Improve perf by avoiding vector reallocation in renderer clusters and VT output graphics options (#6420)

## Summary of the Pull Request
Caches vectors in the class and uses a new helper to opportunistically shrink/grow as viewport sizes change in order to save performance on alloc/free of commonly used vectors.

## PR Checklist
* [x] Scratches a perf itch.
* [x] I work here.
* [x] wil tests added
* [x] No add'l doc.
* [x] Am core contributor.

## Detailed Description of the Pull Request / Additional comments
Two fixes:
1. For outputting lots of text, the base renderer class spent a lot of time allocating and freeing and reallocating the `Cluster` vector that adapts the text buffer information into render clusters. I've now cached this vector in the base render class itself and I shrink/grow it based on the viewport update that happens at the top of every frame. To prevent too much thrashing in the downward/shrink direction, I wrote the `til::manage_vector` helper that contains a threshold to only shrink if it asks for small enough of a size relative to the existing one. I used 80% of the existing size as the threshold for this one.
2. For outputting lots of changing colors, the VT graphics output engine spent a bunch of time allocating and reallocating the vector for `GraphicsOptions`. This one doesn't really have a predictable size, but I never expect it to get extremely big. So I just held it in the base class.

## Validation Steps Performed
* [x] Ran the til unit test
* [x] Checked render cluster vector time before/after against `big.txt` from #1064
* [x] Checked VT graphics output vector time before/after against `cacafire`

Case | Before | After
---|---|---|
`big.txt` | ![image](https://user-images.githubusercontent.com/18221333/84088632-cbaa8400-a9a1-11ea-8932-04b2e12a0477.png) | ![image](https://user-images.githubusercontent.com/18221333/84088996-b6822500-a9a2-11ea-837c-5e32a110156e.png)
`cacafire` | ![image](https://user-images.githubusercontent.com/18221333/84089153-22648d80-a9a3-11ea-8567-c3d80efa16a6.png) | ![image](https://user-images.githubusercontent.com/18221333/84089190-34463080-a9a3-11ea-98e5-a236b12330d6.png)

src/inc/til.h

@@ -18,6 +18,26 @@
 
 namespace til // Terminal Implementation Library. Also: "Today I Learned"
 {
+    template<typename T>
+    void manage_vector(std::vector<T>& vector, typename std::vector<T>::size_type requestedSize, float shrinkThreshold)
+    {
+        const auto existingCapacity = vector.capacity();
+        const auto requiredCapacity = requestedSize;
+
+        // Check by integer first as float math is way more expensive.
+        if (requiredCapacity < existingCapacity)
+        {
+            // Now check if it's even worth shrinking. We don't want to shrink by 1 at a time, so meet a threshold first.
+            if (requiredCapacity <= gsl::narrow_cast<size_t>((static_cast<float>(existingCapacity) * shrinkThreshold)))
+            {
+                // There's no real way to force a shrink, so make a new one.
+                vector = std::vector<T>{};
+            }
+        }
+
+        // Reserve won't shrink on its own and won't grow if we have enough space.
+        vector.reserve(requiredCapacity);
+    }
 }
 
 // These sit outside the namespace because they sit outside for WIL too.

src/renderer/base/renderer.cpp

@@ -28,7 +28,8 @@ Renderer::Renderer(IRenderData* pData,
                    std::unique_ptr<IRenderThread> thread) :
     _pData(pData),
     _pThread{ std::move(thread) },
-    _destructing{ false }
+    _destructing{ false },
+    _clusterBuffer{}
 {
     _srViewportPrevious = { 0 };
 
@@ -370,6 +371,12 @@ bool Renderer::_CheckViewportAndScroll()
 
     _srViewportPrevious = srNewViewport;
 
+    // If we're keeping some buffers between calls, let them know about the viewport size
+    // so they can prepare the buffers for changes to either preallocate memory at once
+    // (instead of growing naturally) or shrink down to reduce usage as appropriate.
+    const size_t lineLength = gsl::narrow_cast<size_t>(til::rectangle{ srNewViewport }.width());
+    til::manage_vector(_clusterBuffer, lineLength, _shrinkThreshold);
+
     if (coordDelta.X != 0 || coordDelta.Y != 0)
     {
         for (auto engine : _rgpEngines)
@@ -683,7 +690,6 @@ void Renderer::_PaintBufferOutputHelper(_In_ IRenderEngine* const pEngine,
         // we should have an iterator/view adapter for the rendering.
         // That would probably also eliminate the RenderData needing to give us the entire TextBuffer as well...
         // Retrieve the iterator for one line of information.
-        std::vector<Cluster> clusters;
         size_t cols = 0;
 
