Re: try_finally_expr in must_not_throw_expr

[Richard Sandiford <richard.sandiford@arm.com>]

Philipp Rimmele via Gcc <gcc@gcc.gnu.org> writes:
> Hi,
>
> i'm developing a GCC-Plugin. And i don't understand why there is a "try_finally_expr" in a must_not_throw-Area in my AST. It happens in the destructors.
> Here is my AST:
> function_decl Exception::__dt_base
>   1: must_not_throw_expr(->void_type{void})[42]
>     0: statement_list(->void_type{void})
>       0: bind_expr(->void_type{void})[42]
>         1: statement_list(->void_type{void})
>           0: cleanup_point_expr(->void_type{void})[42]
>             0: expr_stmt(->void_type{void})
>               0: convert_expr(->void_type{void})
>                 0: modify_expr(->pointer_type->pointer_type{__vtbl_ptr_type}->function_type->integer_type{int})
>                   0: component_ref(->pointer_type->pointer_type{__vtbl_ptr_type}->function_type->integer_type{int})
>                     0: indirect_ref(->record_type{Exception})
>                       0: nop_expr(->pointer_type->record_type{Exception})
>                         0: parm_decl(->pointer_type->record_type{Exception}) : this
>                     1: field_decl(->pointer_type->pointer_type{__vtbl_ptr_type}->function_type->integer_type{int})
>                   1: pointer_plus_expr(->pointer_type->pointer_type{__vtbl_ptr_type}->function_type->integer_type{int})
>                     0: addr_expr(->pointer_type->pointer_type{__vtbl_ptr_type}->function_type->integer_type{int})
>                       0: var_decl(->array_type->pointer_type{__vtbl_ptr_type}->function_type->integer_type{int}) : _ZTV9Exception
>                     1: integer_cst : 16 : 1
>           0: try_finally(->void_type{void})[42]
>             0: statement_list(->void_type{void})
>             1: modify_expr(->void_type{void})
>               0: indirect_ref(->record_type)
>                 0: nop_expr(->reference_type->record_type)
>                   0: parm_decl(->pointer_type->record_type{Exception}) : this
>               1: constructor(->record_type)
>         2: block
>       0: label_expr(->void_type{void})[42]
>         0: label_decl(->void_type{void}) : <unnamed>
>
> What is the reason for this? There should no Exception be thrown, so why handle it with a try_finally-Expression? I'm currently using GCC-8.2.0.
> I would be realy glad if you could answer me this question. And if you can give me some examples, where the try_finally-expression is also used, it would be realy helpfull.

I'm not an expert on this by any means, but since no-one else has
replied yet... I suspect it's simpler to use try_finally whenever
something needs to be run at the end of a scope, regardless of whether
the scope ends through fallthrough, breaking, continuing, or exceptions.

To put it another way: try_finally at this stage doesn't guarantee
that exception handling will actually be needed.  For example:

  try
    {
      int i = 1;
      int j = 2;
      if (i == j)
        foo ();
    }
  finally ...

starts out with a potentially-throwing call to foo, but it (and the
possibility of an exception) will get optimised away later.  It probably
didn't seem worthwhile having the frontend do a similar but separate
analysis of whether statements might throw.

Thanks,
Richard