WASM: Set CONFIG_MAX_INSTANCES to a small value + hack the GLSL.

Temporary fix until spec constants arrive.

Second attempt, fixes #5859.

RELEASE_NOTES.md

@@ -5,6 +5,7 @@ A new header is inserted each time a *tag* is created.
 
 ## main branch
 
+- WebGL: reduce max instance count to work around Chrome issues [⚠️ **Recompile Materials**]
 - engine: rework material/shader sampler binding code [⚠️ **Recompile Materials**]
 
 ## v1.26.0

filament/backend/src/opengl/OpenGLProgram.cpp

@@ -131,6 +131,34 @@ void OpenGLProgram::compileShaders(OpenGLContext& context,
                 }
             }
 
+#ifdef __EMSCRIPTEN__
+            // HACK ALERT
+            // ----------
+            // The values below are based on CONFIG_MAX_INSTANCES, which is not accessible here.
+            // Since matc cannot know if it's building for WebGL, we replace the constant at run
+            // time. This will go away after we support specialization constants. The name of the
+            // struct field might be changed by the GLSL optimizer, so we search for any codeline
+            // that has the pattern: "PerRenderableData(.*)[64]"
+            const std::string_view type = "PerRenderableData";
+            const std::string_view source = "[64]";
+            const std::string_view target = "[8] ";
+            assert_invariant(source.size() == target.size());
+            const size_t npos = std::string_view::npos;
+            for (size_t typePos = shaderView.find(type); typePos != npos;) {
+                size_t newlinePos = shaderView.find('\n', typePos + type.size());
+                std::string_view codeline = shaderView.substr(typePos, newlinePos - typePos);
+                if (size_t sourcePos = codeline.find(source, type.size()); sourcePos != npos) {
+                    if (temp.empty()) {
+                        temp = shaderView;
+                    }
+                    target.copy(temp.data() + typePos + sourcePos, target.size());
+                    shaderView = temp;
+                    break;
+                }
+                typePos = shaderView.find(type, newlinePos + 1);
+            }
+#endif
+
             // Tragically, OpenGL 4.1 doesn't support unpackHalf2x16 and
             // MacOS doesn't support GL_ARB_shading_language_packing
             if constexpr (BACKEND_OPENGL_VERSION == BACKEND_OPENGL_VERSION_GL) {

libs/filabridge/include/private/filament/EngineEnums.h

@@ -65,8 +65,18 @@ constexpr size_t CONFIG_MAX_SHADOW_CASCADES = 4;
 // instances, and shadow casting spotlights.
 constexpr size_t CONFIG_MINSPEC_UBO_SIZE = 16384;
 
+// HACK ALERT
+// ----------
+// If you modify the values below, you will need to update the hack in OpenGLProgram.cpp!
 // The maximum number of instances that Filament automatically creates as an optimization.
+// Use a much smaller number for WebGL as a workaround for the following Chrome issues:
+//     https://crbug.com/1348017 Compiling GLSL is very slow with struct arrays
+//     https://crbug.com/1348363 Lighting looks wrong with D3D11 but not OpenGL
+#if defined(__EMSCRIPTEN__)
+constexpr size_t CONFIG_MAX_INSTANCES = 8;
+#else
 constexpr size_t CONFIG_MAX_INSTANCES = 64;
+#endif
 
 // The maximum number of bones that can be associated with a single renderable.
 // We store 32 bytes per bone. Must be a power-of-two, and must fit within CONFIG_MINSPEC_UBO_SIZE.