Implement NSDate as a small object (tagged pointer) - clang/libobjc2 only (#451)

* Implement GSSmallObject Class

* Remove private concrete class access

* Change secondary bias

* NSDate: Get interval from rhs object in comparison

* Add prefix to CONCRETE_CLASS_NAME  macro

Source/NSCalendarDate.m

@@ -58,9 +58,6 @@
 @class	GSAbsTimeZone;
 @interface	GSAbsTimeZone : NSObject	// Help the compiler
 @end
-@class	NSGDate;
-@interface	NSGDate : NSObject	// Help the compiler
-@end
 
 
 #define DISTANT_FUTURE	63113990400.0
@@ -396,7 +393,6 @@ GSPrivateTimeNow(void)
       absAbrIMP = (NSString* (*)(id,SEL,id))
 	[absClass instanceMethodForSelector: abrSEL];
 
-      GSObjCAddClassBehavior(self, [NSGDate class]);
       [pool release];
     }
 }
@@ -463,6 +459,11 @@ GSPrivateTimeNow(void)
   return AUTORELEASE(d);
 }
 
+- (NSTimeInterval) timeIntervalSinceReferenceDate
+{
+  return _seconds_since_ref;
+}
+
 /**
  * Creates and returns a new NSCalendarDate object by taking the
  * value of the receiver and adding the interval in seconds specified.

Source/NSDatePrivate.h

@@ -0,0 +1,41 @@
+/** NSDate Private Interface
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   Written by: Hugo Melder <hugo@algoriddim.com>
+
+   This file is part of the GNUstep Base Library.
+
+   This library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2 of the License, or (at your option) any later version.
+
+   This library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with this library; if not, write to the Free
+   Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+   Boston, MA 02110 USA.
+*/
+
+#import <Foundation/NSDate.h>
+
+#if defined(OBJC_SMALL_OBJECT_SHIFT) && (OBJC_SMALL_OBJECT_SHIFT == 3)
+#define USE_SMALL_DATE 1
+#define DATE_CONCRETE_CLASS_NAME GSSmallDate
+#else
+#define USE_SMALL_DATE 0
+#define DATE_CONCRETE_CLASS_NAME NSGDate
+#endif
+
+@interface DATE_CONCRETE_CLASS_NAME : NSDate
+#if USE_SMALL_DATE == 0
+{
+@public
+  NSTimeInterval _seconds_since_ref;
+}
+#endif
+@end

Source/NSTimer.m

@@ -35,9 +35,8 @@
 #import "Foundation/NSRunLoop.h"
 #import "Foundation/NSInvocation.h"
 
-@class	NSGDate;
-@interface NSGDate : NSObject	// Help the compiler
-@end
+#import "NSDatePrivate.h"
+
 static Class	NSDate_class;
 
 /**
@@ -58,7 +57,7 @@ static Class	NSDate_class;
 {
   if (self == [NSTimer class])
     {
-      NSDate_class = [NSGDate class];
+      NSDate_class = [DATE_CONCRETE_CLASS_NAME class];
     }
 }