OnHeadersComplete improve speculation and memory ordering (VTune)

Author: benaadams

src/Servers/Kestrel/Core/src/Internal/Http/HttpRequestHeaders.cs

@@ -27,18 +27,22 @@ public HttpRequestHeaders(bool reuseHeaderValues = true, bool useLatin1 = false)
 
         public void OnHeadersComplete()
         {
-            var bitsToClear = _previousBits & ~_bits;
+            var newHeaderFlags = _bits;
+            var previousHeaderFlags = _previousBits;
             _previousBits = 0;
 
-            if (bitsToClear != 0)
+            var headersToClear = (~newHeaderFlags) & previousHeaderFlags;
+            if (headersToClear == 0)
             {
-                // Some previous headers were not reused or overwritten.
-
-                // While they cannot be accessed by the current request (as they were not supplied by it)
-                // there is no point in holding on to them, so clear them now,
-                // to allow them to get collected by the GC.
-                Clear(bitsToClear);
+                // All headers were resued.
+                return;
             }
+
+            // Some previous headers were not reused or overwritten.
+            // While they cannot be accessed by the current request (as they were not supplied by it)
+            // there is no point in holding on to them, so clear them now,
+            // to allow them to get collected by the GC.
+            Clear(headersToClear);
         }
 
         protected override void ClearFast()