[spv-in]: Cull unused builtins inside structs (#1469)

Some compilers like shaderc introduce a full gl_PerVertex struct, this
includes gl_ClipDistance. Normally this isn't a problem since most
drivers optimize it away, but naga zero inits globals if they weren't
previously initialized. This causes gl_ClipDistance to be initialized to
zero which can be really bad for performance.

src/front/spv/function.rs

@@ -376,9 +376,16 @@ impl<I: Iterator<Item = u32>> super::Parser<I> {
  ..
  } => {
  for (index, sm) in sub_members.iter().enumerate() {
- if sm.binding.is_none() {
+ match sm.binding {
+ Some(crate::Binding::BuiltIn(builtin)) => {
+ // Cull unused builtins to preserve performances
+ if !self.builtin_usage.contains(&builtin) {
+ continue;
+ }
+ }
  // unrecognized binding, skip
- continue;
+ None => continue,
+ _ => {}
  }
  members.push(sm.clone());
  components.push(function.expressions.append(

src/front/spv/mod.rs

@@ -37,7 +37,7 @@ use function::*;
 use crate::{
  arena::{Arena, Handle, UniqueArena},
  proc::Layouter,
- FastHashMap,
+ FastHashMap, FastHashSet,
 };
 
 use num_traits::cast::FromPrimitive;
@@ -549,6 +549,10 @@ pub struct Parser<I> {
  (BodyIndex, Vec<i32>),
  std::hash::BuildHasherDefault<fxhash::FxHasher>,
  >,
+
+ /// Tracks usage of builtins, used to cull unused builtins since they can
+ /// have serious performance implications.
+ builtin_usage: FastHashSet<crate::BuiltIn>,
 }
 
 impl<I: Iterator<Item = u32>> Parser<I> {
@@ -582,6 +586,7 @@ impl<I: Iterator<Item = u32>> Parser<I> {
  index_constants: Vec::new(),
  index_constant_expressions: Vec::new(),
  switch_cases: indexmap::IndexMap::default(),
+ builtin_usage: FastHashSet::default(),
  }
  }
 
@@ -1294,9 +1299,10 @@ impl<I: Iterator<Item = u32>> Parser<I> {
  let type_lookup = self.lookup_type.lookup(acex.type_id)?;
  acex = match ctx.type_arena[type_lookup.handle].inner {
  // can only index a struct with a constant
- crate::TypeInner::Struct { .. } => {
+ crate::TypeInner::Struct { ref members, .. } => {
  let index = index_maybe
  .ok_or_else(|| Error::InvalidAccess(index_expr_data.clone()))?;
+
  let lookup_member = self
  .lookup_member
  .get(&(type_lookup.handle, index))
@@ -1308,6 +1314,13 @@ impl<I: Iterator<Item = u32>> Parser<I> {
  },
  span,
  );
+
+ if let Some(crate::Binding::BuiltIn(builtin)) =
+ members[index as usize].binding
+ {
+ self.builtin_usage.insert(builtin);
+ }
+
  AccessExpression {
  base_handle,
  type_id: lookup_member.type_id,

tests/out/glsl/quad-vert.main.Vertex.glsl

@@ -6,9 +6,6 @@ precision highp int;
 struct type9 {
  vec2 member;
  vec4 gen_gl_Position;
- float gen_gl_PointSize;
- float gen_gl_ClipDistance[1];
- float gen_gl_CullDistance[1];
 };
 
 vec2 v_uv = vec2(0.0, 0.0);
@@ -42,12 +39,9 @@ void main() {
  a_uv1 = a_uv;
  a_pos1 = a_pos;
  main2();
- vec2 _e10 = v_uv;
- vec4 _e11 = perVertexStruct.gen_gl_Position;
- float _e12 = perVertexStruct.gen_gl_PointSize;
- float _e13[1] = perVertexStruct.gen_gl_ClipDistance;
- float _e14[1] = perVertexStruct.gen_gl_CullDistance;
- type9 _tmp_return = type9(_e10, _e11, _e12, _e13, _e14);
+ vec2 _e7 = v_uv;
+ vec4 _e8 = perVertexStruct.gen_gl_Position;
+ type9 _tmp_return = type9(_e7, _e8);
  _vs2fs_location0 = _tmp_return.member;
  gl_Position = _tmp_return.gen_gl_Position;
  gl_Position.yz = vec2(-gl_Position.y, gl_Position.z * 2.0 - gl_Position.w);

tests/out/hlsl/quad-vert.hlsl

@@ -9,9 +9,6 @@ struct gl_PerVertex {
 struct type9 {
  linear float2 member : LOC0;
  float4 gl_Position : SV_Position;
- float gl_PointSize : PSIZE;
- float gl_ClipDistance[1] : SV_ClipDistance;
- float gl_CullDistance[1] : SV_CullDistance;
 };
 
