[Impeller] exploit content context options' perfect hash function. (flutter/engine#56360)

The content context options hash function is pefect - meaning a distinct hash guarantees a distinct value and vice versa. We can replace the hashing entirely with an equality check of the hash function.

Additionally, we can remove the hashmap. As most of these maps have fewer than a dozen entries (often just 2 or 3) and the linear search is much faster.

engine/src/flutter/impeller/entity/contents/content_context.cc

@@ -8,7 +8,6 @@
 #include <utility>
 
 #include "fml/trace_event.h"
-#include "impeller/base/strings.h"
 #include "impeller/base/validation.h"
 #include "impeller/core/formats.h"
 #include "impeller/core/texture_descriptor.h"

engine/src/flutter/impeller/entity/contents/content_context.h

@@ -315,47 +315,27 @@ struct ContentContextOptions {
   bool wireframe = false;
   bool is_for_rrect_blur_clear = false;
 
-  struct Hash {
-    constexpr uint64_t operator()(const ContentContextOptions& o) const {
-      static_assert(sizeof(o.sample_count) == 1);
-      static_assert(sizeof(o.blend_mode) == 1);
-      static_assert(sizeof(o.sample_count) == 1);
-      static_assert(sizeof(o.depth_compare) == 1);
-      static_assert(sizeof(o.stencil_mode) == 1);
-      static_assert(sizeof(o.primitive_type) == 1);
-      static_assert(sizeof(o.color_attachment_pixel_format) == 1);
+  constexpr uint64_t ToKey() const {
+    static_assert(sizeof(sample_count) == 1);
+    static_assert(sizeof(blend_mode) == 1);
+    static_assert(sizeof(sample_count) == 1);
+    static_assert(sizeof(depth_compare) == 1);
+    static_assert(sizeof(stencil_mode) == 1);
+    static_assert(sizeof(primitive_type) == 1);
+    static_assert(sizeof(color_attachment_pixel_format) == 1);
 
-      return (o.is_for_rrect_blur_clear ? 1llu : 0llu) << 0 |
-             (o.wireframe ? 1llu : 0llu) << 1 |
-             (o.has_depth_stencil_attachments ? 1llu : 0llu) << 2 |
-             (o.depth_write_enabled ? 1llu : 0llu) << 3 |
-             // enums
-             static_cast<uint64_t>(o.color_attachment_pixel_format) << 8 |
-             static_cast<uint64_t>(o.primitive_type) << 16 |
-             static_cast<uint64_t>(o.stencil_mode) << 24 |
-             static_cast<uint64_t>(o.depth_compare) << 32 |
-             static_cast<uint64_t>(o.blend_mode) << 40 |
-             static_cast<uint64_t>(o.sample_count) << 48;
-    }
-  };
-
-  struct Equal {
-    constexpr bool operator()(const ContentContextOptions& lhs,
-                              const ContentContextOptions& rhs) const {
-      return lhs.sample_count == rhs.sample_count &&
-             lhs.blend_mode == rhs.blend_mode &&
-             lhs.depth_write_enabled == rhs.depth_write_enabled &&
-             lhs.depth_compare == rhs.depth_compare &&
-             lhs.stencil_mode == rhs.stencil_mode &&
-             lhs.primitive_type == rhs.primitive_type &&
-             lhs.color_attachment_pixel_format ==
-                 rhs.color_attachment_pixel_format &&
-             lhs.has_depth_stencil_attachments ==
-                 rhs.has_depth_stencil_attachments &&
-             lhs.wireframe == rhs.wireframe &&
-             lhs.is_for_rrect_blur_clear == rhs.is_for_rrect_blur_clear;
-    }
-  };
+    return (is_for_rrect_blur_clear ? 1llu : 0llu) << 0 |
+           (wireframe ? 1llu : 0llu) << 1 |
+           (has_depth_stencil_attachments ? 1llu : 0llu) << 2 |
+           (depth_write_enabled ? 1llu : 0llu) << 3 |
+           // enums
+           static_cast<uint64_t>(color_attachment_pixel_format) << 8 |
+           static_cast<uint64_t>(primitive_type) << 16 |
+           static_cast<uint64_t>(stencil_mode) << 24 |
+           static_cast<uint64_t>(depth_compare) << 32 |
+           static_cast<uint64_t>(blend_mode) << 40 |
+           static_cast<uint64_t>(sample_count) << 48;
+  }
 
