Avoid unnecessary memory allocations in decodeMapFromStruct

Splits allocates memory. This burdens the garbage collector.
By using string.Index to parse string you will be more cache
aware and do not create gabbage.

mapstructure.go

@@ -840,42 +840,31 @@ func (d *Decoder) decodeMapFromStruct(name string, dataVal reflect.Value, val re
 		}
 
 		tagValue := f.Tag.Get(d.config.TagName)
-		tagParts := strings.Split(tagValue, ",")
-
-		// If "omitempty" is specified in the tag, it ignores empty values.
-		omitempty := false
-		for _, tag := range tagParts[1:] {
-			if tag == "omitempty" {
-				omitempty = true
-				break
-			}
-		}
-		if omitempty && isEmptyValue(v) {
-			continue
-		}
-
-		// Determine the name of the key in the map
 		keyName := f.Name
-		if tagParts[0] != "" {
-			if tagParts[0] == "-" {
-				continue
-			}
-			keyName = tagParts[0]
-		}
 
 		// If Squash is set in the config, we squash the field down.
 		squash := d.config.Squash && v.Kind() == reflect.Struct
-		// If "squash" is specified in the tag, we squash the field down.
-		if !squash {
-			for _, tag := range tagParts[1:] {
-				if tag == "squash" {
-					squash = true
-					break
-				}
+		// Determine the name of the key in the map
+		if index := strings.Index(tagValue, ","); index != -1 {
+			if tagValue[:index] == "-" {
+				continue;
 			}
+			// If "omitempty" is specified in the tag, it ignores empty values.
+			if strings.Index(tagValue[index + 1:], "omitempty") != -1 && isEmptyValue(v) {
+				continue
+			}
+
+			// If "squash" is specified in the tag, we squash the field down.
+			squash = !squash && strings.Index(tagValue[index + 1:], "squash") != -1
 			if squash && v.Kind() != reflect.Struct {
 				return fmt.Errorf("cannot squash non-struct type '%s'", v.Type())
 			}
+			keyName = tagValue[:index]
+		} else if len(tagValue) > 0 {
+			if  tagValue == "-" {
+				continue
+			}
+			keyName = tagValue
 		}
 
 		switch v.Kind() {
@@ -1226,9 +1215,8 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 			}
 
 			// Build our field
-			fieldCurrent := field{fieldType, structVal.Field(i)}
 			if remain {
-				remainField = &fieldCurrent
+				remainField = &field{fieldType, structVal.Field(i)}
 			} else {
 				// Normal struct field, store it away
 				fields = append(fields, field{fieldType, structVal.Field(i)})