[clang] 5d4e072 - [C++20] [Modules] [Reduced BMI] Don't eagerly write static entities in

[Chuanqi Xu via cfe-commits]

Author: Chuanqi Xu
Date: 2024-04-18T15:38:02+08:00
New Revision: 5d4e072a253afac018e312d75290314bd2d03b30

URL: https://github.com/llvm/llvm-project/commit/5d4e072a253afac018e312d75290314bd2d03b30
DIFF: https://github.com/llvm/llvm-project/commit/5d4e072a253afac018e312d75290314bd2d03b30.diff

LOG: [C++20] [Modules] [Reduced BMI] Don't eagerly write static entities in
module purview

For,

```
export module A;
static int impl() { ... }
export int func() { return impl(); }
```

Previously, even with reduced BMI, the function `impl` will be emitted
into the BMI. After the patch, the static entities in module purview
won't get emitted eagerly. Now the static entities may only be emitted
if required.

Note that, this restriction is actually more relaxed than the language
standard required. The language spec said, the program is ill-formed if
any TU-local entities get exposed. However, we can't do this since there
are many static entities in the headers of existing libraries.
Forbidding that will cause many existing program fail immediately.

Another note here is, we can't do this for non-static non-exported
entities, they can be used for other module units within the same
module.

Added: 
    clang/test/Modules/unreached-static-entities.cppm

Modified: 
    clang/lib/Serialization/ASTWriter.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 9ae9648f38a3b7..522b8ce56c3266 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -4892,8 +4892,21 @@ void ASTWriter::PrepareWritingSpecialDecls(Sema &SemaRef) {
     if (D->isFromASTFile())
       continue;
 
-    if (GeneratingReducedBMI && D->isFromExplicitGlobalModule())
-      continue;
+    if (GeneratingReducedBMI) {
+      if (D->isFromExplicitGlobalModule())
+        continue;
+
+      // Don't force emitting static entities.
+      //
+      // Technically, all static entities shouldn't be in reduced BMI. The
+      // language also specifies that the program exposes TU-local entities
+      // is ill-formed. However, in practice, there are a lot of projects
+      // uses `static inline` in the headers. So we can't get rid of all
+      // static entities in reduced BMI now.
+      if (auto *ND = dyn_cast<NamedDecl>(D);
+          ND && ND->getFormalLinkage() == Linkage::Internal)
+        continue;
+    }
 
     GetDeclRef(D);
   }

diff  --git a/clang/test/Modules/unreached-static-entities.cppm b/clang/test/Modules/unreached-static-entities.cppm
new file mode 100644
index 00000000000000..10f70ae09e5a1a
--- /dev/null
+++ b/clang/test/Modules/unreached-static-entities.cppm
@@ -0,0 +1,22 @@
+// Test that the static function only used in non-inline functions won't get emitted
+// into the BMI.
+//
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+//
+// RUN: %clang_cc1 -std=c++20 %s -emit-reduced-module-interface -o %t/S.pcm
+// RUN: llvm-bcanalyzer --dump --disable-histogram --show-binary-blobs %t/S.pcm > %t/S.dump
+// RUN: cat %t/S.dump | FileCheck %s
+
+export module S;
+static int static_func() {
+    return 43;
+}
+
+export int func() {
+    return static_func();
+}
+
+// CHECK: <DECL_FUNCTION
+// Checks that we won't see a second function
+// CHECK-NOT: <DECL_FUNCTION