From 4d6a2ba2a0f8646f0c44034ccf2e21167ab87b0f Mon Sep 17 00:00:00 2001
From: Leonard Hecker <lhecker@microsoft.com>
Date: Fri, 2 Jun 2023 02:12:00 +0200
Subject: [PATCH] Reduce cost of resetting row attributes

---
 src/buffer/out/Row.cpp     |  4 +++-
 src/inc/til/rle.h          |  5 +++++
 src/inc/til/small_vector.h | 18 ++++++++++++++++++
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/src/buffer/out/Row.cpp b/src/buffer/out/Row.cpp
index 9d883b09502..a765e474b75 100644
--- a/src/buffer/out/Row.cpp
+++ b/src/buffer/out/Row.cpp
@@ -128,7 +128,9 @@ void ROW::Reset(const TextAttribute& attr)
 {
     _charsHeap.reset();
     _chars = { _charsBuffer, _columnCount };
-    _attr = { _columnCount, attr };
+    // Constructing and then moving objects into place isn't free.
+    // Modifying the existing object is _much_ faster.
+    *_attr.runs().unsafe_shrink_to_size(1) = til::rle_pair{ attr, _columnCount };
     _lineRendition = LineRendition::SingleWidth;
     _wrapForced = false;
     _doubleBytePadded = false;
diff --git a/src/inc/til/rle.h b/src/inc/til/rle.h
index fa9e3793678..d94c3a744e6 100644
--- a/src/inc/til/rle.h
+++ b/src/inc/til/rle.h
@@ -402,6 +402,11 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
             return _runs;
         }
 
+        container& runs() noexcept
+        {
+            return _runs;
+        }
+
         // Get the value at the position
         const_reference at(size_type position) const
         {
diff --git a/src/inc/til/small_vector.h b/src/inc/til/small_vector.h
index 4a1fe4eab90..6484b96da0f 100644
--- a/src/inc/til/small_vector.h
+++ b/src/inc/til/small_vector.h
@@ -622,6 +622,24 @@ namespace til
             _capacity = capacity;
         }
 
+        // This is a very unsafe shortcut to free the buffer and get a direct
+        // hold to the _buffer. The caller can then fill it with `size` items.
+        [[nodiscard]] T* unsafe_shrink_to_size(size_t size) noexcept
+        {
+            assert(size <= N);
+
+            if (_capacity != N)
+            {
+                _deallocate(_data);
+            }
+
+            _data = &_buffer[0];
+            _capacity = N;
+            _size = size;
+
+            return &_buffer[0];
+        }
+
         void push_back(const T& value)
         {
             emplace_back(value);