Re: Ada and "early return" - opinion/practice question

[Stephen Leake <stephen_leake@stephe-leake.org>]

Niklas Holsti <niklas.holsti@tidorum.invalid> writes:

> Are you sure about that? As I understand it, for the full-runtime GNAT
> systems, the "zero cost exception handling" means that there is no 
> run-time cost to placing an exception handler in a subprogram or
> block, because there is no run-time code to set up such a handler when
> entering the subprogram or block, nor to remove it when leaving the
> subprogram or block.

right

> But there is cost, partly at compile/link time, when the tools set up
> a static mapping from code sections (address ranges) to the exception 
> handlers that handle exceptions raised in those parts of the code, 

right.

> and partly at run-time when an exception is raised. AIUI raising an
> exception is not a mere goto: at run-time, the code has to use the
> static mapping to locate the applicable handler, while also unwinding
> the call-stack to the corresponding point, and then enter the handler
> code

Yes; it is more like a sequence of "return" than "goto".

It is cheaper at run-time than the previous implemententation of
exceptions, which used set-jmp/long-jmp.

> A different zero-cost system is used when GNAT compiles for embedded
> targets with run-times that do not support exception propagation. In 
> those cases, one can write only "local" exception handlers. For
> example, the exception handlers in a subprogram can handle only
> exceptions raised in that same subprogram, but not exceptions raised
> in any callee subprograms. Then raising an exception is indeed the
> same as a goto, because the compiler statically knows if there is a
> local handler, and where it is, and no stack unwinding is needed. 

I did not know about this one.

-- 
-- Stephe