[Impeller] avoid hashing and std::vector growth when binding descriptor sets. (#45070)

We call allocateAndBindDescriptorSets once for every cmd rendered, so this time can bubble up quite a bit. From the traces I've gathered map emplacement (emplace_unique_key_args) and std::vector growth (__push_back_slow_path) contribute the most to this trace.

I think we have physical limits on the number of descriptors we can bind, though I'm not sure if we enforce that anywhere?

Author: Jonah Williams

impeller/renderer/backend/vulkan/render_pass_vk.cc

@@ -6,7 +6,6 @@
 
 #include <array>
 #include <cstdint>
-#include <unordered_map>
 #include <vector>
 
 #include "flutter/fml/logging.h"
@@ -339,9 +338,15 @@ static bool AllocateAndBindDescriptorSets(const ContextVK& context,
 
   auto& allocator = *context.GetResourceAllocator();
 
-  std::unordered_map<uint32_t, vk::DescriptorBufferInfo> buffers;
-  std::unordered_map<uint32_t, vk::DescriptorImageInfo> images;
+  std::vector<vk::DescriptorImageInfo> images;
+  std::vector<vk::DescriptorBufferInfo> buffers;
   std::vector<vk::WriteDescriptorSet> writes;
+  writes.reserve(command.vertex_bindings.buffers.size() +
+                 command.fragment_bindings.buffers.size() +
+                 command.fragment_bindings.sampled_images.size());
+  images.reserve(command.fragment_bindings.sampled_images.size());
+  buffers.reserve(command.vertex_bindings.buffers.size() +
+                  command.fragment_bindings.buffers.size());
 
   auto bind_images = [&encoder,     //
                       &images,      //
@@ -364,13 +369,14 @@ static bool AllocateAndBindDescriptorSets(const ContextVK& context,
       image_info.imageLayout = vk::ImageLayout::eShaderReadOnlyOptimal;
       image_info.sampler = sampler.GetSampler();
       image_info.imageView = texture_vk.GetImageView();
+      images.push_back(image_info);
 
       vk::WriteDescriptorSet write_set;
       write_set.dstSet = vk_desc_set.value();
       write_set.dstBinding = slot.binding;
       write_set.descriptorCount = 1u;
       write_set.descriptorType = vk::DescriptorType::eCombinedImageSampler;
-      write_set.pImageInfo = &(images[slot.binding] = image_info);
+      write_set.pImageInfo = &images.back();
 
       writes.push_back(write_set);
     }
@@ -409,6 +415,7 @@ static bool AllocateAndBindDescriptorSets(const ContextVK& context,
       buffer_info.buffer = buffer;
       buffer_info.offset = offset;
       buffer_info.range = data.view.resource.range.length;
+      buffers.push_back(buffer_info);
 
       const ShaderUniformSlot& uniform = data.slot;
       auto layout_it = std::find_if(desc_set.begin(), desc_set.end(),
@@ -427,7 +434,7 @@ static bool AllocateAndBindDescriptorSets(const ContextVK& context,
       write_set.dstBinding = uniform.binding;
       write_set.descriptorCount = 1u;
       write_set.descriptorType = ToVKDescriptorType(layout.descriptor_type);
-      write_set.pBufferInfo = &(buffers[uniform.binding] = buffer_info);
+      write_set.pBufferInfo = &buffers.back();
 
       writes.push_back(write_set);
     }