         // Retrieve the first color.
@@ -714,7 +720,7 @@ void Renderer::_PaintBufferOutputHelper(_In_ IRenderEngine* const pEngine,
             const auto currentRunTargetStart = screenPoint;
 
             // Ensure that our cluster vector is clear.
-            clusters.clear();
+            _clusterBuffer.clear();
 
             // Reset our flag to know when we're in the special circumstance
             // of attempting to draw only the right-half of a two-column character
@@ -746,7 +752,7 @@ void Renderer::_PaintBufferOutputHelper(_In_ IRenderEngine* const pEngine,
 
                 // If we're on the first cluster to be added and it's marked as "trailing"
                 // (a.k.a. the right half of a two column character), then we need some special handling.
-                if (clusters.empty() && it->DbcsAttr().IsTrailing())
+                if (_clusterBuffer.empty() && it->DbcsAttr().IsTrailing())
                 {
                     // If we have room to move to the left to start drawing...
                     if (screenPoint.X > 0)
@@ -757,7 +763,7 @@ void Renderer::_PaintBufferOutputHelper(_In_ IRenderEngine* const pEngine,
                         trimLeft = true;
                         // And add one to the number of columns we expect it to take as we insert it.
                         columnCount = it->Columns() + 1;
-                        clusters.emplace_back(it->Chars(), columnCount);
+                        _clusterBuffer.emplace_back(it->Chars(), columnCount);
                     }
                     else
                     {
@@ -770,7 +776,7 @@ void Renderer::_PaintBufferOutputHelper(_In_ IRenderEngine* const pEngine,
                 else
                 {
                     columnCount = it->Columns();
-                    clusters.emplace_back(it->Chars(), columnCount);
+                    _clusterBuffer.emplace_back(it->Chars(), columnCount);
                 }
 
                 if (columnCount > 1)
@@ -785,7 +791,7 @@ void Renderer::_PaintBufferOutputHelper(_In_ IRenderEngine* const pEngine,
             } while (it);
 
             // Do the painting.
-            THROW_IF_FAILED(pEngine->PaintBufferLine({ clusters.data(), clusters.size() }, screenPoint, trimLeft, lineWrapped));
+            THROW_IF_FAILED(pEngine->PaintBufferLine({ _clusterBuffer.data(), _clusterBuffer.size() }, screenPoint, trimLeft, lineWrapped));
 
             // If we're allowed to do grid drawing, draw that now too (since it will be coupled with the color data)
             // We're only allowed to draw the grid lines under certain circumstances.

src/renderer/base/renderer.hpp

@@ -121,6 +121,9 @@ namespace Microsoft::Console::Render
 
         SMALL_RECT _srViewportPrevious;
 
+        static constexpr float _shrinkThreshold = 0.8f;
+        std::vector<Cluster> _clusterBuffer;
+
         std::vector<SMALL_RECT> _GetSelectionRects() const;
         void _ScrollPreviousSelection(const til::point delta);
         std::vector<SMALL_RECT> _previousSelection;

