fix: strip leading underscore for __cdecl functions on x86

Author: Nerixyz

lldb/source/Plugins/SymbolFile/NativePDB/PdbUtil.cpp

@@ -1118,3 +1118,21 @@ size_t lldb_private::npdb::GetSizeOfType(PdbTypeSymId id,
   }
   return 0;
 }
+
+llvm::StringRef lldb_private::npdb::StripCDeclPrefix(llvm::StringRef mangled) {
+  // See
+  // https://learn.microsoft.com/en-us/cpp/build/reference/decorated-names#FormatC
+  if (!mangled.starts_with('_'))
+    return mangled;
+
+  // make sure this isn't __stdcall (`_{name}@{sizeof(params)}`) or __vectorcall
+  // (`{name}@@{sizeof(params)}`).
+  size_t last_at_pos = mangled.find_last_of('@');
+  if (last_at_pos != llvm::StringRef::npos &&
+      last_at_pos < mangled.size() - 1 &&
+      llvm::all_of(mangled.slice(last_at_pos + 1, mangled.size()),
+                   llvm::isDigit))
+    return mangled;
+
+  return mangled.drop_front();
+}

lldb/source/Plugins/SymbolFile/NativePDB/PdbUtil.h

@@ -161,6 +161,18 @@ PdbTypeSymId GetBestPossibleDecl(PdbTypeSymId id, llvm::pdb::TpiStream &tpi);
 
 size_t GetSizeOfType(PdbTypeSymId id, llvm::pdb::TpiStream &tpi);
 
+/// Strips the leading underscore of mangled __cdecl functions.
+///
+/// If the name comes from another calling convention, it is returned as-is.
+///
+/// \pre \c mangled must not be from a 64-bit environment as __cdecl names
+///     aren't mangled there.
+///
+/// \param[in] mangled A mangled symbol name
+/// \returns The stripped name if this name is a mangled __cdecl one. Otherwise,
+///     the input is returned.
+llvm::StringRef StripCDeclPrefix(llvm::StringRef mangled);
+
 } // namespace npdb
 } // namespace lldb_private
 

lldb/source/Plugins/SymbolFile/NativePDB/SymbolFileNativePDB.cpp

@@ -2677,6 +2677,7 @@ SymbolFileNativePDB::FindMangledFunctionName(PdbCompilandSymId func_id) {
 
   ProcSym proc(static_cast<SymbolRecordKind>(sym_record.kind()));
   cantFail(SymbolDeserializer::deserializeAs<ProcSym>(sym_record, proc));
+
   return FindMangledSymbol(SegmentOffset(proc.Segment, proc.CodeOffset));
 }
 
@@ -2686,7 +2687,21 @@ SymbolFileNativePDB::FindMangledSymbol(SegmentOffset so) {
                                                  so.segment, so.offset);
   if (!symbol)
     return std::nullopt;
-  return symbol->first.Name;
+
+  llvm::StringRef name = symbol->first.Name;
+  // "In non-64 bit environments" (on x86 in pactice), __cdecl functions get
+  // prefixed with an underscore. For compilers using LLVM, this happens in LLVM
+  // (as opposed to the compiler frontend). Because of this, DWARF doesn't
+  // contain the "full" mangled name in DW_AT_linkage_name for these functions.
+  // We strip the mangling here for compatibility with DWARF. See
+  // llvm.org/pr161676 and
+  // https://learn.microsoft.com/en-us/cpp/build/reference/decorated-names#FormatC
+  if ((symbol->first.Flags & PublicSymFlags::Function) !=
+          PublicSymFlags::None &&
+      m_index->dbi().getMachineType() == PDB_Machine::x86)
+    name = StripCDeclPrefix(name);
+
+  return name;
 }
 
