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

[Timothy Arceri]

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?