Fix a COMDouble::Round() issue (#12210)

* Fix a COMDouble::Round() issue
fixes https://github.com/dotnet/coreclr/issues/12137

* Add a couple tests for Math.Round(double)
based on https://github.com/dotnet/coreclr/issues/12137

* Use G17 format specifier for printing double values

2 files changed, 4 insertions, 6 deletions

diff --git a/src/classlibnative/float/floatdouble.cpp b/src/classlibnative/float/floatdouble.cpp
index 6f593e503c..ba90a57f88 100644
--- a/src/classlibnative/float/floatdouble.cpp
+++ b/src/classlibnative/float/floatdouble.cpp
@@ -183,10 +183,9 @@ FCIMPL1_V(double, COMDouble::Round, double x)
     // We had a number that was equally close to 2 integers.
     // We need to return the even one.
 
-    double tempVal = (x + 0.5);
-    double flrTempVal = floor(tempVal);
+    double flrTempVal = floor(x + 0.5);
 
-    if ((flrTempVal == tempVal) && (fmod(tempVal, 2.0) != 0)) {
+    if ((x == (floor(x) + 0.5)) && (fmod(flrTempVal, 2.0) != 0)) {
         flrTempVal -= 1.0;
     }
 
diff --git a/src/classlibnative/float/floatsingle.cpp b/src/classlibnative/float/floatsingle.cpp
index c7b7d4126b..ba48aea709 100644
--- a/src/classlibnative/float/floatsingle.cpp
+++ b/src/classlibnative/float/floatsingle.cpp
@@ -181,10 +181,9 @@ FCIMPL1_V(float, COMSingle::Round, float x)
     // We had a number that was equally close to 2 integers.
     // We need to return the even one.
 
-    float tempVal = (x + 0.5f);
-    float flrTempVal = floorf(tempVal);
+    float flrTempVal = floorf(x + 0.5f);
 
-    if ((flrTempVal == tempVal) && (fmodf(tempVal, 2.0f) != 0)) {
+    if ((x == (floorf(x) + 0.5f)) && (fmod(flrTempVal, 2.0f) != 0)) {
         flrTempVal -= 1.0f;
     }