Moved font glyph atlas to flat_hash_map (flutter/engine#56847)

In https://github.com/flutter/engine/pull/56844 we saw a 15% increase in performance by switching to absl::flat_hash_map.  I suspect we say see even better results ([source](https://martin.ankerl.com/2022/08/27/hashmap-bench-01)).  The absl guidance is to default to using this.

test exempt: no intentional functional change, just performance

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/impeller/typographer/BUILD.gn

@@ -34,6 +34,11 @@ impeller_component("typographer") {
     "../renderer",
   ]
 
+  if (!is_fuchsia) {
+    public_deps +=
+        [ "//flutter/third_party/abseil-cpp/absl/container:flat_hash_map" ]
+  }
+
   deps = [ "//flutter/fml" ]
 }
 

engine/src/flutter/impeller/typographer/font_glyph_pair.h

@@ -41,11 +41,10 @@ struct ScaledFont {
   Font font;
   Scalar scale;
 
-  struct Hash {
-    constexpr std::size_t operator()(const impeller::ScaledFont& sf) const {
-      return fml::HashCombine(sf.font.GetHash(), sf.scale);
-    }
-  };
+  template <typename H>
+  friend H AbslHashValue(H h, const ScaledFont& sf) {
+    return H::combine(std::move(h), sf.font.GetHash(), sf.scale);
+  }
 
   struct Equal {
     constexpr bool operator()(const impeller::ScaledFont& lhs,
@@ -70,19 +69,18 @@ struct SubpixelGlyph {
         subpixel_offset(p_subpixel_offset),
         properties(p_properties) {}
 
-  struct Hash {
-    constexpr std::size_t operator()(const impeller::SubpixelGlyph& sg) const {
-      if (!sg.properties.has_value()) {
-        return fml::HashCombine(sg.glyph.index, sg.subpixel_offset.x,
-                                sg.subpixel_offset.y);
-      }
-      return fml::HashCombine(
-          sg.glyph.index, sg.subpixel_offset.x, sg.subpixel_offset.y,
-          sg.properties->color.ToARGB(), sg.properties->stroke,
-          sg.properties->stroke_cap, sg.properties->stroke_join,
-          sg.properties->stroke_miter, sg.properties->stroke_width);
+  template <typename H>
+  friend H AbslHashValue(H h, const SubpixelGlyph& sg) {
+    if (!sg.properties.has_value()) {
+      return H::combine(std::move(h), sg.glyph.index, sg.subpixel_offset.x,
+                        sg.subpixel_offset.y);
     }
-  };
+    return H::combine(std::move(h), sg.glyph.index, sg.subpixel_offset.x,
+                      sg.subpixel_offset.y, sg.properties->color.ToARGB(),
+                      sg.properties->stroke, sg.properties->stroke_cap,
+                      sg.properties->stroke_join, sg.properties->stroke_miter,
+                      sg.properties->stroke_width);
+  }
 
   struct Equal {
     constexpr bool operator()(const impeller::SubpixelGlyph& lhs,

engine/src/flutter/impeller/typographer/glyph_atlas.cc

@@ -78,7 +78,9 @@ void GlyphAtlas::SetAtlasGeneration(size_t generation) {
 void GlyphAtlas::AddTypefaceGlyphPositionAndBounds(const FontGlyphPair& pair,
                                                    Rect position,
                                                    Rect bounds) {
-  font_atlas_map_[pair.scaled_font].positions_[pair.glyph] =
+  FontAtlasMap::iterator it = font_atlas_map_.find(pair.scaled_font);
+  FML_DCHECK(it != font_atlas_map_.end());
+  it->second.positions_[pair.glyph] =
       FrameBounds{position, bounds, /*is_placeholder=*/false};
 }
 
