[WHLSL] Implement dynamically uniform expression restrictions

• Component: WebGPU
• Status: RESOLVED MOVED

[Myles C. Maxfield]

Implement dynamically uniform expression restrictions

[Radar WebKit Bug Importer]

<rdar://problem/44403002>

[Myles C. Maxfield]

Ever value (lvalue and rvalue) will be associated with a "uniformity" attribute, which will be assigned by the compiler. These attributes can even survive being assigned to a variable, if the variable is marked as "uniform" in the program source.

Then, after we know whether or not each value is uniform, we can associate each nested scope with a similar uniformity attribute. This is true iff the condition leading to that scope is uniform.

We apply the restriction that any AssignmentExpression inside a non-uniform scope must never assign to a uniform variable that is declared in the non-innermost scope. This should guarantee that all uniform variables are, indeed, uniform.

Then, we can require any IndexExpression that indexes into an array which contains textures to use a uniform index.

[Myles C. Maxfield]

Functions that return uniform variables must only have a single return point

[Myles C. Maxfield]

UAVs can't hold uniform values

[Myles C. Maxfield]

Conditional continues and breaks should contribute to the uniformity of the blocks they target

[Myles C. Maxfield]

(In reply to Myles C. Maxfield from comment #5)
> Conditional continues and breaks should contribute to the uniformity of the
> blocks they target

This isn't quite right - the continue & break can affect just the remainder of the scope.

Also, uniforms should only exist in thread / constant address space.

[Myles C. Maxfield]

(In reply to Myles C. Maxfield from comment #6)
> (In reply to Myles C. Maxfield from comment #5)
> > Conditional continues and breaks should contribute to the uniformity of the
> > blocks they target
> 
> This isn't quite right - the continue & break can affect just the remainder
> of the scope.
> 

and "return"

In general, returns should be modeled as an assignment to a variable that's outside the function scope.

[Myles C. Maxfield]

(In reply to Myles C. Maxfield from comment #3)
> Functions that return uniform variables must only have a single return point

Actually, we can do better - just treat early returns as marking the rest of the scope non-uniform, and treating the return as writing to a value that's outside the scope

[Myles C. Maxfield]

*** Bug 189123 has been marked as a duplicate of this bug. ***

[Myles C. Maxfield]

Created attachment 351650 [details]
WIP

[Myles C. Maxfield]

Created attachment 351711 [details]
WIP

[Myles C. Maxfield]

Created attachment 351866 [details]
WIP

[Myles C. Maxfield]

Created attachment 351868 [details]
WIP

[Myles C. Maxfield]

Migrated to https://github.com/gpuweb/WHLSL/issues/56