[C++20] [Modules] [Reduced BMI] Don't record declarations in functions

by default

For reduced BMI, it is meaningless to record the local declarations in
functions if not required explicitly during the process of writing the
function bodies.

It wastes time for reduced BMI and may be problematic if we want to
avoid transiting unnecessary changes.

Author: ChuanqiXu9

clang/lib/Serialization/ASTWriter.cpp

@@ -3195,6 +3195,10 @@ uint64_t ASTWriter::WriteDeclContextLexicalBlock(ASTContext &Context,
   if (DC->decls_empty())
     return 0;
 
+  // In reduced BMI, we don't care the declarations in functions.
+  if (GeneratingReducedBMI && DC->isFunctionOrMethod())
+    return 0;
+
   uint64_t Offset = Stream.GetCurrentBitNo();
   SmallVector<uint32_t, 128> KindDeclPairs;
   for (const auto *D : DC->decls()) {

clang/test/Modules/no-local-decl-in-reduced-bmi.cppm

@@ -0,0 +1,33 @@
+// Test that we won't record local declarations by default in reduced BMI.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-reduced-module-interface -o %t/a.pcm
+// RUN: llvm-bcanalyzer --dump --disable-histogram --show-binary-blobs %t/a.pcm > %t/a.dump
+// RUN: cat %t/a.dump | FileCheck %t/a.cppm
+//
+// RUN: %clang_cc1 -std=c++20 %t/b.cppm -emit-reduced-module-interface -o %t/b.pcm
+// RUN: llvm-bcanalyzer --dump --disable-histogram --show-binary-blobs %t/b.pcm > %t/b.dump
+// RUN: cat %t/b.dump | FileCheck %t/b.cppm
+
+//--- a.cppm
+export module a;
+export int func() {
+    int v = 43;
+    return 43;
+}
+
+// Test that the variable declaration is not recorded completely.
+// CHECK-NOT: <DECL_VAR
+
+//--- b.cppm
+export module b;
+export inline int func() {
+    int v = 43;
+    return v;
+}
+
+// Check that we still records the declaration from inline functions.
+// CHECK: <DECL_VAR