[C++20] [Modules] Don't record implicitly declarations to BMI by default

[ChuanqiXu9]

I found we may insert unused implciit declarations like AArch SVE declarations by default on AArch64 due to we will insert that by default. But it should be completely redundant and this patch tries to remove that.

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-modules

Author: Chuanqi Xu (ChuanqiXu9)

Changes

I found we may insert unused implciit declarations like AArch SVE declarations by default on AArch64 due to we will insert that by default. But it should be completely redundant and this patch tries to remove that.

---

Full diff: https://github.com/llvm/llvm-project/pull/93459.diff

2 Files Affected:

• (modified) clang/lib/Serialization/ASTWriter.cpp (+11-2)
• (added) clang/test/Modules/no-implicit-declarations.cppm (+26)

diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 00b0e48083217..a85cd94fd5b5a 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -5037,6 +5037,14 @@ void ASTWriter::PrepareWritingSpecialDecls(Sema &SemaRef) {
         continue;
     }
 
+    // If we're writing C++ named modules, don't emit declarations which are
+    // not from modules by default. They may be built in declarations (be
+    // handled above) or implcit declarations (see the implementation of
+    // `Sema::Initialize()` for example).
+    if (isWritingStdCXXNamedModules() && !D->getOwningModule() &&
+        D->isImplicit())
+      continue;
+
     GetDeclRef(D);
   }
 
@@ -6197,8 +6205,9 @@ bool ASTWriter::wasDeclEmitted(const Decl *D) const {
     return true;
 
   bool Emitted = DeclIDs.contains(D);
-  assert((Emitted || GeneratingReducedBMI) &&
-         "The declaration can only be omitted in reduced BMI.");
+  assert((Emitted || (!D->getOwningModule() && isWritingStdCXXNamedModules()) ||
+          GeneratingReducedBMI) &&
+         "The declaration within modules can only be omitted in reduced BMI.");
   return Emitted;
 }
 
diff --git a/clang/test/Modules/no-implicit-declarations.cppm b/clang/test/Modules/no-implicit-declarations.cppm
new file mode 100644
index 0000000000000..319d3a432ea23
--- /dev/null
+++ b/clang/test/Modules/no-implicit-declarations.cppm
@@ -0,0 +1,26 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+//
+// RUN: %clang_cc1 -std=c++20 %s -emit-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 %s
+//
+// RUN: %clang_cc1 -std=c++20 %s -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 %s
+
+export module a;
+// Contain something at least to make sure the compiler won't
+// optimize this out.
+export int a = 43;
+
+// CHECK:  <DECLTYPES_BLOCK
+// CHECK-NOT: <DECL_TYPEDEF
+// CHECK:    <DECL_CONTEXT_LEXICAL
+// CHECK:    <UnknownCode
+// CHECK:    <TYPE_TYPEDEF
+// CHECK:    <DECL_VAR
+// CHECK:    <EXPR_INTEGER_LITERAL
+// CHECK:    <STMT_STOP
+// CHECK:    <TYPE_RECORD
+// CHECK:  </DECLTYPES_BLOCK>