[Impeller] leave glyph atlas in transfer dst to improve vulkan throughput.

impeller/renderer/backend/gles/blit_pass_gles.cc

@@ -132,7 +132,8 @@ bool BlitPassGLES::OnCopyBufferToTextureCommand(
     std::shared_ptr<Texture> destination,
     IRect destination_region,
     std::string label,
-    uint32_t slice) {
+    uint32_t slice,
+    bool convert_to_read) {
   auto command = std::make_unique<BlitCopyBufferToTextureCommandGLES>();
   command->label = label;
   command->source = std::move(source);

impeller/renderer/backend/gles/blit_pass_gles.h

@@ -59,7 +59,8 @@ class BlitPassGLES final : public BlitPass,
                                     std::shared_ptr<Texture> destination,
                                     IRect destination_region,
                                     std::string label,
-                                    uint32_t slice) override;
+                                    uint32_t slice,
+                                    bool convert_to_read) override;
 
   // |BlitPass|
   bool OnGenerateMipmapCommand(std::shared_ptr<Texture> texture,

impeller/renderer/backend/metal/blit_pass_mtl.h

@@ -58,7 +58,8 @@ class BlitPassMTL final : public BlitPass {
                                     std::shared_ptr<Texture> destination,
                                     IRect destination_region,
                                     std::string label,
-                                    uint32_t slice) override;
+                                    uint32_t slice,
+                                    bool convert_to_read) override;
 
   // |BlitPass|
   bool OnGenerateMipmapCommand(std::shared_ptr<Texture> texture,

impeller/renderer/backend/metal/blit_pass_mtl.mm

@@ -166,7 +166,8 @@
     std::shared_ptr<Texture> destination,
     IRect destination_region,
     std::string label,
-    uint32_t slice) {
+    uint32_t slice,
+    bool convert_to_read) {
   auto source_mtl = DeviceBufferMTL::Cast(*source.buffer).GetMTLBuffer();
   if (!source_mtl) {
     return false;

impeller/renderer/backend/vulkan/blit_pass_vk.cc

@@ -221,13 +221,37 @@ bool BlitPassVK::OnCopyTextureToBufferCommand(
   return true;
 }
 
+bool BlitPassVK::ConvertTextureToShaderRead(
+    const std::shared_ptr<Texture>& texture) {
+  auto& encoder = *command_buffer_->GetEncoder();
+  const auto& cmd_buffer = encoder.GetCommandBuffer();
+
+  BarrierVK barrier;
+  barrier.cmd_buffer = cmd_buffer;
+  barrier.src_access = vk::AccessFlagBits::eTransferWrite;
+  barrier.src_stage = vk::PipelineStageFlagBits::eTransfer;
+  barrier.dst_access = vk::AccessFlagBits::eShaderRead;
+  barrier.dst_stage = vk::PipelineStageFlagBits::eFragmentShader;
+
+  barrier.new_layout = vk::ImageLayout::eShaderReadOnlyOptimal;
+
+  const auto& texture_vk = TextureVK::Cast(*texture);
+
+  if (!encoder.Track(texture)) {
+    return false;
+  }
+
+  return texture_vk.SetLayout(barrier);
+}
+
 // |BlitPass|
 bool BlitPassVK::OnCopyBufferToTextureCommand(
     BufferView source,
     std::shared_ptr<Texture> destination,
     IRect destination_region,
     std::string label,
-    uint32_t slice) {
+    uint32_t slice,
+    bool convert_to_read) {
   auto& encoder = *command_buffer_->GetEncoder();
   const auto& cmd_buffer = encoder.GetCommandBuffer();
 
@@ -262,6 +286,9 @@ bool BlitPassVK::OnCopyBufferToTextureCommand(
   image_copy.imageExtent.height = destination_region.GetHeight();
   image_copy.imageExtent.depth = 1u;
 
+  // Note: this barrier should do nothing if we're already in the transfer dst
+  // optimal state. This is important for performance of repeated blit pass
+  // encoding.
   if (!dst.SetLayout(dst_barrier)) {
     VALIDATION_LOG << "Could not encode layout transition.";
     return false;
@@ -274,7 +301,7 @@ bool BlitPassVK::OnCopyBufferToTextureCommand(
   );
 
   // Transition to shader-read.
-  {
+  if (convert_to_read) {
     BarrierVK barrier;
     barrier.cmd_buffer = cmd_buffer;
     barrier.src_access = vk::AccessFlagBits::eTransferWrite;

impeller/renderer/backend/vulkan/blit_pass_vk.h

@@ -37,6 +37,10 @@ class BlitPassVK final : public BlitPass {
   bool EncodeCommands(
       const std::shared_ptr<Allocator>& transients_allocator) const override;
 
