false),
+        ModulesSkipHeaderSearchPaths(false),
+        ModulesSkipPragmaDiagnosticMappings(false), ModulesHashContent(false),
         ModulesStrictContextHash(false) {}
 
   /// AddPath - Add the \p Path path to the specified \p Group list.
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 1e86566d81fbc02..0161ad10f3f2381 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -1261,8 +1261,8 @@ 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 d1d3cc50cb25b83..c54e6d523800b6b 100644
--- a/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
+++ b/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
@@ -255,6 +255,8 @@ class DependencyScanningAction : public tooling::ToolAction {
     ScanInstance.getHeaderSearchOpts().ModulesStrictContextHash = true;
     ScanInstance.getHeaderSearchOpts().ModulesSkipDiagnosticOptions = true;
     ScanInstance.getHeaderSearchOpts().ModulesSkipHeaderSearchPaths = true;
+    ScanInstance.getHeaderSearchOpts().ModulesSkipPragmaDiagnosticMappings =
+        true;
 
     // Avoid some checks and module map parsing when loading PCM files.
     ScanInstance.getPreprocessorOpts().ModulesCheckRelocated = false;