[Mesa-dev] [PATCH] mesa: Define helper function to get the number of texture layers.

[Francisco Jerez]

Paul Berry <stereotype441 at gmail.com> writes:

> On 10 December 2013 08:42, Francisco Jerez <currojerez at riseup.net> wrote:
>
>> Brian Paul <brianp at vmware.com> writes:
>>
>> > On 12/07/2013 06:17 PM, Francisco Jerez wrote:
>> >>[...]
>> >> +   default:
>> >> +      unreachable();
>> >
>> > I think I'd like to see an assertion or _mesa_problem() call to catch
>> > unhandled cases if new texture targets are added in the future.
>> >
>> >
>> How about having the unreachable() macro print out an error and abort if
>> it's ever reached?  See the attached patch.
>>
>
> I would prefer for us to simply do this:
>
> assert(!"Unexpected texture target");
> unreachable();
>
> That would have the exact same semantics as your proposed patch (assert in
> debug builds, allow compiler optimizations in release builds), and it would
> have the added advantage that it's obvious what's going on to someone
> reading the code.  With your proposed patch it's not obvious--the reader
> has to be aware that Mesa has a non-standard meaning for unreachable().

The message adds no additional useful information IMHO, and it's already
obvious for anyone reading the code that something marked with
unreachable() shouldn't ever be reached -- What else do we have to say?
I have the impression that using the assert(!"") idiom before every
occurrence of unreachable() just increases the amount of noise in the
code and the likelihood of making mistakes.

Regardless of what we do in this specific case, I believe that making
the unreachable() macro abort is a good thing because it will catch
cases where we have forgotten to pair it with an assert call [I have
already, a few times], and mistakes like writing 'assert("!...' [see
61143b87c16231a2df0d69324d531503027f9aca].

If you would like to include some additional explanation in an
unreachable block you can just write a comment, or make unreachable() a
variadic macro that prints its (optionally passed) argument to stderr
when the unreachable block is executed.
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