[Lldb-commits] [lldb] 8056d92 - [lldb][ClangExpressionParser] Implement ExternalSemaSource::ReadUndefinedButUsed (#104817)

[via lldb-commits]

Author: Michael Buch
Date: 2024-08-20T18:42:46+01:00
New Revision: 8056d92bc201906edbb27f4efa1cc80a18359880

URL: https://github.com/llvm/llvm-project/commit/8056d92bc201906edbb27f4efa1cc80a18359880
DIFF: https://github.com/llvm/llvm-project/commit/8056d92bc201906edbb27f4efa1cc80a18359880.diff

LOG: [lldb][ClangExpressionParser] Implement ExternalSemaSource::ReadUndefinedButUsed (#104817)

While parsing an expression, Clang tries to diagnose usage of decls
(with possibly non-external linkage) for which it hasn't been provided
with a definition. This is the case, e.g., for functions with parameters
that live in an anonymous namespace (those will have `UniqueExternal`
linkage, this is computed [here in
computeTypeLinkageInfo](https://github.com/llvm/llvm-project/blob/ea8bb4d633683f5cbfd82491620be3056f347a02/clang/lib/AST/Type.cpp#L4647-L4653)).
Before diagnosing such situations, Clang calls
`ExternalSemaSource::ReadUndefinedButUsed`. The intended use of this API
is to extend the set of "used but not defined" decls with additional
ones that the external source knows about. However, in LLDB's case, we
never provide `FunctionDecl`s with a definition, and instead rely on the
expression parser to resolve those symbols by linkage name. Thus, to
avoid the Clang parser from erroring out in these situations, this patch
implements `ReadUndefinedButUsed` which just removes the "undefined"
non-external `FunctionDecl`s that Clang found.

We also had to add an `ExternalSemaSource` to the `clang::Sema` instance
LLDB creates. We previously didn't have any source on `Sema`. Because we
add the `ExternalASTSourceWrapper` here, that means we'd also
technically be adding the `ClangExpressionDeclMap` as an
`ExternalASTSource` to `Sema`, which is fine because `Sema` will only be
calling into the `ExternalSemaSource` APIs (though nothing currently
strictly enforces this, which is a bit worrying).

Note, the decision for whether to put a function into `UndefinedButUsed`
is done in
[Sema::MarkFunctionReferenced](https://github.com/llvm/llvm-project/blob/ea8bb4d633683f5cbfd82491620be3056f347a02/clang/lib/Sema/SemaExpr.cpp#L18083-L18087).
The `UniqueExternal` linkage computation is done in
[getLVForNamespaceScopeDecl](https://github.com/llvm/llvm-project/blob/ea8bb4d633683f5cbfd82491620be3056f347a02/clang/lib/AST/Decl.cpp#L821-L833).

Fixes https://github.com/llvm/llvm-project/issues/104712

Added: 
    lldb/test/Shell/Expr/TestAnonNamespaceParamFunc.cpp

Modified: 
    lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
    lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp

Removed: 
    


################################################################################
diff  --git a/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h b/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
index 44e52e8dd65113..5669c31f1c402e 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
@@ -15,6 +15,7 @@
 #include "clang/Sema/Sema.h"
 #include "clang/Sema/SemaConsumer.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
+#include "llvm/Support/Casting.h"
 #include <optional>
 
 namespace clang {
@@ -138,6 +139,24 @@ class ExternalASTSourceWrapper : public clang::ExternalSemaSource {
     return m_Source->layoutRecordType(Record, Size, Alignment, FieldOffsets,
                                       BaseOffsets, VirtualBaseOffsets);
   }
+
+  /// This gets called when Sema is reconciling undefined but used decls.
+  /// For LLDB's use-case, we never provide Clang with function definitions,
+  /// instead we rely on linkage names and symbol resolution to call the
+  /// correct funcitons during JITting. So this implementation clears
+  /// any "undefined" FunctionDecls that Clang found while parsing.
+  ///
+  /// \param[in,out] Undefined A set of used decls for which Clang has not
+  ///                          been provided a definition with.
+  ///
+  void ReadUndefinedButUsed(
+      llvm::MapVector<clang::NamedDecl *, clang::SourceLocation> &Undefined)
+      override {
+    Undefined.remove_if([](auto const &decl_loc_pair) {
+      const clang::NamedDecl *ND = decl_loc_pair.first;
+      return llvm::isa_and_present<clang::FunctionDecl>(ND);
+    });
+  }
 };
 
 /// Wraps an ASTConsumer into an SemaConsumer. Doesn't take ownership of the

diff  --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
index d37787ebae26c3..38fe9fa10493fa 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
@@ -1223,6 +1223,8 @@ ClangExpressionParser::ParseInternal(DiagnosticManager &diagnostic_manager,
 
     clang::ExternalASTSource *ast_source = decl_map->CreateProxy();
 
+    auto *ast_source_wrapper = new ExternalASTSourceWrapper(ast_source);
+
     if (ast_context.getExternalSource()) {
       auto *module_wrapper =
           new ExternalASTSourceWrapper(ast_context.getExternalSource());
@@ -1236,6 +1238,7 @@ ClangExpressionParser::ParseInternal(DiagnosticManager &diagnostic_manager,
     } else {
       ast_context.setExternalSource(ast_source);
     }
+    m_compiler->getSema().addExternalSource(ast_source_wrapper);
     decl_map->InstallASTContext(*m_ast_context);
   }
 

diff  --git a/lldb/test/Shell/Expr/TestAnonNamespaceParamFunc.cpp b/lldb/test/Shell/Expr/TestAnonNamespaceParamFunc.cpp
new file mode 100644
index 00000000000000..1882be1b31a899
--- /dev/null
+++ b/lldb/test/Shell/Expr/TestAnonNamespaceParamFunc.cpp
@@ -0,0 +1,22 @@
+// Tests that we can evaluate functions that Clang
+// classifies as having clang::Linkage::UniqueExternal
+// linkage. In this case, a function whose argument
+// is not legally usable outside this TU.
+
+// RUN: %build %s -o %t
+// RUN: %lldb %t -o run -o "expression func(a)" -o exit | FileCheck %s
+
+// CHECK: expression func(a)
+// CHECK: (int) $0 = 15
+
+namespace {
+struct InAnon {};
+} // namespace
+
+int func(InAnon a) { return 15; }
+
+int main() {
+  InAnon a;
+  __builtin_debugtrap();
+  return func(a);
+}