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

[Giovanni Mascellani]

Hi,

On 11/11/21 12:14, Matteo Bruni wrote:
>> Notice that variables can have more than four components. Matrices can
>> have up to 16 and arrays even more.
> Right, but we probably don't want or need to do copy propagation on
> those i.e. copy propagation should probably happen after matrix /
> struct / array splitting.

Mmh, then there is something about splitting that I'm not understanding.

My understanding so far was that variables themselves are not splitted: 
they are just there, and do not appear in the code as themselves.  What 
gets splitted are the temporaries that appear when some piece of code 
actually does something with (say) a matrix. So, for example, if you 
have this fragment of code:

---
float4x4 a;
float4x4 b;
float4x4 c;
c = a + b;
---

the compile first naively represents it as:

---
float4x4 a
float4x4 b
float4x4 c
@1 = load(a) of type float4x4
@2 = load(b) of type float4x4
@3 = + (@1 @2) of type float4x4
store(c, @3)
---

and then this gets splitted as:

---
float4x4 a
float4x4 b
float4x4 c
@1 = load(a, 0) of type float4
@2 = load(b, 0) of type float4
@3 = + (@1 @2) of type float4
store(c, 0, @3)
@5 = load(a, 4) of type float4
@6 = load(b, 4) of type float4
@7 = + (@5 @6) of type float4
store(c, 4, @7)
...
---

That is, the variables keep their type, even though the accesses (loads 
and stores) to the variables have a smaller type. That's my 
understanding of what we want. My code mirrors this, therefore allows a 
variable to have more than four registers.

What is the advantage of splitting variables themselves?

In the specific case of my copy propagation pass, this would make things 
more complicated. For example, if right now I cannot reconstruct the 
offset of a store, I can just invalidate the whole variable. In your 
model, as I get it, I'd have to also invalidate other variables, that 
are unrelated by that point.

Thanks, Giovanni.