[Impeller] add compute pass API for memory barriers, re-enable for Vulkan. (#49946)

Adds two new APIs that insert memory barriers for compute -> compute dependencies.

```c++
  // |ComputePass|
  void AddBufferMemoryBarrier() override;

  // |ComputePass|
  void AddTextureMemoryBarrier() override;
```

Also makes the ComputePassVK automatically insert a compute -> vertex dependency when encoding.  This change is sufficient to let the GPU compute implementation of draw points work on Pixel and Samsung Android devices.

For more explaination on these specific barriers, see the documentation added in this PR.

Fixes #49946

Author: Jonah Williams

impeller/entity/entity_unittests.cc

@@ -2545,11 +2545,6 @@ TEST_P(EntityTest, PointFieldGeometryCoverage) {
             Rect::MakeLTRB(35, 15, 135, 205));
 }
 
-TEST_P(EntityTest, PointFieldCanUseCompute) {
-  EXPECT_EQ(PointFieldGeometry::CanUseCompute(*GetContentContext()),
-            GetContext()->GetBackendType() == Context::BackendType::kMetal);
-}
-
 TEST_P(EntityTest, ColorFilterContentsWithLargeGeometry) {
   Entity entity;
   entity.SetTransform(Matrix::MakeScale(GetContentScale()));

impeller/entity/geometry/point_field_geometry.cc

@@ -17,7 +17,7 @@ GeometryResult PointFieldGeometry::GetPositionBuffer(
     const ContentContext& renderer,
     const Entity& entity,
     RenderPass& pass) const {
-  if (CanUseCompute(renderer)) {
+  if (renderer.GetDeviceCapabilities().SupportsCompute()) {
     return GetPositionBufferGPU(renderer, entity, pass);
   }
   auto vtx_builder = GetPositionBufferCPU(renderer, entity, pass);
@@ -40,7 +40,7 @@ GeometryResult PointFieldGeometry::GetPositionUVBuffer(
     const ContentContext& renderer,
     const Entity& entity,
     RenderPass& pass) const {
-  if (CanUseCompute(renderer)) {
+  if (renderer.GetDeviceCapabilities().SupportsCompute()) {
     return GetPositionBufferGPU(renderer, entity, pass, texture_coverage,
                                 effect_transform);
   }
@@ -200,6 +200,7 @@ GeometryResult PointFieldGeometry::GetPositionBufferGPU(
 
     using UV = UvComputeShader;
 
+    compute_pass->AddBufferMemoryBarrier();
     compute_pass->SetCommandLabel("UV Geometry");
     compute_pass->SetPipeline(renderer.GetUvComputePipeline());
 
@@ -264,14 +265,6 @@ GeometryVertexType PointFieldGeometry::GetVertexType() const {
   return GeometryVertexType::kPosition;
 }
 
-// Compute is disabled for Vulkan because the barriers are incorrect, see
-// also: https://github.com/flutter/flutter/issues/140798 .
-bool PointFieldGeometry::CanUseCompute(const ContentContext& renderer) {
-  return renderer.GetDeviceCapabilities().SupportsCompute() &&
-         renderer.GetContext()->GetBackendType() ==
-             Context::BackendType::kMetal;
-}
-
 // |Geometry|
 std::optional<Rect> PointFieldGeometry::GetCoverage(
     const Matrix& transform) const {

impeller/entity/geometry/point_field_geometry.h

@@ -17,9 +17,6 @@ class PointFieldGeometry final : public Geometry {
 
   static size_t ComputeCircleDivisions(Scalar scaled_radius, bool round);
 
-  /// If the platform can use compute safely.
-  static bool CanUseCompute(const ContentContext& renderer);
-
  private:
   // |Geometry|
   GeometryResult GetPositionBuffer(const ContentContext& renderer,

impeller/renderer/backend/metal/compute_pass_mtl.h

@@ -66,6 +66,12 @@ class ComputePassMTL final : public ComputePass {
   // |ComputePass|
   bool EncodeCommands() const override;
 
+  // |ComputePass|
+  void AddBufferMemoryBarrier() override;
+
+  // |ComputePass|
+  void AddTextureMemoryBarrier() override;
+
   ComputePassMTL(const ComputePassMTL&) = delete;
 
   ComputePassMTL& operator=(const ComputePassMTL&) = delete;

impeller/renderer/backend/metal/compute_pass_mtl.mm

@@ -32,7 +32,8 @@
   if (!buffer_) {
     return;
   }
-  encoder_ = [buffer_ computeCommandEncoder];
+  encoder_ = [buffer_ computeCommandEncoderWithDispatchType:
+                          MTLDispatchType::MTLDispatchTypeConcurrent];
   if (!encoder_) {
     return;
   }
@@ -67,6 +68,16 @@
       ComputePipelineMTL::Cast(*pipeline).GetMTLComputePipelineState());
 }
 
+// |ComputePass|
+void ComputePassMTL::AddBufferMemoryBarrier() {
+  [encoder_ memoryBarrierWithScope:MTLBarrierScopeBuffers];
+}
+
+// |ComputePass|
+void ComputePassMTL::AddTextureMemoryBarrier() {
+  [encoder_ memoryBarrierWithScope:MTLBarrierScopeTextures];
+}
+
 // |ComputePass|
 bool ComputePassMTL::BindResource(ShaderStage stage,
                                   DescriptorType type,

impeller/renderer/backend/vulkan/compute_pass_vk.cc

@@ -104,11 +104,7 @@ fml::Status ComputePassVK::Compute(const ISize& grid_size) {
 
