[PATCH vkd3d 04/17] vkd3d-shader/hlsl: Propagate constant loads in copy propagation.

[Zebediah Figura]

On 12/2/21 09:02, Giovanni Mascellani wrote:
> Hi,
> 
> On 01/12/21 17:55, Matteo Bruni wrote:
>> IIRC this patch gets rid of unnecessary copies from an immediate to a
>> temporary variable. I already forgot the details but I'm pretty sure
>> that it did something...
> 
> I already replied privately, but for the sake of having a publicly 
> archived reference, this patch converts this:
> 
> trace:hlsl_dump_function:    2:      float | 1.00000000e+00
> trace:hlsl_dump_function:    3:      float | 2.00000000e+00
> trace:hlsl_dump_function:    4:      float | 3.00000000e+00
> trace:hlsl_dump_function:    5:      float | 4.00000000e+00
> trace:hlsl_dump_function:    6:            | = (<constructor-0>.x @2)
> trace:hlsl_dump_function:    7:            | = (<constructor-0>.y @3)
> trace:hlsl_dump_function:    8:            | = (<constructor-0>.z @4)
> trace:hlsl_dump_function:    9:            | = (<constructor-0>.w @5)
> trace:hlsl_dump_function:   10:     float4 | <constructor-0>
> trace:hlsl_dump_function:   11:            | return
> 
> to this:
> 
> trace:hlsl_dump_function:    2:     float4 | {1.00000000e+00 
> 2.00000000e+00 3.00000000e+00 4.00000000e+00 }
> trace:hlsl_dump_function:    3:            | return
> 
> It is not something 1/17 can (or should) be able to do. There might be a 
> better way to do this with more generality, but this is here right now, 
> so I think it makes sense to have it in master.

Right. And for the sake of further reference, my argument is that we 
should have a generic coalescing/vectorization pass, which would convert

2: 1.0
3. 1.0
4: = (var.x @2)
5: = (var.y @3)

into

2: (1.0, 1.0)
3: = (var.xy @2)

without depending on there being a following load, whereupon the 
existing copy-prop pass should be able to handle the rest of the 
optimization.

If this pass is going to be hard to write, I'm fine enough with using 
this patch as a temporary solution. It's not clear to me that the pass 
*is* hard to write, but it probably won't make it into 1.3 if that's 
going to be released soon. Of course, it's also not clear to me that we 
really *need* a temporary solution...