summaryrefslogtreecommitdiff
path: root/clang/docs/Block-ABI-Apple.txt
blob: 917059b4829c36e3517d81f4b6becf62bd2e9f9d (about) (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
Block Implementation Specification

Copyright 2008-2010 Apple, Inc.
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

0. History

2008/7/14  - created
2008/8/21  - revised, C++
2008/9/24  - add NULL isa field to __block storage
2008/10/1  - revise block layout to use a static descriptor structure
2008/10/6  - revise block layout to use an unsigned long int flags
2008/10/28 - specify use of _Block_object_assign/dispose for all "Object" types in helper functions
2008/10/30 - revise new layout to have invoke function in same place
2008/10/30 - add __weak support

2010/3/16  - rev for stret return, signature field
2010/4/6   - improved wording

This document describes the Apple ABI implementation specification of Blocks.

The first shipping version of this ABI is found in Mac OS X 10.6, and shall be referred to as 10.6.ABI. As of 2010/3/16, the following describes the ABI contract with the runtime and the compiler, and, as necessary, will be referred to as ABI.2010.3.16.

Since the Apple ABI references symbols from other elements of the system, any attempt to use this ABI on systems prior to SnowLeopard is undefined.

1. High Level

The ABI of blocks consist of their layout and the runtime functions required by the compiler.
A Block consists of a structure of the following form:

struct Block_literal_1 {
    void *isa; // initialized to &_NSConcreteStackBlock or &_NSConcreteGlobalBlock
    int flags;
    int reserved; 
    void (*invoke)(void *, ...);
    struct Block_descriptor_1 {
	unsigned long int reserved;	// NULL
    	unsigned long int size;         // sizeof(struct Block_literal_1)
	// optional helper functions
    	void (*copy_helper)(void *dst, void *src);     // IFF (1<<25)
    	void (*dispose_helper)(void *src);             // IFF (1<<25)
        // required ABI.2010.3.16
        const char *signature;                         // IFF (1<<30)
    } *descriptor;
    // imported variables
};

The following flags bits are in use thusly for a possible ABI.2010.3.16:

enum {
    BLOCK_HAS_COPY_DISPOSE =  (1 << 25),
    BLOCK_HAS_CTOR =          (1 << 26), // helpers have C++ code
    BLOCK_IS_GLOBAL =         (1 << 28),
    BLOCK_HAS_STRET =         (1 << 29), // IFF BLOCK_HAS_SIGNATURE
    BLOCK_HAS_SIGNATURE =     (1 << 30), 
};

In 10.6.ABI the (1<<29) was usually set and was always ignored by the runtime - it had been a transitional marker that did not get deleted after the transition. This bit is now paired with (1<<30), and represented as the pair (3<<30), for the following combinations of valid bit settings, and their meanings.

switch (flags & (3<<29)) {
  case (0<<29):      10.6.ABI, no signature field available
  case (1<<29):      10.6.ABI, no signature field available
  case (2<<29): ABI.2010.3.16, regular calling convention, presence of signature field
  case (3<<29): ABI.2010.3.16, stret calling convention, presence of signature field,
}

The signature field is not always populated.

The following discussions are presented as 10.6.ABI otherwise.

Block literals may occur within functions where the structure is created in stack local memory.  They may also appear as initialization expressions for Block variables of global or static local variables.

When a Block literal expression is evaluated the stack based structure is initialized as follows:

1) static descriptor structure is declared and initialized as follows:
1a) the invoke function pointer is set to a function that takes the Block structure as its first argument and the rest of the arguments (if any) to the Block and executes the Block compound statement.
1b) the size field is set to the size of the following Block literal structure.
1c) the copy_helper and dispose_helper function pointers are set to respective helper functions if they are required by the Block literal
2) a stack (or global) Block literal data structure is created and initialized as follows:
2a) the isa field is set to the address of the external _NSConcreteStackBlock, which is a block of uninitialized memory supplied in libSystem, or _NSConcreteGlobalBlock if this is a static or file level block literal.
2) The flags field is set to zero unless there are variables imported into the block that need helper functions for program level Block_copy() and Block_release() operations, in which case the (1<<25) flags bit is set.


As an example, the Block literal expression
   ^ { printf("hello world\n"); }
would cause to be created on a 32-bit system:

struct __block_literal_1 {
    void *isa;
    int flags;
    int reserved; 
    void (*invoke)(struct __block_literal_1 *);
    struct __block_descriptor_1 *descriptor;
};

void __block_invoke_1(struct __block_literal_1 *_block) {
    printf("hello world\n");
}

static struct __block_descriptor_1 {
    unsigned long int reserved;
    unsigned long int Block_size;
} __block_descriptor_1 = { 0, sizeof(struct __block_literal_1), __block_invoke_1 };

and where the block literal appeared

  struct __block_literal_1 _block_literal = {
	&_NSConcreteStackBlock,
	(1<<29), <uninitialized>,
	__block_invoke_1,
	&__block_descriptor_1
   };

Blocks import other Block references, const copies of other variables, and variables marked __block.  In Objective-C variables may additionally be objects.