 static float2 v_uv = (float2)0;
@@ -22,9 +19,6 @@ static float2 a_pos1 = (float2)0;
 struct VertexOutput_main {
  float2 member : LOC0;
  float4 gl_Position : SV_Position;
- float gl_ClipDistance : SV_ClipDistance;
- float gl_CullDistance : SV_CullDistance;
- float gl_PointSize : PSIZE;
 };
 
 void main1()
@@ -36,13 +30,10 @@ void main1()
  return;
 }
 
-type9 Constructtype9(float2 arg0, float4 arg1, float arg2, float arg3[1], float arg4[1]) {
+type9 Constructtype9(float2 arg0, float4 arg1) {
  type9 ret;
  ret.member = arg0;
  ret.gl_Position = arg1;
- ret.gl_PointSize = arg2;
- ret.gl_ClipDistance = arg3;
- ret.gl_CullDistance = arg4;
  return ret;
 }
 
@@ -51,12 +42,9 @@ VertexOutput_main main(float2 a_uv : LOC1, float2 a_pos : LOC0)
  a_uv1 = a_uv;
  a_pos1 = a_pos;
  main1();
- float2 _expr10 = v_uv;
- float4 _expr11 = perVertexStruct.gl_Position;
- float _expr12 = perVertexStruct.gl_PointSize;
- float _expr13[1] = perVertexStruct.gl_ClipDistance;
- float _expr14[1] = perVertexStruct.gl_CullDistance;
- const type9 type9 = Constructtype9(_expr10, _expr11, _expr12, _expr13, _expr14);
- const VertexOutput_main type9_1 = { type9.member, type9.gl_Position, type9.gl_ClipDistance, type9.gl_CullDistance, type9.gl_PointSize };
+ float2 _expr7 = v_uv;
+ float4 _expr8 = perVertexStruct.gl_Position;
+ const type9 type9 = Constructtype9(_expr7, _expr8);
+ const VertexOutput_main type9_1 = { type9.member, type9.gl_Position };
  return type9_1;
 }

tests/out/msl/quad-vert.msl

@@ -14,9 +14,6 @@ struct gl_PerVertex {
 struct type9 {
  metal::float2 member;
  metal::float4 gl_Position;
- float gl_PointSize;
- type5 gl_ClipDistance;
- type5 gl_CullDistance;
 };
 constant metal::float4 const_type3 = {0.0, 0.0, 0.0, 1.0};
 constant type5 const_type5 = {0.0};
@@ -42,8 +39,6 @@ struct main1Input {
 struct main1Output {
  metal::float2 member [[user(loc0), center_perspective]];
  metal::float4 gl_Position [[position]];
- float gl_PointSize [[point_size]];
- float gl_ClipDistance [[clip_distance]] [1];
 };
 vertex main1Output main1(
  main1Input varyings [[stage_in]]
@@ -57,11 +52,8 @@ vertex main1Output main1(
  a_uv1 = a_uv;
  a_pos1 = a_pos;
  main2(v_uv, a_uv1, perVertexStruct, a_pos1);
- metal::float2 _e10 = v_uv;
- metal::float4 _e11 = perVertexStruct.gl_Position;
- float _e12 = perVertexStruct.gl_PointSize;
- type5 _e13 = perVertexStruct.gl_ClipDistance;
- type5 _e14 = perVertexStruct.gl_CullDistance;
- const auto _tmp = type9 {_e10, _e11, _e12, _e13, _e14};
- return main1Output { _tmp.member, _tmp.gl_Position, _tmp.gl_PointSize, {_tmp.gl_ClipDistance.inner[0]} };
+ metal::float2 _e7 = v_uv;
+ metal::float4 _e8 = perVertexStruct.gl_Position;
+ const auto _tmp = type9 {_e7, _e8};
+ return main1Output { _tmp.member, _tmp.gl_Position };
 }

tests/out/wgsl/quad-vert.wgsl

@@ -26,7 +26,7 @@ fn main([[location(1)]] a_uv: vec2<f32>, [[location(0)]] a_pos: vec2<f32>) -> Ve
  a_uv1 = a_uv;
  a_pos1 = a_pos;
  main1();
- let e10: vec2<f32> = v_uv;
- let e11: vec4<f32> = perVertexStruct.gl_Position;
- return VertexOutput(e10, e11);
+ let e7: vec2<f32> = v_uv;
+ let e8: vec4<f32> = perVertexStruct.gl_Position;
+ return VertexOutput(e7, e8);
 }