[perf-prof] Adjust source position addresses according to ELF header size

src/perf-jit.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 304 matching lines @@
   debug_info.size_ = size + padding;
   LogWriteBytes(reinterpret_cast<const char*>(&debug_info), sizeof(debug_info));
 
   Address code_start = code->instruction_start();
 
   for (SourcePositionTableIterator iterator(code->source_position_table());
        !iterator.done(); iterator.Advance()) {
     SourcePositionInfo info(GetSourcePositionInfo(code_handle, function_handle,
                                                   iterator.source_position()));
     PerfJitDebugEntry entry;
-    // TODO(danno): There seems to be a bug in the dwarf handling of JIT code in
-    // the perf tool. It seems to erroneously believe that the first instruction
-    // of functions is at offset 0x40 when displayed in "perf report". To
-    // compensate for this, add a magic constant to the position addresses when
-    // writing them out.
-    entry.address_ =
-        reinterpret_cast<intptr_t>(code_start + iterator.code_offset() + 0x40);
+    // The entry point of the function will be placed straight after the ELF
+    // header when processed by "perf inject". Adjust the position addresses
+    // accordingly.
+    entry.address_ = reinterpret_cast<intptr_t>(
+        code_start + iterator.code_offset() + kElfHeaderSize);
     entry.line_number_ = info.line + 1;
     entry.column_ = info.column + 1;
     LogWriteBytes(reinterpret_cast<const char*>(&entry), sizeof(entry));
     Handle<Script> script(Script::cast(info.function->script()));
     std::unique_ptr<char[]> name_string = GetScriptName(script);
     LogWriteBytes(name_string.get(),
                   static_cast<uint32_t>(strlen(name_string.get())) + 1);
   }
   char padding_bytes[] = "\0\0\0\0\0\0\0\0";
   LogWriteBytes(padding_bytes, padding);
@@ skipping 58 matching lines @@
   header.time_stamp_ =
       static_cast<uint64_t>(base::OS::TimeCurrentMillis() * 1000.0);
   header.flags_ = 0;
 
   LogWriteBytes(reinterpret_cast<const char*>(&header), sizeof(header));
 }
 
 #endif  // V8_OS_LINUX
 }  // namespace internal
 }  // namespace v8