[naga wgsl-in] Do not eagerly concretize global const declarations of…

… abstract types Instead allow the const to be converted and each time it is const evaluated as part of another expression. This allows an abstract const to be used as a different type depending on the context. A consequence of this is that abstract types may now find their way in to the IR, which we don't want. We therefore additionally now ensure that the compact pass removes global constants of abstract types. This will have no *functional* effect on shaders generated by the backends, as the expressions belonging to the abstract consts in the IR will not actually be used, as any usage in the input shader will have been const-evaluated away. Certain unused const declarations will now be removed, however, as can be seen by the effect on the snapshot outputs.
jamienicol · Feb 12, 2025 · efe6402 · efe6402
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -85,6 +85,7 @@ By @brodycj in [#6924](https://github.com/gfx-rs/wgpu/pull/6924).
 - Fix some instances of functions which have a return type but don't return a value being incorrectly validated. By @jamienicol in [#7013](https://github.com/gfx-rs/wgpu/pull/7013).
 - Allow abstract expressions to be used in WGSL function return statements. By @jamienicol in [#7035](https://github.com/gfx-rs/wgpu/pull/7035).
 - Error if structs have two fields with the same name. By @SparkyPotato in [#7088](https://github.com/gfx-rs/wgpu/pull/7088).
+- Allow WGSL const declarations to have abstract types. By @jamienicol in [#7055](https://github.com/gfx-rs/wgpu/pull/7055).
 
 #### General
 

diff --git a/naga/src/compact/mod.rs b/naga/src/compact/mod.rs
@@ -48,13 +48,24 @@ pub fn compact(module: &mut crate::Module) {
     log::trace!("tracing special types");
     module_tracer.trace_special_types(&module.special_types);
 
-    // We treat all named constants as used by definition.
+    // We treat all named constants as used by definition, unless they have an
+    // abstract type as we do not want those reaching the IR.
     log::trace!("tracing named constants");
     for (handle, constant) in module.constants.iter() {
         if constant.name.is_some() {
             log::trace!("tracing constant {:?}", constant.name.as_ref().unwrap());
-            module_tracer.constants_used.insert(handle);
-            module_tracer.global_expressions_used.insert(constant.init);
+            let mut ty = constant.ty;
+            while let crate::TypeInner::Array { base, .. } = module.types[ty].inner {
+                ty = base;
+            }
+            if !module.types[ty]
+                .inner
+                .scalar()
+                .is_some_and(|s| s.is_abstract())
+            {
+                module_tracer.constants_used.insert(handle);
+                module_tracer.global_expressions_used.insert(constant.init);
+            }
         }
     }
 

diff --git a/naga/src/front/wgsl/lower/mod.rs b/naga/src/front/wgsl/lower/mod.rs
@@ -1072,8 +1072,13 @@ impl<'source, 'temp> Lowerer<'source, 'temp> {
                         v.ty.map(|ast| self.resolve_ast_type(ast, &mut ctx))
                             .transpose()?;
 
-                    let (ty, initializer) =
-                        self.type_and_init(v.name, v.init, explicit_ty, &mut ctx.as_override())?;
+                    let (ty, initializer) = self.type_and_init(
+                        v.name,
+                        v.init,
+                        explicit_ty,
+                        false,
+                        &mut ctx.as_override(),
+                    )?;
 
                     let binding = if let Some(ref binding) = v.binding {
                         Some(crate::ResourceBinding {
@@ -1106,7 +1111,7 @@ impl<'source, 'temp> Lowerer<'source, 'temp> {
                             .transpose()?;
 
                     let (ty, init) =
-                        self.type_and_init(c.name, Some(c.init), explicit_ty, &mut ectx)?;
+                        self.type_and_init(c.name, Some(c.init), explicit_ty, true, &mut ectx)?;
                     let init = init.expect("Global const must have init");
 
                     let handle = ctx.module.constants.append(
@@ -1128,7 +1133,8 @@ impl<'source, 'temp> Lowerer<'source, 'temp> {
 
                     let mut ectx = ctx.as_override();
 
