[Mesa-dev] [PATCH 42/61] nir: Add a concept of per-member structs and a lowering pass

[Timothy Arceri]

On 29/03/18 05:34, Jason Ekstrand wrote:
> On March 27, 2018 21:16:25 Timothy Arceri <tarceri at itsqueeze.com> wrote:
> 
> So I've been thinking about structs and I'm pretty sure we should be
> able to write some passes to completely lower them away.
> 
> vertex shader inputs, buffer block and shader interface blocks cannot
> contain structs so it seems to me the only blocker is the way we assign
> uniform and varying locations.
> 
> Currently for a struct like this:
> 
> struct S2 {
> int b;
> int c;
> };
> 
> struct S1 {
> int a;
> S2 s2[3];
> int d;
> };
> 
> uniform S1 s[2][2];
> 
> We store things like so:
> 
> s[0][0].a = location 0
> s[0][0].s[0].b = location 1
> s[0][0].s[0].c = location 2
> s[0][0].s[1].b = location 3
> s[0][0].s[1].c = location 4
> s[0][0].s[2].b = location 5
> s[0][0].s[2].c = location 6
> 
> ...
> 
> If we had a GLSL IR pass that pushed the arrays down to the innermost
> member like so:
> 
> struct S2 {
> int b[2][2][3];
> int c[2][2][3];
> };
> 
> struct S1 {
> int a[2][2];
> S2 s2;
> int d[2][2];
> };
> 
> uniform S1 s;
> 
> 
> We would instead store things like so:
> 
> s[0][0].a = location 0
> s[0][1].a = location 1
> s[1][0].a = location 2
> s[1][1].a = location 3
> s[0][0].s[0].b = location 4
> s[0][0].s[1].b = location 5
> s[0][0].s[2].b = location 6
> 
> ...
> 
> This allows us to easily split the members out into independent arrays
> of arrays.
> 
> To do this we might want to create the uniform (resource) name before
> pushing the arrays down so that we still match up the correct uniforms
> with the names passed to the API but that shouldn't be to difficult.
> 
> With this in place we should be able to generate better shaders when
> structs are used and be able to delete a whole bunch of struct handing
> code (and avoid this new code/concept?).
> 
> Does anyone see any holes in my analysis?
> 
> Only that it doesn't help for SPIR-V which is the whole reason for this 
> patch.
> 
> 

How does it not help SPIR-V? As per above it would seem the only reason 
we need to handle structs in NIR is because of the way we assign storage 
in OpenGL. I'm not as overly familiar with SPIR-V still but is there any 
reason we can't do the struct splitting pass in NIR and be done with 
structs?