src/terminal/parser/OutputStateMachineEngine.cpp

@@ -463,7 +463,9 @@ bool OutputStateMachineEngine::ActionCsiDispatch(const wchar_t wch,
     size_t clearType = 0;
     unsigned int function = 0;
     DispatchTypes::EraseType eraseType = DispatchTypes::EraseType::ToEnd;
-    std::vector<DispatchTypes::GraphicsOptions> graphicsOptions;
+    // We hold the vector in the class because client applications that do a lot of color work
+    // would spend a lot of time reallocating/resizing the vector.
+    _graphicsOptions.clear();
     DispatchTypes::AnsiStatusType deviceStatusType = static_cast<DispatchTypes::AnsiStatusType>(0); // there is no default status type.
     size_t repeatCount = 0;
     // This is all the args after the first arg, and the count of args not including the first one.
@@ -502,7 +504,7 @@ bool OutputStateMachineEngine::ActionCsiDispatch(const wchar_t wch,
             success = _GetEraseOperation(parameters, eraseType);
             break;
         case VTActionCodes::SGR_SetGraphicsRendition:
-            success = _GetGraphicsOptions(parameters, graphicsOptions);
+            success = _GetGraphicsOptions(parameters, _graphicsOptions);
             break;
         case VTActionCodes::DSR_DeviceStatusReport:
             success = _GetDeviceStatusOperation(parameters, deviceStatusType);
@@ -613,7 +615,7 @@ bool OutputStateMachineEngine::ActionCsiDispatch(const wchar_t wch,
                 TermTelemetry::Instance().Log(TermTelemetry::Codes::EL);
                 break;
             case VTActionCodes::SGR_SetGraphicsRendition:
-                success = _dispatch->SetGraphicsRendition({ graphicsOptions.data(), graphicsOptions.size() });
+                success = _dispatch->SetGraphicsRendition({ _graphicsOptions.data(), _graphicsOptions.size() });
                 TermTelemetry::Instance().Log(TermTelemetry::Codes::SGR);
                 break;
             case VTActionCodes::DSR_DeviceStatusReport:

src/terminal/parser/OutputStateMachineEngine.hpp

@@ -71,6 +71,7 @@ namespace Microsoft::Console::VirtualTerminal
         Microsoft::Console::ITerminalOutputConnection* _pTtyConnection;
         std::function<bool()> _pfnFlushToTerminal;
         wchar_t _lastPrintedChar;
+        std::vector<DispatchTypes::GraphicsOptions> _graphicsOptions;
 
         bool _IntermediateScsDispatch(const wchar_t wch,
                                       const std::basic_string_view<wchar_t> intermediates);

src/til/ut_til/BaseTests.cpp

@@ -0,0 +1,36 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+#include "precomp.h"
+
+#include "til.h"
+
+using namespace WEX::Common;
+using namespace WEX::Logging;
+using namespace WEX::TestExecution;
+
+// Tests for things in the TIL base class.
+class BaseTests
+{
+    TEST_CLASS(BaseTests);
+
+    TEST_METHOD(ManageVector)
+    {
+        constexpr float shrinkThreshold = 0.5f;
+
+        std::vector<int> foo;
+        foo.reserve(20);
+
+        Log::Comment(L"Expand vector.");
+        til::manage_vector(foo, 30, shrinkThreshold);
+        VERIFY_ARE_EQUAL(30, foo.capacity());
+
+        Log::Comment(L"Try shrink but by not enough for threshold.");
+        til::manage_vector(foo, 18, shrinkThreshold);
+        VERIFY_ARE_EQUAL(30, foo.capacity());
+
+        Log::Comment(L"Shrink because it is meeting threshold.");
+        til::manage_vector(foo, 15, shrinkThreshold);
+        VERIFY_ARE_EQUAL(15, foo.capacity());
+    }
+};

src/til/ut_til/sources

@@ -14,6 +14,7 @@ DLLDEF                  =
 
 SOURCES = \
     $(SOURCES) \
+    BaseTests.cpp \
     BitmapTests.cpp \
     ColorTests.cpp \
     OperatorTests.cpp \

src/til/ut_til/til.unit.tests.vcxproj

@@ -10,6 +10,7 @@
   </PropertyGroup>
   <Import Project="$(SolutionDir)src\common.build.pre.props" />
   <ItemGroup>
+    <ClCompile Include="BaseTests.cpp" />
     <ClCompile Include="BitmapTests.cpp" />
     <ClCompile Include="OperatorTests.cpp" />
     <ClCompile Include="PointTests.cpp" />

src/til/ut_til/til.unit.tests.vcxproj.filters

@@ -13,6 +13,8 @@
     <ClCompile Include="RectangleTests.cpp" />
     <ClCompile Include="BitmapTests.cpp" />
     <ClCompile Include="OperatorTests.cpp" />
+    <ClCompile Include="MathTests.cpp" />
+    <ClCompile Include="BaseTests.cpp" />
   </ItemGroup>
   <ItemGroup>
     <ClInclude Include="..\precomp.h" />