Improve throughput of TimeSpan.ToString/TryFormat with "g"/"G" (#19051)

* Improve throughput of TimeSpan.ToString/TryFormat with "g"/"G"

TimeSpan has three standard formats: "c", "g", and "G".  Yesterday I updated its implementation with throughput improvements for "c" (the default) based on porting the design from Utf8Formatter; this PR does so for "g"/"G".

Initially I wasn't going to handle "g"/"G" as they factor in culture (Utf8Formatter doesn't), but even with accessing the current culture there are still significant wins to be had.  I was also going to keep the "c" and "g"/"G" implementations separate, to avoid bogging down the default "c" formatting with additional conditions needed to support "g"/"G", but the overhead incurred for that turns out to be minimal enough that it's worth keeping one implementation rather than two mostly-similar ones... the impact on "c" is mostly within noise.

This PR makes a significant throughput improvement for "g"/"G" formatting.  It also removes several unnecessary allocations, such that TryFormat with "g"/"G" is now allocation-free (and ToString just allocates the asked-for string).

* Address PR feedback

1 files changed, 29 insertions, 0 deletions

diff --git a/src/System.Private.CoreLib/shared/System/Buffers/Text/FormattingHelpers.CountDigits.cs b/src/System.Private.CoreLib/shared/System/Buffers/Text/FormattingHelpers.CountDigits.cs
index 709ac4fba6..b6140adbad 100644
--- a/src/System.Private.CoreLib/shared/System/Buffers/Text/FormattingHelpers.CountDigits.cs
+++ b/src/System.Private.CoreLib/shared/System/Buffers/Text/FormattingHelpers.CountDigits.cs
@@ -128,5 +128,34 @@ namespace System.Buffers.Text
 
             return digits;
         }
+
+        
+        // Counts the number of trailing '0' digits in a decimal number.
+        // e.g., value =      0 => retVal = 0, valueWithoutTrailingZeros = 0
+        //       value =   1234 => retVal = 0, valueWithoutTrailingZeros = 1234
+        //       value = 320900 => retVal = 2, valueWithoutTrailingZeros = 3209
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int CountDecimalTrailingZeros(uint value, out uint valueWithoutTrailingZeros)
+        {
+            int zeroCount = 0;
+
+            if (value != 0)
+            {
+                while (true)
+                {
+                    uint temp = value / 10;
+                    if (value != (temp * 10))
+                    {
+                        break;
+                    }
+
+                    value = temp;
+                    zeroCount++;
+                }
+            }
+
+            valueWithoutTrailingZeros = value;
+            return zeroCount;
+        }
     }
 }