Cocoaheads Smalltalk : Tagged Pointers

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## Cocoaheads Smalltalk
# Tagged Pointers

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# Principe

En 64-bit sur Intel, les pointeurs sont alignés sur 16 octets.

.no-highlight
	   6         5         4         3         2         1         0
	3210987654321098765432109876543210987654321098765432109876543210
	............................................................0000

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# Principe

En 64-bit sur Intel, les pointeurs sont alignés sur 16 octets.

.no-highlight
	   6         5         4         3         2         1         0
	3210987654321098765432109876543210987654321098765432109876543210
	........................................................ccccttt1
	|                                                       |   |  +- 1 bit
	|                                                       |   +---- 3 bits
	|                                                       +-------- 4 bits
	+---------------------------------------------------------------- 56 bits

* 1 bit : flag, 1 pour les tagged pointers
* 7 bits d'infos de type
	* 3 bits pour la classe Objective-C
	
	* 4 bits d'infos réservés pour la classe
* 56 bits (7 octets) de payload

---

# CFInternal.h

.objectivec
	#define CF_IS_TAGGED_OBJ(PTR)	((uintptr_t)(PTR) & 0x1)
	#define CF_TAGGED_OBJ_TYPE(PTR)	((uintptr_t)(PTR) & 0xF)
	 
	enum {
	    kCFTaggedObjectID_Invalid = 0,
	    kCFTaggedObjectID_Undefined0 = (0 << 1) + 1,
	    kCFTaggedObjectID_Integer = (1 << 1) + 1,
	    kCFTaggedObjectID_Undefined2 = (2 << 1) + 1,
	    kCFTaggedObjectID_Undefined3 = (3 << 1) + 1,
	    kCFTaggedObjectID_Undefined4 = (4 << 1) + 1,
	    kCFTaggedObjectID_ManagedObjectID = (5 << 1) + 1, // Core Data
	    kCFTaggedObjectID_Date = (6 << 1) + 1,
	    kCFTaggedObjectID_DateTS = (7 << 1) + 1,
	};

<!--
	* 3 octets donc 8 classes possibles
		* c'est l'implémentation actuelle, on peut changer
		* le bit de poids faible est utilisé comme flag, alors qu'il suffit que le dernier 4-bit ne soit pas 0

# Aujourd'hui 4 valeurs utilisées
	* enum de CFInternal (slide)
		* NSNUmber (dans CFNumber.c)
		* NSDate (mais pas CFDateRef?)
		* NSDateTimeStamp (?)
		* NSManagedObjectID (pas vu)

-->

---

# objc-private.h

.objectivec
	#define OBJC_IS_TAGGED_PTR(PTR)		((uintptr_t)(PTR) & 0x1)
	extern Class _objc_tagged_isa_table[16];

static inline Class _object_getClass(id obj)
	{
	    if (OBJC_IS_TAGGED_PTR(obj)) {
	        uint8_t slotNumber = ((uint8_t) (uint64_t) obj) & 0x0F;
	        Class isa = _objc_tagged_isa_table[slotNumber];
	        return isa;
	    }
	}

---

# Integers

.objectivec
	- (void) testSanity
	{
		NSNumber * obj1 = [NSNumber numberWithInt:42];
		NSNumber * obj2 = [NSNumber numberWithInt:42];
		STAssertEqualObjects(obj1, obj2,
							 @"both objects are equal");
	}
	
	- (void) testPointerEquality
	{
		NSNumber * obj1 = [NSNumber numberWithInt:42];
		NSNumber * obj2 = [NSNumber numberWithInt:42];
		STAssertEquals(obj1, obj2,
					   @"both objects are **the same**");
	}
	
	- (void) testPointerValue
	{
		NSNumber * object = [NSNumber numberWithInt:42];
		// shifting the last byte reveals the value
		STAssertEquals(42, (int)((long)object>>8),
					   @"the value is in the pointer");
	}

---

# NSDate

**NSDate <=> NSTimeInterval**

.objectivec
	typedef union _DoubleOrPointer {
	    NSTimeInterval interval;
	    void * p;
	} DoubleOrPointer;

- (void) testPointerValue
	{
	    DoubleOrPointer dop = {.interval = 42.0};
	    NSDate * object = [NSDate dateWithTimeIntervalSinceReferenceDate:
						   dop.interval];
	    // Masking out the last byte gives the passed interval
	    STAssertEquals(dop.p, (void*)((long)object&0xFFFFFFFFFFFFFF00),
					   @"the pointer is the value");
	}

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# Links

* objectivistc.tumblr.com
	* [Tagged pointers and fast-pathed CFNumber integers in Lion](http://objectivistc.tumblr.com/post/7872364181/tagged-pointers-and-fast-pathed-cfnumber-integers-in)
	* [A CHOCKING MYSTERY: po [NSNumber numberWithBool:NO] outputs 1](http://objectivistc.tumblr.com/post/8992822737/a-chocking-mystery-po-nsnumber-numberwithbool-no)
* lists.apple.com
	* [Objc-langage](http://lists.apple.com/archives/Objc-language/2011/Nov/msg00014.html)
* opensource.apple.com
	* [ObjC](http://opensource.apple.com/source/objc4/objc4-493.9/)
	* [CoreFoundation](http://opensource.apple.com/source/CF/CF-635/)

---

# Bonus #1 : Singleton NSNumbers

.objectivec
	- (void) testNSNumberSingletons
	{
	    for (int i = -1000; i<=1000; i++) {
	        // NSNumbers between -1 and 12 are singletons
	        if(i>=-1 && i<=12)
	            STAssertTrue( [NSNumber numberWithInt:i]
							  == [NSNumber numberWithInt:i],
	                       	  @"objects should be the same pointer");
	        else
	            STAssertTrue( [NSNumber numberWithInt:i]
					 		  != [NSNumber numberWithInt:i],
	                          @"objects should not be the same pointer");
	    }
	}

---

# Bonus #2 : objc4/runtime/hashtable2.m

.objectivec
	/*
		hashtable2.m
	  	Copyright 1989-1996 NeXT Software, Inc.
		Created by Bertrand Serlet, Feb 89
	 */
	
	...

/**********************************************************************
	 *
	 *	On z'y va
	 *	
	 **********************************************************************/
	
	NXHashTable *NXCreateHashTable (NXHashTablePrototype prototype,
	...

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.footnote[*Made with [remark](gnab.github.com/remark)*]