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
