[Mesa-dev] [PATCH 1/2] nir/dead_cf: Don't crash on unreachable after-loop blocks

[Matt Turner]

On Thu, Sep 1, 2016 at 3:19 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> On Thu, Sep 1, 2016 at 12:41 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
>>
>> As-is, this change will make us delete trivially infinite loops (i.e.
>> loops with no break statement). The most likely scenario where we get
>> one of those (besides for silly Piglit tests) is a developer with an
>> accidental bug in their shader. In that case, it seems kinda mean to
>> then delete the entire loop and pretend like it doesn't exist. Maybe
>> add something like
>>
>> if (!after->imm_dom)
>>    return false;
>>
>> with a comment explaining what's going on? This would also help us
>> catch other places where we don't handle infinite loops correctly that
>> might've been hidden by this.
>
>
> I wrote that patch and kicked it off to Jenkins and it passed just fine.
> However, I don't know if I like it.  Matt and I were talking about this
> yesterday and his suggestion was to, if they have an infinite loop, delete
> the entire shader and draw pink.  That's more useful to developers than
> hanging their GPU.

Yeah, FWIW the GLSL spec says

> Non-terminating loops are allowed. The consequences of very long or non-terminating loops are platform dependent.

So I think we're safe to remove infinite loops as far as the spec is concerned.

I suppose Connor is right that removing the loop is mean in the same
way that NVIDIA zero initializing uninitialized variables is mean. In
practice, we're going to see far fewer infinite loops than
uninitialized variables. :)