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

[Jason Ekstrand]

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.