HLSL offsetting

[Francisco Casas]

So, as Matteo summarized, we are between 2 main options:

a) Multiple register offsets.
b) Component offsets with structured dereference info.

How I see it, (a) changes hlsl_deref to:

---
enum register_set {
     HLSL_REGSET_OBJ,
     HLSL_REGSET_NUM,
     /* ... add more as needed, to cover for all SMs. */

     HLSL_REGSET_COUNT,
};

struct hlsl_deref
{
     struct hlsl_ir_var *var;
     struct hlsl_src offset[HLSL_REGSET_COUNT];
};
---

Also, the types' reg_size becomes reg_size[HLSL_REGSET_COUNT], and so do 
field offsets. Many functions have to receive an additional register_set 
argument or an array of offsets instead of a single offset.


On the other hand, the version of (b) I imagine changes hlsl_deref to:
---
struct hlsl_deref
{
     struct hlsl_ir_var *var;
     unsigned int route_len;
     struct hlsl_src *route;
};
---
Where route is intended to be a variable size array of component 
offsets. It would make sense to remove reg_size from the types and also 
field offsets. Functions that cannot receive structs or arrays may 
receive a "flattened" component offset that can then be translated into 
a route, other functions would require the route as an array.



At this point I can see the benefits of (b) over (a), but also, several 
complications that may arise (you have pointed most of them):
- We will have to translate many things that are already in terms of 
register offsets into component offsets.
- We will have to move all optimization passes (like vectorization) that 
require register offsets to their specific SMxIR, RA too.
- We will have to do the proper translation to register offsets, on each 
SMxIR level, and probably some sort of constant folding for them.
- Once we start supporting non-constant offsets, we may also want to 
introduce a common subexpression elimination pass for the registers 
offsets expressions that will arise (currently, the creation of common 
expressions is mainly avoided by the recursive structure of the split 
passes).


Solely because I have spent a considerable amount of time implementing 
option (a) (and some time giving up on implementing component offsets as 
a single hlsl_src, instead of a path) I am rushing (a) to see how the 
patch turns out in the end, before trying (b).

I so think that (a) can be cleansed one step at the time. Even if the 
register sizes and offsets depend on the SM, we can write an interface 
to abstract the rest of the code of using them directly, and gradually 
migrate the code that does to use this interface instead.


But so far, yeah, I am being convinced that (b) is better, however more 
difficult.
If we do (b), I suggest we try to do it in separate steps, with some 
"scaffolding code":

- Add the component offset route to hlsl_deref without deleting the 
register offset.
- Create a simple pass that initializes the register offset field using 
the route in all the hlsl_derefs.
- Translate all the parse code first to work with the routes, and apply 
the pass just after parsing.
- Translate some more compilation passes and move the translation pass 
forward.
- Repeat until all the passes that can be written in terms of component 
offsets are.
- Write the SMxIR·s and the SMxIR translations.
- Only then, remove the the register offset from hlsl_deref and the 
translation pass from the codebase.


Otherwise we may end up writing a very big patch that may take too long 
to complete (!).


Best regards,
Francisco.