[C++20] [Modules] Don't import non-inline function bodies even if it is always-inline

Recommit
llvm@1ecbab5

Close llvm#80949

The new thing in this commit is to allow to import the function body
from instantiations if it is marked with always-inline. See the
discussion in llvm#86893 for
details.

Author: ChuanqiXu9

clang/lib/CodeGen/CodeGenModule.cpp

@@ -3952,9 +3952,20 @@ bool CodeGenModule::shouldEmitFunction(GlobalDecl GD) {
   // behavior may break ABI compatibility of the current unit.
   if (const Module *M = F->getOwningModule();
       M && M->getTopLevelModule()->isNamedModule() &&
-      getContext().getCurrentNamedModule() != M->getTopLevelModule() &&
-      !F->hasAttr<AlwaysInlineAttr>())
-    return false;
+      getContext().getCurrentNamedModule() != M->getTopLevelModule()) {
+    // There are practices to mark template member function as always-inline
+    // and mark the template as extern explicit instantiation but not give
+    // the definition for member function. So we have to emit the function
+    // from explicitly instantiation with always-inline.
+    //
+    // See https://github.com/llvm/llvm-project/issues/86893 for details.
+    //
+    // TODO: Maybe it is better to give it a warning if we call a non-inline
+    // function from other module units which is marked as always-inline.
+    if (!F->isTemplateInstantiation() || !F->hasAttr<AlwaysInlineAttr>()) {
+      return false;
+    }
+  }
 
   if (F->hasAttr<NoInlineAttr>())
     return false;

clang/test/CodeGenCXX/module-funcs-from-imports.cppm

@@ -23,6 +23,21 @@ int func_in_gmf_not_called() {
     return 44;
 }
 
+template <class T>
+class A {
+public:
+    __attribute__((always_inline))
+    inline constexpr int getValue() {
+        return 43;
+    }
+
+    inline constexpr int getValue2() {
+        return 43;
+    }
+};
+
+extern template class A<char>;
+
 //--- M.cppm
 module;
 #include "foo.h"
@@ -47,17 +62,21 @@ int always_inline_func() {
     return 45;
 }
 
+export using ::A;
+
 //--- Use.cpp
 import M;
 int use() {
-    return exported_func() + always_inline_func();
+    A<char> a;
+    return exported_func() + always_inline_func() +
+           a.getValue() + a.getValue2();
 }
 
-// Checks that none of the function (except the always_inline_func) in the importees
-// are generated in the importer's code.
 // CHECK-O0: define{{.*}}_Z3usev(
 // CHECK-O0: declare{{.*}}_ZW1M13exported_funcv(
-// CHECK-O0: define{{.*}}available_externally{{.*}}_ZW1M18always_inline_funcv(
+// CHECK-O0: declare{{.*}}_ZW1M18always_inline_funcv(
+// CHECK-O0: define{{.*}}@_ZN1AIcE8getValueEv(
+// CHECK-O0: declare{{.*}}@_ZN1AIcE9getValue2Ev(
 // CHECK-O0-NOT: func_in_gmf
 // CHECK-O0-NOT: func_in_gmf_not_called
 // CHECK-O0-NOT: non_exported_func
@@ -68,7 +87,9 @@ int use() {
 // O0 to keep consistent ABI.
 // CHECK-O1: define{{.*}}_Z3usev(
 // CHECK-O1: declare{{.*}}_ZW1M13exported_funcv(
-// CHECK-O1: define{{.*}}available_externally{{.*}}_ZW1M18always_inline_funcv(
+// CHECK-O1: declare{{.*}}_ZW1M18always_inline_funcv(
+// CHECK-O1: define{{.*}}@_ZN1AIcE8getValueEv(
+// CHECK-O1: declare{{.*}}@_ZN1AIcE9getValue2Ev(
 // CHECK-O1-NOT: func_in_gmf
 // CHECK-O1-NOT: func_in_gmf_not_called
 // CHECK-O1-NOT: non_exported_func