From 4cd3d1917b52bb1befd077661a739ff35f19b7db Mon Sep 17 00:00:00 2001
From: Andy Hovingh <6198728+AndyHovingh@users.noreply.github.com>
Date: Fri, 2 May 2025 14:09:00 -0500
Subject: [PATCH] webgpu: do not include constants that are absent in the WGSL

---
 filament/backend/src/webgpu/WGPUProgram.cpp | 67 +++++++++++++--------
 1 file changed, 41 insertions(+), 26 deletions(-)

diff --git a/filament/backend/src/webgpu/WGPUProgram.cpp b/filament/backend/src/webgpu/WGPUProgram.cpp
index 5a62874557..6828ad123b 100644
--- a/filament/backend/src/webgpu/WGPUProgram.cpp
+++ b/filament/backend/src/webgpu/WGPUProgram.cpp
@@ -27,6 +27,7 @@
 
 #include <webgpu/webgpu_cpp.h>
 
+#include <algorithm>
 #include <sstream>
 #include <string_view>
 #include <vector>
@@ -125,50 +126,64 @@ namespace {
     return module;
 }
 
-wgpu::StringView getSpecConstantStringValue(uint32_t value) {
-    switch (value) {
+// This is a 1 to 1 mapping of the ReservedSpecializationConstants enum in EngineEnums.h
+// The _hack is a workaround until https://issues.chromium.org/issues/42250586 is resolved
+// This workaround is the same one being used on the generateSpecializationConstant() function
+wgpu::StringView getSpecConstantStringId(uint32_t id) {
+    switch (id) {
         case 0:
-            return "BACKEND_FEATURE_LEVEL_hack";
+            return "0";// BACKEND_FEATURE_LEVEL_hack
         case 1:
-            return "CONFIG_MAX_INSTANCES_hack";
+            return "1";// CONFIG_MAX_INSTANCES_hack
         case 2:
-            return "CONFIG_STATIC_TEXTURE_TARGET_WORKAROUND_hack";
+            return "2";// ONFIG_STATIC_TEXTURE_TARGET_WORKAROUND_hack
         case 3:
-            return "CONFIG_SRGB_SWAPCHAIN_EMULATION_hack";
+            return "3";// CONFIG_SRGB_SWAPCHAIN_EMULATION_hack
         case 4:
-            return "CONFIG_FROXEL_BUFFER_HEIGHT_hack";
+            return "4";// CONFIG_FROXEL_BUFFER_HEIGHT_hack
         case 5:
-            return "CONFIG_POWER_VR_SHADER_WORKAROUNDS_hack";
+            return "5";// CONFIG_POWER_VR_SHADER_WORKAROUNDS_hack
         case 6:
-            return "CONFIG_DEBUG_DIRECTIONAL_SHADOWMAP_hack";
+            return "6";// CONFIG_DEBUG_DIRECTIONAL_SHADOWMAP_hack
         case 7:
-            return "CONFIG_DEBUG_FROXEL_VISUALIZATION_hack";
+            return "7";// CONFIG_DEBUG_FROXEL_VISUALIZATION_hack
         case 8:
-            return "CONFIG_STEREO_EYE_COUNT_hack";
+            return "8";// CONFIG_STEREO_EYE_COUNT_hack
         case 9:
-            return "CONFIG_SH_BANDS_COUNT_hack";
+            return "9";// CONFIG_SH_BANDS_COUNT_hack
         case 10:
-            return "CONFIG_SHADOW_SAMPLING_METHOD_hack";
+            return "10";// CONFIG_SHADOW_SAMPLING_METHOD_hack
+        default:
+            PANIC_POSTCONDITION("Unknown/unhandled spec constant key/id: %d", id);
     }
-    return "";
 }
 
 std::vector<wgpu::ConstantEntry> convertConstants(
         utils::FixedCapacityVector<filament::backend::Program::SpecializationConstant> const&
                 constantsInfo) {
-    std::vector<wgpu::ConstantEntry> constants(constantsInfo.size());
-    return constants;
-    for (size_t i = 0; i < constantsInfo.size(); i++) {
-        filament::backend::Program::SpecializationConstant const& specConstant = constantsInfo[i];
-        wgpu::ConstantEntry& constantEntry = constants[i];
-        constantEntry.key = getSpecConstantStringValue(specConstant.id);
-        if (auto* v = std::get_if<int32_t>(&specConstant.value)) {
-            constantEntry.value = static_cast<double>(*v);
-        } else if (auto* f = std::get_if<float>(&specConstant.value)) {
-            constantEntry.value = static_cast<double>(*f);
-        } else if (auto* b = std::get_if<bool>(&specConstant.value)) {
-            constantEntry.value = *b ? 0.0 : 1.0;
+    std::vector<wgpu::ConstantEntry> constants;
+    constants.reserve(constantsInfo.size());
+    for (filament::backend::Program::SpecializationConstant const& constant: constantsInfo) {
+        // CONFIG_MAX_INSTANCES (1) and CONFIG_FROXEL_BUFFER_HEIGHT (4) will not be present
+        // as constant overrides in the generated WGSL, because WGSL doesn't support specialization
+        // constants as an array length
+        // More information at https://github.com/gpuweb/gpuweb/issues/572#issuecomment-649760005
+        // CONFIG_SRGB_SWAPCHAIN_EMULATION (3) is being skipped all together since it's only
+        // included for the case of mFeatureLevel == FeatureLevel::FEATURE_LEVEL_0, which should
+        // not be possible for WebGPU
+        if (constant.id == 1 || constant.id == 3 || constant.id == 4) {
+            continue;
         }
+        double value = 0.0;
+        if (auto* v = std::get_if<int32_t>(&constant.value)) {
+            value = static_cast<double>(*v);
+        } else if (auto* f = std::get_if<float>(&constant.value)) {
+            value = static_cast<double>(*f);
+        } else if (auto* b = std::get_if<bool>(&constant.value)) {
+            value = *b ? 0.0 : 1.0;
+        }
+        constants.push_back(
+                wgpu::ConstantEntry{ .key = getSpecConstantStringId(constant.id), .value = value });
     }
     return constants;
 }