-                    let (ty, init) = self.type_and_init(o.name, o.init, explicit_ty, &mut ectx)?;
+                    let (ty, init) =
+                        self.type_and_init(o.name, o.init, explicit_ty, false, &mut ectx)?;
 
                     let id =
                         o.id.map(|id| self.const_u32(id, &mut ctx.as_const()))
@@ -1201,6 +1207,7 @@ impl<'source, 'temp> Lowerer<'source, 'temp> {
         name: ast::Ident<'source>,
         init: Option<Handle<ast::Expression<'source>>>,
         explicit_ty: Option<Handle<crate::Type>>,
+        allow_abstract: bool,
         ectx: &mut ExpressionContext<'source, '_, '_>,
     ) -> Result<(Handle<crate::Type>, Option<Handle<crate::Expression>>), Error<'source>> {
         let ty;
@@ -1234,9 +1241,12 @@ impl<'source, 'temp> Lowerer<'source, 'temp> {
                 initializer = Some(init);
             }
             (Some(init), None) => {
-                let concretized = self.expression(init, ectx)?;
-                ty = ectx.register_type(concretized)?;
-                initializer = Some(concretized);
+                let mut init = self.expression_for_abstract(init, ectx)?;
+                if !allow_abstract {
+                    init = ectx.concretize(init)?;
+                }
+                ty = ectx.register_type(init)?;
+                initializer = Some(init);
             }
             (None, Some(explicit_ty)) => {
                 ty = explicit_ty;
@@ -1477,7 +1487,7 @@ impl<'source, 'temp> Lowerer<'source, 'temp> {
                     emitter.start(&ctx.function.expressions);
                     let mut ectx = ctx.as_expression(block, &mut emitter);
                     let (ty, initializer) =
-                        self.type_and_init(v.name, v.init, explicit_ty, &mut ectx)?;
+                        self.type_and_init(v.name, v.init, explicit_ty, false, &mut ectx)?;
 
                     let (const_initializer, initializer) = {
                         match initializer {
@@ -1537,7 +1547,7 @@ impl<'source, 'temp> Lowerer<'source, 'temp> {
                             .transpose()?;
 
                     let (_ty, init) =
-                        self.type_and_init(c.name, Some(c.init), explicit_ty, ectx)?;
+                        self.type_and_init(c.name, Some(c.init), explicit_ty, false, ectx)?;
                     let init = init.expect("Local const must have init");
 
                     block.extend(emitter.finish(&ctx.function.expressions));

diff --git a/naga/tests/out/glsl/constructors.main.Compute.glsl b/naga/tests/out/glsl/constructors.main.Compute.glsl
@@ -9,7 +9,6 @@ struct Foo {
     vec4 a;
     int b;
 };
-const vec3 const2_ = vec3(0.0, 1.0, 2.0);
 const mat2x2 const3_ = mat2x2(vec2(0.0, 1.0), vec2(2.0, 3.0));
 const mat2x2 const4_[1] = mat2x2[1](mat2x2(vec2(0.0, 1.0), vec2(2.0, 3.0)));
 const bool cz0_ = false;
@@ -20,7 +19,6 @@ const uvec2 cz4_ = uvec2(0u);
 const mat2x2 cz5_ = mat2x2(0.0);
 const Foo cz6_[3] = Foo[3](Foo(vec4(0.0), 0), Foo(vec4(0.0), 0), Foo(vec4(0.0), 0));
 const Foo cz7_ = Foo(vec4(0.0), 0);
-const int cp3_[4] = int[4](0, 1, 2, 3);
 
 
 void main() {

diff --git a/naga/tests/out/hlsl/constructors.hlsl b/naga/tests/out/hlsl/constructors.hlsl
@@ -12,12 +12,6 @@ ret_Constructarray1_float2x2_ Constructarray1_float2x2_(float2x2 arg0) {
     return ret;
 }
 
-typedef int ret_Constructarray4_int_[4];
-ret_Constructarray4_int_ Constructarray4_int_(int arg0, int arg1, int arg2, int arg3) {
-    int ret[4] = { arg0, arg1, arg2, arg3 };
-    return ret;
-}
-
 bool ZeroValuebool() {
     return (bool)0;
 }
@@ -51,7 +45,6 @@ Foo ZeroValueFoo() {
     return (Foo)0;
 }
 
-static const float3 const2_ = float3(0.0, 1.0, 2.0);
 static const float2x2 const3_ = float2x2(float2(0.0, 1.0), float2(2.0, 3.0));
 static const float2x2 const4_[1] = Constructarray1_float2x2_(float2x2(float2(0.0, 1.0), float2(2.0, 3.0)));
 static const bool cz0_ = ZeroValuebool();
@@ -62,7 +55,6 @@ static const uint2 cz4_ = ZeroValueuint2();
 static const float2x2 cz5_ = ZeroValuefloat2x2();
 static const Foo cz6_[3] = ZeroValuearray3_Foo_();
 static const Foo cz7_ = ZeroValueFoo();
-static const int cp3_[4] = Constructarray4_int_(0, 1, 2, 3);
 
