[lldb] Don't create instance of SymbolFileDWARFDebugMap for non-Mach-O files (#139170)

# Change

`SymbolFileDWARFDebugMap::CreateInstance()` will return `nullptr` if the
file is not a Mach-O.


# Benefit

This may improve **Linux** debugger launch time by skipping the creation
of `SymbolFileDWARFDebugMap` during the [`SymbolFile::FindPlugin()`
call](https://fburl.com/hi1w8dil), which loops through a list of
`SymbolFile` plugins and tries to find the one that provides the best
abilities. If the `SymbolFileDWARFDebugMap` is created during this loop,
it will load the symbol table of the file in question and loop through
all the compile units in the debug map (the OSO entries) to calculate
the abilities.


# Tests

See PR.

Author: royitaqi

lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.cpp

@@ -246,6 +246,10 @@ llvm::StringRef SymbolFileDWARFDebugMap::GetPluginDescriptionStatic() {
 }
 
 SymbolFile *SymbolFileDWARFDebugMap::CreateInstance(ObjectFileSP objfile_sp) {
+  assert(objfile_sp);
+  // Don't create a debug map if the object file isn't a Mach-O.
+  if (!objfile_sp->GetArchitecture().GetTriple().isAppleMachO())
+    return nullptr;
   return new SymbolFileDWARFDebugMap(std::move(objfile_sp));
 }
 

lldb/unittests/SymbolFile/DWARF/CMakeLists.txt

@@ -4,6 +4,7 @@ add_lldb_unittest(SymbolFileDWARFTests
   DWARFDIETest.cpp
   DWARFIndexCachingTest.cpp
   DWARFUnitTest.cpp
+  SymbolFileDWARFDebugMapTests.cpp
   SymbolFileDWARFTests.cpp
   XcodeSDKModuleTests.cpp
 

lldb/unittests/SymbolFile/DWARF/SymbolFileDWARFDebugMapTests.cpp

@@ -0,0 +1,154 @@
+//===-- SymbolFileDWARFDebugMapTests.cpp ----------------------------------===//
+//
+// 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/ObjectFile/ELF/ObjectFileELF.h"
+#include "Plugins/ObjectFile/Mach-O/ObjectFileMachO.h"
+#include "Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.h"
+#include "TestingSupport/SubsystemRAII.h"
+#include "TestingSupport/TestUtilities.h"
+
+#include "lldb/Core/Module.h"
+#include "llvm/Testing/Support/Error.h"
+
+#include "gtest/gtest.h"
+
+using namespace lldb;
+using namespace lldb_private;
+using namespace lldb_private::plugin::dwarf;
+
+class SymbolFileDWARFDebugMapTests : public testing::Test {
+  SubsystemRAII<ObjectFileELF, ObjectFileMachO> subsystems;
+};
+
+TEST_F(SymbolFileDWARFDebugMapTests, CreateInstanceReturnNonNullForMachOFile) {
+  // A Mach-O file built for arm64 CPU type and macOS platform.
+  const char *yamldata = R"(
+--- !mach-o
+FileHeader:
+  magic:           0xFEEDFACF
+  cputype:         0x0100000C
+  cpusubtype:      0x00000000
+  filetype:        0x00000001
+  ncmds:           1
+  sizeofcmds:      176
+  flags:           0x00002000
+  reserved:        0x00000000
+LoadCommands:
+  - cmd:             LC_BUILD_VERSION
+    cmdsize:         24
+    platform:        1
+    minos:           658944
+    sdk:             658944
+    ntools:          0
+  - cmd:             LC_SEGMENT_64
+    cmdsize:         152
+    segname:         __TEXT
+    vmaddr:          0
+    vmsize:          4
+    fileoff:         208
+    filesize:        4
+    maxprot:         7
+    initprot:        7
+    nsects:          1
+    flags:           0
+    Sections:
+      - sectname:        __text
+        segname:         __TEXT
+        addr:            0x0000000000000000
+        content:         'AABBCCDD'
+        size:            4
+        offset:          208
+        align:           0
+        reloff:          0x00000000
+        nreloc:          0
+        flags:           0x80000400
+        reserved1:       0x00000000
+        reserved2:       0x00000000
+        reserved3:       0x00000000
+...
+)";
+
+  // Perform setup.
+  llvm::Expected<TestFile> file = TestFile::fromYaml(yamldata);
+  EXPECT_THAT_EXPECTED(file, llvm::Succeeded());
+  auto module_sp = std::make_shared<Module>(file->moduleSpec());
+  ASSERT_NE(module_sp, nullptr);
+  auto object_file = module_sp->GetObjectFile();
+  ASSERT_NE(object_file, nullptr);
+
+  // The debug map should be non-null for Mach-O file.
+  auto debug_map =
+      SymbolFileDWARFDebugMap::CreateInstance(object_file->shared_from_this());
+  ASSERT_NE(debug_map, nullptr);
+}
+
+TEST_F(SymbolFileDWARFDebugMapTests, CreateInstanceReturnNullForNonMachOFile) {
+  // An ELF file.
+  const char *yamldata = R"(
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_EXEC
+  Machine: EM_386
+DWARF:
+  debug_abbrev:
+    - Table:
+        - Code:            0x00000001
+          Tag:             DW_TAG_compile_unit
+          Children:        DW_CHILDREN_no
+          Attributes:
+            - Attribute:       DW_AT_addr_base
+              Form:            DW_FORM_sec_offset
+  debug_info:
+    - Version:         5
+      AddrSize:        4
+      UnitType:        DW_UT_compile
+      Entries:
+        - AbbrCode:        0x00000001
+          Values:
+            - Value:           0x8 # Offset of the first Address past the header
+        - AbbrCode:        0x0
+  debug_addr:
+    - Version: 5
+      AddressSize: 4
+      Entries:
+        - Address: 0x1234
+        - Address: 0x5678
+  debug_line:
+    - Length:          42
+      Version:         2
+      PrologueLength:  36
+      MinInstLength:   1
+      DefaultIsStmt:   1
+      LineBase:        251
+      LineRange:       14
+      OpcodeBase:      13
+      StandardOpcodeLengths: [ 0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 1 ]
+      IncludeDirs:
+        - '/tmp'
+      Files:
+        - Name:            main.cpp
+          DirIdx:          1
+          ModTime:         0
+          Length:          0
+)";
+
+  // Perform setup.
+  llvm::Expected<TestFile> file = TestFile::fromYaml(yamldata);
+  EXPECT_THAT_EXPECTED(file, llvm::Succeeded());
+  auto module_sp = std::make_shared<Module>(file->moduleSpec());
+  ASSERT_NE(module_sp, nullptr);
+  auto object_file = module_sp->GetObjectFile();
+  ASSERT_NE(object_file, nullptr);
+
+  // The debug map should be null for non-Mach-O (ELF) file.
+  auto debug_map =
+      SymbolFileDWARFDebugMap::CreateInstance(object_file->shared_from_this());
+  ASSERT_EQ(debug_map, nullptr);
+}