   // Special case for linear processing.
   if (height == 1) {
-    int64_t minimum = 1;
-    int64_t threadGroups = std::max(
-        static_cast<int64_t>(std::ceil(width * 1.0 / max_wg_size_[0] * 1.0)),
-        minimum);
-    command_buffer_vk.dispatch(threadGroups, 1, 1);
+    command_buffer_vk.dispatch(width, 1, 1);
   } else {
     while (width > max_wg_size_[0]) {
       width = std::max(static_cast<int64_t>(1), width / 2);
@@ -216,8 +212,53 @@ bool ComputePassVK::BindResource(size_t binding,
   return true;
 }
 
+// Note:
+// https://github.com/KhronosGroup/Vulkan-Docs/wiki/Synchronization-Examples
+// Seems to suggest that anything more finely grained than a global memory
+// barrier is likely to be weakened into a global barrier. Confirming this on
+// mobile devices will require some experimentation.
+
+// |ComputePass|
+void ComputePassVK::AddBufferMemoryBarrier() {
+  vk::MemoryBarrier barrier;
+  barrier.srcAccessMask = vk::AccessFlagBits::eShaderWrite;
+  barrier.dstAccessMask = vk::AccessFlagBits::eShaderRead;
+
+  command_buffer_->GetEncoder()->GetCommandBuffer().pipelineBarrier(
+      vk::PipelineStageFlagBits::eComputeShader,
+      vk::PipelineStageFlagBits::eComputeShader, {}, 1, &barrier, 0, {}, 0, {});
+}
+
+// |ComputePass|
+void ComputePassVK::AddTextureMemoryBarrier() {
+  vk::MemoryBarrier barrier;
+  barrier.srcAccessMask = vk::AccessFlagBits::eShaderWrite;
+  barrier.dstAccessMask = vk::AccessFlagBits::eShaderRead;
+
+  command_buffer_->GetEncoder()->GetCommandBuffer().pipelineBarrier(
+      vk::PipelineStageFlagBits::eComputeShader,
+      vk::PipelineStageFlagBits::eComputeShader, {}, 1, &barrier, 0, {}, 0, {});
+}
+
 // |ComputePass|
 bool ComputePassVK::EncodeCommands() const {
+  // Since we only use global memory barrier, we don't have to worry about
+  // compute to compute dependencies across cmd buffers. Instead, we pessimize
+  // here and assume that we wrote to a storage image or buffer and that a
+  // render pass will read from it. if there are ever scenarios where we end up
+  // with compute to compute dependencies this should be revisited.
+
+  // This does not currently handle image barriers as we do not use them
+  // for anything.
+  vk::MemoryBarrier barrier;
+  barrier.srcAccessMask = vk::AccessFlagBits::eShaderWrite;
+  barrier.dstAccessMask =
+      vk::AccessFlagBits::eIndexRead | vk::AccessFlagBits::eVertexAttributeRead;
+
+  command_buffer_->GetEncoder()->GetCommandBuffer().pipelineBarrier(
+      vk::PipelineStageFlagBits::eComputeShader,
+      vk::PipelineStageFlagBits::eVertexInput, {}, 1, &barrier, 0, {}, 0, {});
+
   return true;
 }
 

impeller/renderer/backend/vulkan/compute_pass_vk.h

@@ -57,6 +57,12 @@ class ComputePassVK final : public ComputePass {
   void SetPipeline(const std::shared_ptr<Pipeline<ComputePipelineDescriptor>>&
                        pipeline) override;
 
+  // |ComputePass|
+  void AddBufferMemoryBarrier() override;
+
+  // |ComputePass|
+  void AddTextureMemoryBarrier() override;
+
   // |ComputePass|
   fml::Status Compute(const ISize& grid_size) override;
 

impeller/renderer/compute_pass.h

@@ -35,6 +35,22 @@ class ComputePass : public ResourceBinder {
 
   virtual fml::Status Compute(const ISize& grid_size) = 0;
 
+  /// @brief Ensures all previously encoded compute command's buffer writes are
+  ///        visible to any subsequent compute commands.
+  ///
+  ///        On Vulkan, it does not matter if the compute command is in a
+  ///        different command buffer, only that it is executed later in queue
+  ///        order.
+  virtual void AddBufferMemoryBarrier() = 0;
+
+  /// @brief Ensures all previously encoded compute command's texture writes are
+  ///        visible to any subsequent compute commands.
+  ///
+  ///        On Vulkan, it does not matter if the compute command is in a
+  ///        different command buffer, only that it is executed later in queue
+  ///        order.
+  virtual void AddTextureMemoryBarrier() = 0;
+
   //----------------------------------------------------------------------------
   /// @brief      Encode the recorded commands to the underlying command buffer.
   ///
@@ -43,6 +59,8 @@ class ComputePass : public ResourceBinder {
   ///
   virtual bool EncodeCommands() const = 0;
 
+  const Context& GetContext() const { return *context_; }
+
  protected:
   const std::shared_ptr<const Context> context_;
 

impeller/renderer/compute_unittests.cc

@@ -308,6 +308,7 @@ TEST_P(ComputeTest, MultiStageInputAndOutput) {
     CS1::BindOutput(*pass, DeviceBuffer::AsBufferView(output_buffer_1));
 
     ASSERT_TRUE(pass->Compute(ISize(512, 1)).ok());
+    pass->AddBufferMemoryBarrier();
   }
 
   {