@@ -93,12 +95,9 @@ std::optional<FrameBounds> GlyphAtlas::FindFontGlyphBounds(
 
 FontGlyphAtlas* GlyphAtlas::GetOrCreateFontGlyphAtlas(
     const ScaledFont& scaled_font) {
-  const auto& found = font_atlas_map_.find(scaled_font);
-  if (found != font_atlas_map_.end()) {
-    return &found->second;
-  }
-  font_atlas_map_[scaled_font] = FontGlyphAtlas();
-  return &font_atlas_map_[scaled_font];
+  auto [iter, inserted] =
+      font_atlas_map_.try_emplace(scaled_font, FontGlyphAtlas());
+  return &iter->second;
 }
 
 size_t GlyphAtlas::GetGlyphCount() const {

engine/src/flutter/impeller/typographer/glyph_atlas.h

@@ -8,7 +8,16 @@
 #include <functional>
 #include <memory>
 #include <optional>
-#include <unordered_map>
+
+#include "flutter/fml/build_config.h"
+
+#if defined(OS_FUCHSIA)
+// TODO(gaaclarke): Migrate to use absl. I couldn't get it working since absl
+// has special logic in its GN files for Fuchsia that I couldn't sort out.
+#define IMPELLER_TYPOGRAPHER_USE_STD_HASH
+#else
+#include "flutter/third_party/abseil-cpp/absl/container/flat_hash_map.h"
+#endif
 
 #include "impeller/core/texture.h"
 #include "impeller/geometry/rect.h"
@@ -19,6 +28,30 @@ namespace impeller {
 
 class FontGlyphAtlas;
 
+/// Helper for AbslHashAdapter. Tallies a hash value with fml::HashCombine.
+template <typename T>
+struct AbslHashAdapterCombiner {
+  std::size_t value = 0;
+
+  template <typename... Args>
+  static AbslHashAdapterCombiner combine(AbslHashAdapterCombiner combiner,
+                                         const Args&... args) {
+    combiner.value = fml::HashCombine(combiner.value, args...);
+    return combiner;
+  }
+};
+
+/// Adapts AbslHashValue functions to be used with std::unordered_map and the
+/// fml hash functions.
+template <typename T>
+struct AbslHashAdapter {
+  constexpr std::size_t operator()(const T& element) const {
+    AbslHashAdapterCombiner<T> combiner;
+    combiner = AbslHashValue(std::move(combiner), element);
+    return combiner.value;
+  }
+};
+
 struct FrameBounds {
   /// The bounds of the glyph within the glyph atlas.
   Rect atlas_bounds;
@@ -160,11 +193,19 @@ class GlyphAtlas {
   std::shared_ptr<Texture> texture_;
   size_t generation_ = 0;
 
-  std::unordered_map<ScaledFont,
-                     FontGlyphAtlas,
-                     ScaledFont::Hash,
-                     ScaledFont::Equal>
-      font_atlas_map_;
+#if defined(IMPELLER_TYPOGRAPHER_USE_STD_HASH)
+  using FontAtlasMap = std::unordered_map<ScaledFont,
+                                          FontGlyphAtlas,
+                                          AbslHashAdapter<ScaledFont>,
+                                          ScaledFont::Equal>;
+#else
+  using FontAtlasMap = absl::flat_hash_map<ScaledFont,
+                                           FontGlyphAtlas,
+                                           absl::Hash<ScaledFont>,
+                                           ScaledFont::Equal>;
+#endif
+
+  FontAtlasMap font_atlas_map_;
 
   GlyphAtlas(const GlyphAtlas&) = delete;
 
@@ -228,6 +269,7 @@ class GlyphAtlasContext {
 class FontGlyphAtlas {
  public:
   FontGlyphAtlas() = default;
+  FontGlyphAtlas(FontGlyphAtlas&&) = default;
 
   //----------------------------------------------------------------------------
   /// @brief      Find the location of a glyph in the atlas.
@@ -249,12 +291,19 @@ class FontGlyphAtlas {
  private:
   friend class GlyphAtlas;
 
-  std::unordered_map<SubpixelGlyph,
-                     FrameBounds,
-                     SubpixelGlyph::Hash,
-                     SubpixelGlyph::Equal>
-      positions_;
+#if defined(IMPELLER_TYPOGRAPHER_USE_STD_HASH)
+  using PositionsMap = std::unordered_map<SubpixelGlyph,
+                                          FrameBounds,
+                                          AbslHashAdapter<SubpixelGlyph>,
+                                          SubpixelGlyph::Equal>;
+#else
+  using PositionsMap = absl::flat_hash_map<SubpixelGlyph,
+                                           FrameBounds,
+                                           absl::Hash<SubpixelGlyph>,
+                                           SubpixelGlyph::Equal>;
+#endif
 
+  PositionsMap positions_;
   FontGlyphAtlas(const FontGlyphAtlas&) = delete;
 };