 void SymbolFileNativePDB::CacheUdtDeclarations() {

lldb/test/Shell/SymbolFile/NativePDB/c-calling-conventions.cpp

@@ -0,0 +1,51 @@
+// clang-format off
+// REQUIRES: lld, x86
+
+// RUN: %build --compiler=clang-cl --arch=32 --nodefaultlib --output=%t-32.exe %s
+// RUN: lldb-test symbols %t-32.exe | FileCheck --check-prefixes CHECK-32,CHECK-BOTH %s
+// RUN: %build --compiler=clang-cl --arch=64 --nodefaultlib --output=%t-64.exe %s
+// RUN: lldb-test symbols %t-64.exe | FileCheck --check-prefixes CHECK-64,CHECK-BOTH %s
+
+extern "C" {
+int FuncCCall() { return 0; }
+int __stdcall FuncStdCall() { return 0; }
+int __fastcall FuncFastCall() { return 0; }
+int __vectorcall FuncVectorCall() { return 0; }
+
+int __cdecl _underscoreCdecl() { return 0; }
+int __stdcall _underscoreStdcall() { return 0; }
+int __fastcall _underscoreFastcall() { return 0; }
+int __vectorcall _underscoreVectorcall() { return 0; }
+}
+
+int main() {
+  FuncCCall();
+  FuncStdCall();
+  FuncFastCall();
+  FuncVectorCall();
+  _underscoreCdecl();
+  _underscoreStdcall();
+  _underscoreFastcall();
+  _underscoreVectorcall();
+  return 0;
+}
+
+// CHECK-BOTH-DAG: Function{{.*}}, demangled = FuncCCall,
+// CHECK-BOTH-DAG: Function{{.*}}, demangled = FuncVectorCall@@0,
+// CHECK-BOTH-DAG: Function{{.*}}, demangled = _underscoreCdecl,
+// CHECK-BOTH-DAG: Function{{.*}}, demangled = _underscoreVectorcall@@0,
+// CHECK-BOTH-DAG: Function{{.*}}, demangled = main,
+
+// __stdcall and __fastcall aren't available on 64 bit
+
+// CHECK-32-DAG: Function{{.*}}, demangled = _FuncStdCall@0,
+// CHECK-64-DAG: Function{{.*}}, demangled = FuncStdCall,
+
+// CHECK-32-DAG: Function{{.*}}, demangled = @FuncFastCall@0,
+// CHECK-64-DAG: Function{{.*}}, demangled = FuncFastCall,
+
+// CHECK-32-DAG: Function{{.*}}, demangled = __underscoreStdcall@0,
+// CHECK-64-DAG: Function{{.*}}, demangled = _underscoreStdcall,
+
+// CHECK-32-DAG: Function{{.*}}, demangled = @_underscoreFastcall@0,
+// CHECK-64-DAG: Function{{.*}}, demangled = _underscoreFastcall,

lldb/unittests/SymbolFile/NativePDB/CMakeLists.txt

@@ -1,5 +1,6 @@
 add_lldb_unittest(SymbolFileNativePDBTests
   PdbFPOProgramToDWARFExpressionTests.cpp
+  PdbUtilTests.cpp
   UdtRecordCompleterTests.cpp
 
   LINK_COMPONENTS

lldb/unittests/SymbolFile/NativePDB/PdbUtilTests.cpp

@@ -0,0 +1,47 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Plugins/SymbolFile/NativePDB/PdbUtil.h"
+#include "gtest/gtest.h"
+
+using namespace lldb_private::npdb;
+
+TEST(PdbUtil, StripCDeclPrefix) {
+  ASSERT_EQ(StripCDeclPrefix("main"), "main");
+
+  // __cdecl
+  ASSERT_EQ(StripCDeclPrefix("_main"), "main");
+  ASSERT_EQ(StripCDeclPrefix("__main"), "_main");
+  ASSERT_EQ(StripCDeclPrefix("_main@"), "main@");
+  ASSERT_EQ(StripCDeclPrefix("_main@foo"), "main@foo");
+  ASSERT_EQ(StripCDeclPrefix("_main@4@foo"), "main@4@foo");
+
+  // __stdcall
+  ASSERT_EQ(StripCDeclPrefix("_main@4"), "_main@4");
+  ASSERT_EQ(StripCDeclPrefix("_main@foo@4"), "_main@foo@4");
+  ASSERT_EQ(StripCDeclPrefix("_main@4@5"), "_main@4@5");
+
+  // __fastcall
+  ASSERT_EQ(StripCDeclPrefix("@main@4"), "@main@4");
+
+  // __vectorcall
+  ASSERT_EQ(StripCDeclPrefix("main@@4"), "main@@4");
+  ASSERT_EQ(StripCDeclPrefix("_main@@4"), "_main@@4");
+
+  // MS C++ mangling
+  ASSERT_EQ(StripCDeclPrefix("?a@@YAHD@Z"), "?a@@YAHD@Z");
+  // Itanium mangling (e.g. on MinGW)
+  ASSERT_EQ(StripCDeclPrefix("__Z7recursei"), "_Z7recursei");
+
+  ASSERT_EQ(StripCDeclPrefix("_"), "");
+  ASSERT_EQ(StripCDeclPrefix("_@"), "@");
+  ASSERT_EQ(StripCDeclPrefix(""), "");
+  ASSERT_EQ(StripCDeclPrefix("_@4"), "_@4");
+  ASSERT_EQ(StripCDeclPrefix("@4"), "@4");
+  ASSERT_EQ(StripCDeclPrefix("@"), "@");
+}