[lldb-dev] Evaluate expression for template class

[Scott Funkenhauser via lldb-dev]

I built with both clang and gcc, the behavior is the same (lldb fails to
evaluate the expression, gdb succeeds).

Trying to evaluate a template type that hasn't been instantiated fails in
both lldb and gdb.
ie.
(gdb) p sizeof(foo<double>)
No symbol "foo<double>" in current context.
(lldb) p sizeof(foo<double>)
error: implicit instantiation of undefined template 'foo<double>'
template is declared here

Looking at the debug info, both template instantiations are there, it seems
like LLDB just isn't finding it (while GDB is).

 $readelf --debug-dump a.out | grep foo
  0x00000000 666f6f00 5400696e 7400666f 6f3c696e foo.T.int.foo<in
  0x00000080 00746573 74320066 6f6f3c76 6f69643e .test2.foo<void>
    <41>   DW_AT_name        : (indirect string, offset: 0xa): foo<int>
    <49>   DW_AT_name        : (indirect string, offset: 0x0): foo
    <8f>   DW_AT_name        : (indirect string, offset: 0x87): foo<void>
    <97>   DW_AT_name        : (indirect string, offset: 0x0): foo
    8d     foo<void>
    3f     foo<int>

On Tue, Jul 23, 2019 at 8:35 PM Jim Ingham <jingham at apple.com> wrote:

> lldb can't currently create new template instantiations.  It can only
> access ones that were generated in the binary you were debugging.  The
> debug information doesn't have any code, so we can't create new
> instantiations from there.  Having the debugger try to include headers into
> the expression context in the same way the compiler did originally is also
> tricky since we don't know the header search paths or defines that the
> compiler used when it built the template instantiations...  Raphael has
> been working on doing this for the case where the C++ code is build into a
> clang module, but I don't think that work is done yet, and then you have to
> build your code with modules to use it...
>
> The <int> expression is failing because clang is being smart and sees that
> though you mention foo<int> in reference to "test" you never actually use
> "test", so it doesn't actually have to make that instantiation. In the case
> where this works with gdb, did you build with clang or did you build with
> gcc?  If the latter, this might be working for gdb because gcc emits code
> for the int template instantiation when you mention it, rather than only
> when you use it?
>
> Does it also work for gdb when you try:
>
> (gdb) p sizeof(foo<double>)
>
> or any other type you haven't mentioned in your code?
>
> Not that that really matters for what lldb would have to do, but it would
> be interesting to know...
>
> Jim
>
>
>
> > On Jul 23, 2019, at 4:22 PM, Scott Funkenhauser via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >
> > Hey,
> >
> > I've noticed that evaluating expressions involving templated classes
> seems to have some unexpected behavior.
> >
> > I've created the following sample code to reproduce the issue:
> >
> > template <typename T> class foo
> > {
> >   uint32_t data;
> > };
> >
> > foo<int> test;
> >
> > int main() {
> >   foo<void> test2;
> >   return 0;
> > }
> >
> > I've set a breakpoint on main and evaluated the following expressions:
> > (lldb) p sizeof(foo<void>)
> > (unsigned long) $0 = 4
> > (lldb) p sizeof(foo<int>)
> > error: implicit instantiation of undefined template 'foo<int>'
> > template is declared here
> >
> > It seems like expression evaluation can only find the specialized
> templated classes that were used in the current scope. Running the same
> example using GDB works as expected (both expressions are evaluated
> correctly).
> >
> > Is this a known issue?
> > Does anybody have any insight, or familiarity with expression evaluation
> that would be able to point me in the right direction?
> >
> > Thanks,
> > Scott
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
>
>
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