+  // |BlitPass|
+  bool ConvertTextureToShaderRead(
+      const std::shared_ptr<Texture>& texture) override;
+
   // |BlitPass|
   bool OnCopyTextureToTextureCommand(std::shared_ptr<Texture> source,
                                      std::shared_ptr<Texture> destination,
@@ -56,7 +60,8 @@ class BlitPassVK final : public BlitPass {
                                     std::shared_ptr<Texture> destination,
                                     IRect destination_region,
                                     std::string label,
-                                    uint32_t slice) override;
+                                    uint32_t slice,
+                                    bool convert_to_read) override;
   // |BlitPass|
   bool OnGenerateMipmapCommand(std::shared_ptr<Texture> texture,
                                std::string label) override;

impeller/renderer/blit_pass.cc

@@ -124,7 +124,8 @@ bool BlitPass::AddCopy(BufferView source,
                        std::shared_ptr<Texture> destination,
                        std::optional<IRect> destination_region,
                        std::string label,
-                       uint32_t slice) {
+                       uint32_t slice,
+                       bool convert_to_read) {
   if (!destination) {
     VALIDATION_LOG << "Attempted to add a texture blit with no destination.";
     return false;
@@ -156,7 +157,12 @@ bool BlitPass::AddCopy(BufferView source,
 
   return OnCopyBufferToTextureCommand(std::move(source), std::move(destination),
                                       destination_region_value,
-                                      std::move(label), slice);
+                                      std::move(label), slice, convert_to_read);
+}
+
+bool BlitPass::ConvertTextureToShaderRead(
+    const std::shared_ptr<Texture>& texture) {
+  return true;
 }
 
 bool BlitPass::GenerateMipmap(std::shared_ptr<Texture> texture,

impeller/renderer/blit_pass.h

@@ -31,12 +31,19 @@ class BlitPass {
 
   void SetLabel(std::string label);
 
+  //----------------------------------------------------------------------------
+  /// @brief      If the texture is not already in a shader read internal
+  ///             state, then convert it to that state.
+  ///
+  ///             This API is only used by Vulkan.
+  virtual bool ConvertTextureToShaderRead(
+      const std::shared_ptr<Texture>& texture);
+
   //----------------------------------------------------------------------------
   /// @brief      Record a command to copy the contents of one texture to
   ///             another texture. The blit area is limited by the intersection
   ///             of the texture coverage with respect the source region and
   ///             destination origin.
-  ///             No work is encoded into the command buffer at this time.
   ///
   /// @param[in]  source              The texture to read for copying.
   /// @param[in]  destination         The texture to overwrite using the source
@@ -60,7 +67,6 @@ class BlitPass {
   //----------------------------------------------------------------------------
   /// @brief      Record a command to copy the contents of the buffer to
   ///             the texture.
-  ///             No work is encoded into the command buffer at this time.
   ///
   /// @param[in]  source              The texture to read for copying.
   /// @param[in]  destination         The buffer to overwrite using the source
@@ -84,7 +90,6 @@ class BlitPass {
   //----------------------------------------------------------------------------
   /// @brief      Record a command to copy the contents of the buffer to
   ///             the texture.
-  ///             No work is encoded into the command buffer at this time.
   ///
   /// @param[in]  source              The buffer view to read for copying.
   /// @param[in]  destination         The texture to overwrite using the source
@@ -96,6 +101,8 @@ class BlitPass {
   ///                                 command.
   /// @param[in]  slice               For cubemap textures, the slice to write
   ///                                 data to.
+  /// @param[in]  convert_to_read     Whether to convert the texture to a shader
+  ///                                 read state. Defaults to true.
   ///
   /// @return     If the command was valid for subsequent commitment.
   ///
@@ -111,11 +118,11 @@ class BlitPass {
                std::shared_ptr<Texture> destination,
                std::optional<IRect> destination_region = std::nullopt,
                std::string label = "",
-               uint32_t slice = 0);
+               uint32_t slice = 0,
+               bool convert_to_read = true);
 
   //----------------------------------------------------------------------------
   /// @brief      Record a command to generate all mip levels for a texture.
-  ///             No work is encoded into the command buffer at this time.
   ///
   /// @param[in]  texture  The texture to generate mipmaps for.
   /// @param[in]  label    The optional debug label to give the command.
@@ -159,7 +166,8 @@ class BlitPass {
       std::shared_ptr<Texture> destination,
       IRect destination_region,
       std::string label,
-      uint32_t slice) = 0;
+      uint32_t slice,
+      bool convert_to_read) = 0;
 
   virtual bool OnGenerateMipmapCommand(std::shared_ptr<Texture> texture,
                                        std::string label) = 0;

impeller/renderer/testing/mocks.h

@@ -84,7 +84,8 @@ class MockBlitPass : public BlitPass {
                std::shared_ptr<Texture> destination,
                IRect destination_rect,
                std::string label,
-               uint32_t slice),
+               uint32_t slice,
+               bool convert_to_read),
               (override));
   MOCK_METHOD(bool,
               OnGenerateMipmapCommand,

impeller/typographer/backends/skia/typographer_context_skia.cc

@@ -298,13 +298,21 @@ static bool UpdateAtlasBitmap(const GlyphAtlas& atlas,
 
     DrawGlyph(canvas, pair.scaled_font, pair.glyph, has_color);
 
-    if (!blit_pass->AddCopy(allocator.TakeBufferView(), texture,
+    // convert_to_read is set to false so that the texture remains in a transfer
+    // dst layout until we finish writing to it below. This only has an impact
+    // on Vulkan where we are responsible for managing image layouts.
+    if (!blit_pass->AddCopy(allocator.TakeBufferView(),  //
+                            texture,                     //
                             IRect::MakeXYWH(pos->GetLeft(), pos->GetTop(),
-                                            size.width, size.height))) {
+                                            size.width, size.height),  //
+                            /*label=*/"",                              //
+                            /*slice=*/0,                               //
+                            /*convert_to_read=*/false                  //
+                            )) {
       return false;
     }
   }
-  return true;
+  return blit_pass->ConvertTextureToShaderRead(texture);
 }
 
 std::shared_ptr<GlyphAtlas> TypographerContextSkia::CreateGlyphAtlas(