[lldb][ObjC] Don't query objective-c runtime for decls in C++ contexts

[Michael137]

When LLDB isn't able to find a clang::Decl in response to a FindExternalVisibleDeclsByName, it will fall-back to looking into the Objective-C runtime for that decl. This ends up doing a lot of work which isn't necessary when we're debugging a C++ program. This patch makes the ObjC lookup conditional on the language that the ExpressionParser deduced (which can be explicitly set using the expr --language option or is set implicitly if we're stopped in an ObjC frame or a C++ frame without debug-info).

rdar://96236519

[llvmbot]

@llvm/pr-subscribers-lldb

Author: Michael Buch (Michael137)

Changes

When LLDB isn't able to find a clang::Decl in response to a FindExternalVisibleDeclsByName, it will fall-back to looking into the Objective-C runtime for that decl. This ends up doing a lot of work which isn't necessary when we're debugging a C++ program. This patch makes the ObjC lookup conditional on the language that the ExpressionParser deduced (which can be explicitly set using the expr --language option or is set implicitly if we're stopped in an ObjC frame or a C++ frame without debug-info).

rdar://96236519

---

Full diff: https://github.com/llvm/llvm-project/pull/95963.diff

3 Files Affected:

• (modified) lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.cpp (+1-1)
• (modified) lldb/test/API/lang/objcxx/objc-from-cpp-frames-without-debuginfo/TestObjCFromCppFramesWithoutDebugInfo.py (+5)
• (added) lldb/test/Shell/Expr/TestObjCInCXXContext.test (+21)

diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.cpp
index 82a7a2cc3f1ef..1fdd272dcbece 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.cpp
@@ -637,7 +637,7 @@ void ClangASTSource::FindExternalVisibleDecls(
     FindDeclInModules(context, name);
   }
 
-  if (!context.m_found_type) {
+  if (!context.m_found_type && m_ast_context->getLangOpts().ObjC) {
     FindDeclInObjCRuntime(context, name);
   }
 }
diff --git a/lldb/test/API/lang/objcxx/objc-from-cpp-frames-without-debuginfo/TestObjCFromCppFramesWithoutDebugInfo.py b/lldb/test/API/lang/objcxx/objc-from-cpp-frames-without-debuginfo/TestObjCFromCppFramesWithoutDebugInfo.py
index ef8d5540fa4ef..2c2e072bba686 100644
--- a/lldb/test/API/lang/objcxx/objc-from-cpp-frames-without-debuginfo/TestObjCFromCppFramesWithoutDebugInfo.py
+++ b/lldb/test/API/lang/objcxx/objc-from-cpp-frames-without-debuginfo/TestObjCFromCppFramesWithoutDebugInfo.py
@@ -15,4 +15,9 @@ def test(self):
         (_, process, _, _) = lldbutil.run_to_name_breakpoint(self, "main")
 
         self.assertState(process.GetState(), lldb.eStateStopped)
+
+        # Tests that we can use builtin Objective-C identifiers.
         self.expect("expr id", error=False)
+
+        # Tests that we can lookup Objective-C decls in the ObjC runtime plugin.
+        self.expect_expr("NSString *c; c == nullptr", result_value="true", result_type="bool")
diff --git a/lldb/test/Shell/Expr/TestObjCInCXXContext.test b/lldb/test/Shell/Expr/TestObjCInCXXContext.test
new file mode 100644
index 0000000000000..8537799bdeb67
--- /dev/null
+++ b/lldb/test/Shell/Expr/TestObjCInCXXContext.test
@@ -0,0 +1,21 @@
+// UNSUPPORTED: system-linux, system-windows
+
+// Tests that we don't consult the the Objective-C runtime
+// plugin when in a purely C++ context.
+//
+// RUN: %clangxx_host %p/Inputs/objc-cast.cpp -g -o %t
+// RUN: %lldb %t \
+// RUN:   -o "b main" -o run \
+// RUN:   -o "expression --language objective-c -- NSString * a; a" \
+// RUN:   -o "expression --language objective-c++ -- NSString * b; b" \
+// RUN:   -o "expression NSString" \
+// RUN:   2>&1 | FileCheck %s
+
+// CHECK:      (lldb) expression --language objective-c -- NSString * a; a
+// CHECK-NEXT: (NSString *){{.*}}= nil
+
+// CHECK:      (lldb) expression --language objective-c++ -- NSString * b; b
+// CHECK-NEXT: (NSString *){{.*}}= nil
+
+// CHECK:      (lldb) expression NSString
+// CHECK-NEXT: error:{{.*}} use of undeclared identifier 'NSString'

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