[lldb-dev] RTTI does not work stable in LLDB.
Robinson, Paul via lldb-dev
lldb-dev at lists.llvm.org
Mon Feb 6 15:38:48 PST 2017
It's not practical for the DWARF to try to identify the actual address of the vtable; that address might not be available.
it seems like we could hang onto the linkage_name of the vtable though, somewhere, so you wouldn't be relying on the demangler you have available at runtime to produce the same string that the compiler did at compile time. The symbolic name of the vtable should be unambiguous (one hopes!) but not depend on the existence of the vtable in any particular place.
Doesn't solve the problem for today's compilers, granted.
--paulr
P.S. It would be helpful to have these things come up *before* the next rev of the spec is frozen. Just sayin'. ☺
From: Greg Clayton [mailto:gclayton at apple.com]
Sent: Monday, February 06, 2017 2:35 PM
To: Zachary Turner
Cc: Robinson, Paul; LLDB Dev (lldb-dev at lists.llvm.org)
Subject: Re: [lldb-dev] RTTI does not work stable in LLDB.
It just says where it is inside the class itself, not what the value of the vtable pointer is:
0x00006628: TAG_structure_type [112] *
AT_containing_type( {0x0000000000006628} )
AT_name( "base_type" )
AT_byte_size( 0x08 )
AT_decl_file( "/private/tmp/main.cpp" )
AT_decl_line( 6 )
0x00006634: TAG_member [4]
AT_name( "_vptr$base_type" )
AT_type( {0x0000000000004a41} ( __vtbl_ptr_type* ) )
AT_data_member_location( 0x00 )
AT_artificial( true )
Just says “the vtable is at offset zero inside the class”. Not helpful for reading any vtable pointer and trying to figure out which class it belongs to.
Greg
On Feb 6, 2017, at 2:25 PM, Zachary Turner <zturner at google.com<mailto:zturner at google.com>> wrote:
Doesn't the DWARF have a record for the VTable itself? I know PDB does, you can look up the class name through the VTable debug info record rather than trying to demangle the name.
On Mon, Feb 6, 2017 at 2:21 PM Greg Clayton via lldb-dev <lldb-dev at lists.llvm.org<mailto:lldb-dev at lists.llvm.org>> wrote:
Yeah, when doing dynamic type resolution, we look at the first pointer inside the pointer and see if it resolves to a virtual table symbol. If it does, we extract the class name from the demangled symbol name and try to look up. GDB does the same thing. All debuggers do AFAIK.
If the DWARF specified the vtable address in the DWARF on the class definition this would help, but without that the only thing we can really do is to try and figure out the class and look it up by name. Also, even if this is added to future DWARF, it doesn’t fix the problem that we have many compilers that don’t have the info so we would still need to do what we do.
If anyone has any better ideas I am all ears?
Greg
On Feb 6, 2017, at 11:48 AM, Robinson, Paul <paul.robinson at sony.com<mailto:paul.robinson at sony.com>> wrote:
So is LLDB expecting the name in the DWARF info to match the demangled name of the vtable pointer? The DWARF spec does not really specify what the name of a template instantiation should be, and in particular does not *want* to specify whether it matches any given demangler's opinion of the name.
--paulr
From: lldb-dev [mailto:lldb-dev-bounces at lists.llvm.org] On Behalf Of Greg Clayton via lldb-dev
Sent: Monday, February 06, 2017 11:08 AM
To: Greg Clayton
Cc: lldb-dev at lists.llvm.org<mailto:lldb-dev at lists.llvm.org>
Subject: Re: [lldb-dev] RTTI does not work stable in LLDB.
So I found the problem. This is a compiler bug. The DWARF for this type looks like:
0x000065da: TAG_structure_type [112] *
AT_containing_type( {0x0000000000006628} )
AT_name( "derived0<int, int, 1024>" )
AT_byte_size( 0x08 )
AT_decl_file( "/private/tmp/main.cpp" )
AT_decl_line( 9 )
But all of the type info in the symbol table has has the type named as "derived0<int, int, 1024u>”. Note the extra “u” that follows 1024. This stops LLDB from being able to lookup the type correctly so we can show the dynamic type. In LLDB we check the first pointer inside of a class to see if it is a symbol whose name is “vtable for TYPENAME”. If it is, we lookup the type “TYPENAME” to find it. In this case we try to lookup "derived0<int, int, 1024u>” and we fail since the DWARF has it as "derived0<int, int, 1024>”.
I have filed a radar on the compiler here at Apple for the fix.
Greg
On Feb 6, 2017, at 10:22 AM, Greg Clayton via lldb-dev <lldb-dev at lists.llvm.org<mailto:lldb-dev at lists.llvm.org>> wrote:
I am looking at this now. I will let you know what I find.
Greg
On Feb 6, 2017, at 10:00 AM, Roman Popov <ripopov at gmail.com<mailto:ripopov at gmail.com>> wrote:
Yes, that was my thought.
FYI, checked in GDB: it's working correctly on this testcase showing correct dynamic type in both cases.
2017-02-06 9:48 GMT-08:00 Greg Clayton <gclayton at apple.com<mailto:gclayton at apple.com>>:
You have found a bug. It should be reporting this correctly but it isn’t. I verified it fails on MacOSX.
Greg Clayton
On Feb 5, 2017, at 1:19 PM, Roman Popov via lldb-dev <lldb-dev at lists.llvm.org<mailto:lldb-dev at lists.llvm.org>> wrote:
Hello,
I'm observing very strange LLDB behavior: it does not always shows a correct dynamic type when I ask for.
Originally I was working with LLDB 3.9, but it looks like trunk version behaves the same strange way.
I was able to capture this behavior in a small code snippet:
#include <iostream>
#include <typeinfo>
using namespace std;
struct base_type { virtual ~base_type(){} };
template <class T1, class T2, unsigned SIZE>
struct derived0 : base_type {};
template <class T1, class T2>
struct derived1 : base_type {};
int main(int argc, char ** argv) {
base_type * bptr0 = new derived0<int, int, 1024>();
base_type * bptr1 = new derived1<int, int >();
cout << typeid(*bptr0).name() << endl;
cout << typeid(*bptr1).name() << endl;
return 0;
}
lldb --version
lldb version 5.0.0 (http://llvm.org/svn/llvm-project/lldb/trunk revision 293398)
clang revision 293398
llvm revision 293398
Testing in LLDB:
(lldb) break set --file main.cpp --line 22
(lldb) expression -d no-run -- bptr1
(derived1<int, int> *) $2 = 0x0000000000614c40
(lldb) expression -d no-run -- bptr0
(base_type *) $3 = 0x0000000000614c20
Can someone explain me why for bptr0 I dont get a derived0<int, int, 1024> * as I expected?
Thanks,
Roman
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