[Impeller] order metal samplers according to declared order and not usage order

impeller/compiler/compiler.cc

@@ -20,6 +20,24 @@
 namespace impeller {
 namespace compiler {
 
+static std::optional<spv::ExecutionModel> SourceTypeToExecutionModel(
+    const SourceType& type) {
+  switch (type) {
+    case SourceType::kUnknown:
+      return std::nullopt;
+    case SourceType::kVertexShader:
+      return spv::ExecutionModel::ExecutionModelVertex;
+    case SourceType::kFragmentShader:
+      return spv::ExecutionModel::ExecutionModelFragment;
+    case SourceType::kTessellationControlShader:
+      return spv::ExecutionModel::ExecutionModelTessellationControl;
+    case SourceType::kTessellationEvaluationShader:
+      return spv::ExecutionModel::ExecutionModelTessellationEvaluation;
+    case SourceType::kComputeShader:
+      return spv::ExecutionModel::ExecutionModelGLCompute;
+  }
+}
+
 const uint32_t kFragBindingBase = 128;
 const size_t kNumUniformKinds =
     static_cast<int>(shaderc_uniform_kind::shaderc_uniform_kind_buffer) + 1;
@@ -34,7 +52,43 @@ static CompilerBackend CreateMSLCompiler(const spirv_cross::ParsedIR& ir,
   // Metal to AIR must be updated as well.
   sl_options.msl_version =
       spirv_cross::CompilerMSL::Options::make_msl_version(1, 2);
+  sl_options.enable_decoration_binding = true;
   sl_compiler->set_msl_options(sl_options);
+
+  // Set metal resource mappings to be consistent with location based mapping
+  // used on other backends when creating shaders.
+  std::vector<uint32_t> sampler_offsets;
+  ir.for_each_typed_id<spirv_cross::SPIRVariable>(
+      [&](uint32_t, const spirv_cross::SPIRVariable& var) {
+        if (var.storage != spv::StorageClassUniformConstant) {
+          return;
+        }
+        const auto spir_type = sl_compiler->get_type(var.basetype);
+        if (spir_type.basetype !=
+            spirv_cross::SPIRType::BaseType::SampledImage) {
+          return;
+        }
+        auto location = sl_compiler->get_decoration(
+            var.self, spv::Decoration::DecorationLocation);
+        sampler_offsets.push_back(location);
+      });
+  auto stage = SourceTypeToExecutionModel(source_options.type);
+  if (stage.has_value() && sampler_offsets.size() > 0) {
+    auto start_offset =
+        *std::min_element(sampler_offsets.begin(), sampler_offsets.end());
+    for (auto offset : sampler_offsets) {
+      sl_compiler->add_msl_resource_binding({
+          .stage = stage.value(),
+          .basetype = spirv_cross::SPIRType::BaseType::SampledImage,
+          .binding = offset,
+          .count = 1u,
+          .msl_buffer = offset - start_offset,
+          .msl_texture = offset - start_offset,
+          .msl_sampler = offset - start_offset,
+      });
+    }
+  }
+
   return CompilerBackend(sl_compiler);
 }