Add support for pipeline-overridable constants in web backend (#5688)

* Add support for pipeline-overridable constants in WebGPU

* Add utility function for setting constants map

* Panic on failure to set constants map

---------

Co-authored-by: Andreas Reich <r_andreas2@web.de>

CHANGELOG.md

@@ -111,6 +111,10 @@ By @stefnotch in [#5410](https://github.com/gfx-rs/wgpu/pull/5410)
 - Replace `glClear` with `glClearBufferF` because `glDrawBuffers` requires that the ith buffer must be `COLOR_ATTACHMENTi` or `NONE` [#5666](https://github.com/gfx-rs/wgpu/pull/5666)
 - Return the unmodified version in driver_info. By @Valaphee in [#5753](https://github.com/gfx-rs/wgpu/pull/5753)
 
+#### WebGPU
+
+- Added support for pipeline-overridable constants to the WebGPU backend by @DouglasDwyer in [#5688](https://github.com/gfx-rs/wgpu/pull/5688)
+
 ## v0.20.0 (2024-04-28)
 
 ### Major Changes

wgpu/src/backend/webgpu.rs

@@ -7,6 +7,7 @@ use js_sys::Promise;
 use std::{
     any::Any,
     cell::RefCell,
+    collections::HashMap,
     fmt,
     future::Future,
     marker::PhantomData,
@@ -1876,6 +1877,10 @@ impl crate::context::Context for ContextWebGpu {
         let module: &<ContextWebGpu as crate::Context>::ShaderModuleData =
             downcast_ref(desc.vertex.module.data.as_ref());
         let mut mapped_vertex_state = webgpu_sys::GpuVertexState::new(&module.0.module);
+        insert_constants_map(
+            &mapped_vertex_state,
+            desc.vertex.compilation_options.constants,
+        );
         mapped_vertex_state.entry_point(desc.vertex.entry_point);
 
         let buffers = desc
@@ -1952,6 +1957,7 @@ impl crate::context::Context for ContextWebGpu {
                 downcast_ref(frag.module.data.as_ref());
             let mut mapped_fragment_desc =
                 webgpu_sys::GpuFragmentState::new(&module.0.module, &targets);
+            insert_constants_map(&mapped_vertex_state, frag.compilation_options.constants);
             mapped_fragment_desc.entry_point(frag.entry_point);
             mapped_desc.fragment(&mapped_fragment_desc);
         }
@@ -1978,6 +1984,7 @@ impl crate::context::Context for ContextWebGpu {
             downcast_ref(desc.module.data.as_ref());
         let mut mapped_compute_stage =
             webgpu_sys::GpuProgrammableStage::new(&shader_module.0.module);
+        insert_constants_map(&mapped_compute_stage, desc.compilation_options.constants);
         mapped_compute_stage.entry_point(desc.entry_point);
         let auto_layout = wasm_bindgen::JsValue::from(webgpu_sys::GpuAutoLayoutMode::Auto);
         let mut mapped_desc = webgpu_sys::GpuComputePipelineDescriptor::new(
@@ -1994,6 +2001,7 @@ impl crate::context::Context for ContextWebGpu {
         if let Some(label) = desc.label {
             mapped_desc.label(label);
         }
+
         create_identified(device_data.0.create_compute_pipeline(&mapped_desc))
     }
 
@@ -3824,3 +3832,29 @@ impl Drop for BufferMappedRange {
         }
     }
 }
+
+/// Adds the constants map to the given pipeline descriptor if the map is nonempty.
+/// Panics if the map cannot be set.
+///
+/// This function is necessary because the constants array is not currently
+/// exposed by `wasm-bindgen`. See the following issues for details:
+/// - [gfx-rs/wgpu#5688](https://github.com/gfx-rs/wgpu/pull/5688)
+/// - [rustwasm/wasm-bindgen#3587](https://github.com/rustwasm/wasm-bindgen/issues/3587)
+fn insert_constants_map(target: &JsValue, map: &HashMap<String, f64>) {
+    if !map.is_empty() {
+        js_sys::Reflect::set(target, &"constants".into(), &hashmap_to_jsvalue(map))
+            .expect("Setting the values in a Javascript pipeline descriptor should never fail");
+    }
+}
+
+/// Converts a hashmap to a Javascript object.
+fn hashmap_to_jsvalue(map: &HashMap<String, f64>) -> JsValue {
+    let obj = js_sys::Object::new();
+
+    for (k, v) in map.iter() {
+        js_sys::Reflect::set(&obj, &k.into(), &(*v).into())
+            .expect("Setting the values in a Javascript map should never fail");
+    }
+
+    JsValue::from(obj)
+}