   void ApplyToPipelineDescriptor(PipelineDescriptor& desc) const;
 };
@@ -771,7 +751,7 @@ class ContentContext {
     struct Hash {
       std::size_t operator()(const RuntimeEffectPipelineKey& key) const {
         return fml::HashCombine(key.unique_entrypoint_name,
-                                ContentContextOptions::Hash{}(key.options));
+                                key.options.ToKey());
       }
     };
 
@@ -779,7 +759,7 @@ class ContentContext {
       constexpr bool operator()(const RuntimeEffectPipelineKey& lhs,
                                 const RuntimeEffectPipelineKey& rhs) const {
         return lhs.unique_entrypoint_name == rhs.unique_entrypoint_name &&
-               ContentContextOptions::Equal{}(lhs.options, rhs.options);
+               lhs.options.ToKey() == rhs.options.ToKey();
       }
     };
   };
@@ -810,7 +790,13 @@ class ContentContext {
 
     void Set(const ContentContextOptions& options,
              std::unique_ptr<PipelineHandleT> pipeline) {
-      pipelines_[options] = std::move(pipeline);
+      uint64_t p_key = options.ToKey();
+      for (const auto& [key, pipeline] : pipelines_) {
+        if (key == p_key) {
+          return;
+        }
+      }
+      pipelines_.push_back(std::make_pair(p_key, std::move(pipeline)));
     }
 
     void SetDefault(const ContentContextOptions& options,
@@ -833,8 +819,11 @@ class ContentContext {
     }
 
     PipelineHandleT* Get(const ContentContextOptions& options) const {
-      if (auto found = pipelines_.find(options); found != pipelines_.end()) {
-        return found->second.get();
+      uint64_t p_key = options.ToKey();
+      for (const auto& [key, pipeline] : pipelines_) {
+        if (key == p_key) {
+          return pipeline.get();
+        }
       }
       return nullptr;
     }
@@ -850,10 +839,7 @@ class ContentContext {
 
    private:
     std::optional<ContentContextOptions> default_options_;
-    std::unordered_map<ContentContextOptions,
-                       std::unique_ptr<PipelineHandleT>,
-                       ContentContextOptions::Hash,
-                       ContentContextOptions::Equal>
+    std::vector<std::pair<uint64_t, std::unique_ptr<PipelineHandleT>>>
         pipelines_;
 
     Variants(const Variants&) = delete;

engine/src/flutter/impeller/entity/entity_unittests.cc

@@ -2170,16 +2170,16 @@ TEST_P(EntityTest, DecalSpecializationAppliedToMorphologyFilter) {
 // set of options, we can just compare benchmarks.
 TEST_P(EntityTest, ContentContextOptionsHasReasonableHashFunctions) {
   ContentContextOptions opts;
-  auto hash_a = ContentContextOptions::Hash{}(opts);
+  auto hash_a = opts.ToKey();
 
   opts.blend_mode = BlendMode::kColorBurn;
-  auto hash_b = ContentContextOptions::Hash{}(opts);
+  auto hash_b = opts.ToKey();
 
   opts.has_depth_stencil_attachments = false;
-  auto hash_c = ContentContextOptions::Hash{}(opts);
+  auto hash_c = opts.ToKey();
 
   opts.primitive_type = PrimitiveType::kPoint;
-  auto hash_d = ContentContextOptions::Hash{}(opts);
+  auto hash_d = opts.ToKey();
 
   EXPECT_NE(hash_a, hash_b);
   EXPECT_NE(hash_b, hash_c);