 Foo ConstructFoo(float4 arg0, int arg1) {
     Foo ret = (Foo)0;
@@ -71,6 +63,12 @@ Foo ConstructFoo(float4 arg0, int arg1) {
     return ret;
 }
 
+typedef int ret_Constructarray4_int_[4];
+ret_Constructarray4_int_ Constructarray4_int_(int arg0, int arg1, int arg2, int arg3) {
+    int ret[4] = { arg0, arg1, arg2, arg3 };
+    return ret;
+}
+
 float2x3 ZeroValuefloat2x3() {
     return (float2x3)0;
 }

diff --git a/naga/tests/out/ir/const_assert.compact.ron b/naga/tests/out/ir/const_assert.compact.ron
@@ -1,36 +1,14 @@
 (
-    types: [
-        (
-            name: None,
-            inner: Scalar((
-                kind: Sint,
-                width: 4,
-            )),
-        ),
-    ],
+    types: [],
     special_types: (
         ray_desc: None,
         ray_intersection: None,
         predeclared_types: {},
     ),
-    constants: [
-        (
-            name: Some("x"),
-            ty: 0,
-            init: 0,
-        ),
-        (
-            name: Some("y"),
-            ty: 0,
-            init: 1,
-        ),
-    ],
+    constants: [],
     overrides: [],
     global_variables: [],
-    global_expressions: [
-        Literal(I32(1)),
-        Literal(I32(2)),
-    ],
+    global_expressions: [],
     functions: [
         (
             name: Some("foo"),

diff --git a/naga/tests/out/ir/const_assert.ron b/naga/tests/out/ir/const_assert.ron
@@ -1,36 +1,14 @@
 (
-    types: [
-        (
-            name: None,
-            inner: Scalar((
-                kind: Sint,
-                width: 4,
-            )),
-        ),
-    ],
+    types: [],
     special_types: (
         ray_desc: None,
         ray_intersection: None,
         predeclared_types: {},
     ),
-    constants: [
-        (
-            name: Some("x"),
-            ty: 0,
-            init: 0,
-        ),
-        (
-            name: Some("y"),
-            ty: 0,
-            init: 1,
-        ),
-    ],
+    constants: [],
     overrides: [],
     global_variables: [],
-    global_expressions: [
-        Literal(I32(1)),
-        Literal(I32(2)),
-    ],
+    global_expressions: [],
     functions: [
         (
             name: Some("foo"),

diff --git a/naga/tests/out/ir/local-const.compact.ron b/naga/tests/out/ir/local-const.compact.ron
@@ -38,53 +38,28 @@
         predeclared_types: {},
     ),
     constants: [
-        (
-            name: Some("ga"),
-            ty: 0,
-            init: 0,
-        ),
         (
             name: Some("gb"),
             ty: 0,
-            init: 1,
+            init: 0,
         ),
         (
             name: Some("gc"),
             ty: 1,
-            init: 2,
+            init: 1,
         ),
         (
             name: Some("gd"),
             ty: 2,
-            init: 3,
-        ),
-        (
-            name: Some("ge"),
-            ty: 3,
-            init: 4,
-        ),
-        (
-            name: Some("gf"),
-            ty: 2,
-            init: 5,
+            init: 2,
         ),
     ],
     overrides: [],
     global_variables: [],
     global_expressions: [
-        Literal(I32(4)),
         Literal(I32(4)),
         Literal(U32(4)),
         Literal(F32(4.0)),
-        Compose(
-            ty: 3,
-            components: [
-                0,
-                0,
-                0,
-            ],
-        ),
-        Literal(F32(2.0)),
     ],
     functions: [
         (
@@ -106,12 +81,22 @@
                     ],
                 ),
                 Literal(F32(2.0)),
+                Literal(I32(4)),
                 Constant(0),
                 Constant(1),
                 Constant(2),
-                Constant(3),
-                Constant(4),
-                Constant(5),
+                Literal(I32(4)),
+                Literal(I32(4)),
+                Literal(I32(4)),
+                Compose(
+                    ty: 3,
+                    components: [
+                        10,
+                        11,
+                        12,
+                    ],
+                ),
+                Literal(F32(2.0)),
             ],
             named_expressions: {
                 0: "a",
@@ -124,14 +109,22 @@
                 7: "bg",
                 8: "cg",
                 9: "dg",
-                10: "eg",
-                11: "fg",
+                13: "eg",
+                14: "fg",
             },
             body: [
                 Emit((
                     start: 4,
                     end: 5,
                 )),
+                Emit((
+                    start: 0,
+                    end: 0,
+                )),
+                Emit((
+                    start: 13,
+                    end: 14,
+                )),
                 Return(
                     value: None,
                 ),