[PATCH vkd3d 2/5] vkd3d-shader/hlsl: Perform a copy propagation pass.

[Giovanni Mascellani]

Hi,

On 10/11/21 10:57, Giovanni Mascellani wrote:
>> The CF part is probably going to be a lot trickier, but fortunately I 
>> think that this patch alone is the one that really matters. Frankly, 
>> if we were to take this patch, and then a second patch that does 
>> copy-prop on interior CF blocks with a fresh copy_propagation_state(), 
>> we might even cover enough that it's not even worrying about CF...
> 
> Mmh, note that you have to handle control flow in some way or another, 
> you cannot just ignore it. This patch doesn't know about control flow, 
> but the only way it can do it is by bailing out at the first control 
> flow instruction in the program. It's not enough to ignore the control 
> flow block and resume processing after it, because the inner block might 
> invalidate some of your knowledge about the program state (in 3/5 patch 
> this is done by copy_propagation_invalidate_from_block).

I think I misread your comment, even though my point is still valid: in 
your model, you have to refresh copy_propagation_state not only when you 
enter a block, but also when you leave it (actually, you might avoid 
refreshing when entering a conditional, though not when entering a 
loop). I think this means dropping quite a lot of useful information, 
and given that I don't find it particularly hard to avoid dropping it, 
it seems quite a waste.

Notice, in particular, that if you count on this copy propagation pass 
to fix texture loading, then you need to ensure, at a minimum, that all 
loads from a variable that is stored once at the beginning of the 
program (before any control flow) are correctly recognized and 
simplified. This does not happen with your proposal, I believe, while it 
happens with mine 3/5 (which currently has a bug which makes it unsound, 
but I will fix it in my next submission).

Well, technically at this point mine proposal wouldn't work as well, 
because it ignores object variables, but that can be addressed.

Giovanni.