When a Block literal expression used as the initial value of a global or static local variable it is initialized as follows:
  struct __block_literal_1 __block_literal_1 = {
	&_NSConcreteGlobalBlock,
	(1<<28)|(1<<29), <uninitialized>,
	__block_invoke_1,
	&__block_descriptor_1
   };
that is, a different address is provided as the first value and a particular (1<<28) bit is set in the flags field, and otherwise it is the same as for stack based Block literals.  This is an optimization that can be used for any Block literal that imports no const or __block storage variables.


2. Imported Variables

Variables of "auto" storage class are imported as const copies.  Variables of "__block" storage class are imported as a pointer to an enclosing data structure.  Global variables are simply referenced and not considered as imported.

2.1 Imported const copy variables

Automatic storage variables not marked with __block are imported as const copies.

The simplest example is that of importing a variable of type int.

   int x = 10;
   void (^vv)(void) = ^{ printf("x is %d\n", x); }
   x = 11;
   vv();

would be compiled

struct __block_literal_2 {
    void *isa;
    int flags;
    int reserved; 
    void (*invoke)(struct __block_literal_2 *);
    struct __block_descriptor_2 *descriptor;
    const int x;
};

void __block_invoke_2(struct __block_literal_2 *_block) {
    printf("x is %d\n", _block->x);
}

static struct __block_descriptor_2 {
    unsigned long int reserved;
    unsigned long int Block_size;
} __block_descriptor_2 = { 0, sizeof(struct __block_literal_2) };

and

  struct __block_literal_2 __block_literal_2 = {
	&_NSConcreteStackBlock,
	(1<<29), <uninitialized>,
	__block_invoke_2,
	&__block_descriptor_2,
        x
   };

In summary, scalars, structures, unions, and function pointers are generally imported as const copies with no need for helper functions.

2.2 Imported const copy of Block reference

The first case where copy and dispose helper functions are required is for the case of when a block itself is imported.  In this case both a copy_helper function and a dispose_helper function are needed.  The copy_helper function is passed both the existing stack based pointer and the pointer to the new heap version and should call back into the runtime to actually do the copy operation on the imported fields within the block.  The runtime functions are all described in Section 5.0 Runtime Helper Functions.

An example:

   void (^existingBlock)(void) = ...;
   void (^vv)(void) = ^{ existingBlock(); }
   vv();

struct __block_literal_3 {
   ...; // existing block
};

struct __block_literal_4 {
    void *isa;
    int flags;
    int reserved; 
    void (*invoke)(struct __block_literal_4 *);
    struct __block_literal_3 *const existingBlock;
};

void __block_invoke_4(struct __block_literal_2 *_block) {
   __block->existingBlock->invoke(__block->existingBlock);
}

void __block_copy_4(struct __block_literal_4 *dst, struct __block_literal_4 *src) {
     //_Block_copy_assign(&dst->existingBlock, src->existingBlock, 0);
     _Block_object_assign(&dst->existingBlock, src->existingBlock, BLOCK_FIELD_IS_BLOCK);
}

void __block_dispose_4(struct __block_literal_4 *src) {
     // was _Block_destroy
     _Block_object_dispose(src->existingBlock, BLOCK_FIELD_IS_BLOCK);
}

static struct __block_descriptor_4 {
    unsigned long int reserved;
    unsigned long int Block_size;
    void (*copy_helper)(struct __block_literal_4 *dst, struct __block_literal_4 *src);
    void (*dispose_helper)(struct __block_literal_4 *);
} __block_descriptor_4 = {
	0,
	sizeof(struct __block_literal_4),
	__block_copy_4,
	__block_dispose_4,
};

and where it is used

  struct __block_literal_4 _block_literal = {
	&_NSConcreteStackBlock,
	(1<<25)|(1<<29), <uninitialized>
	__block_invoke_4,
	& __block_descriptor_4
        existingBlock,
   };

2.2.1 Importing __attribute__((NSObject)) variables.

GCC introduces __attribute__((NSObject)) on structure pointers to mean "this is an object".  This is useful because many low level data structures are declared as opaque structure pointers, e.g. CFStringRef, CFArrayRef, etc.  When used from C, however, these are still really objects and are the second case where that requires copy and dispose helper functions to be generated.  The copy helper functions generated by the compiler should use the _Block_object_assign runtime helper function and in the dispose helper the _Block_object_dispose runtime helper function should be called.

For example, block xyzzy in the following

    struct Opaque *__attribute__((NSObject)) objectPointer = ...;
    ...
    void (^xyzzy)(void) = ^{  CFPrint(objectPointer); };

would have helper functions

void __block_copy_xyzzy(struct __block_literal_5 *dst, struct __block_literal_5 *src) {
     _Block_object_assign(&dst->objectPointer, src-> objectPointer, BLOCK_FIELD_IS_OBJECT);
}

void __block_dispose_xyzzy(struct __block_literal_5 *src) {
     _Block_object_dispose(src->objectPointer, BLOCK_FIELD_IS_OBJECT);
}

generated.


2.3 Imported __block marked variables.

2.3.1 Layout of __block marked variables

The compiler must embed variables that are marked __block in a specialized structure of the form:

struct _block_byref_xxxx {
    void *isa;
    struct Block_byref *forwarding;
    int flags;   //refcount;
    int size;
    typeof(marked_variable) marked_variable;
};

Variables of certain types require helper functions for when Block_copy() and Block_release() are performed upon a referencing Block.  At the "C" level only variables that are of type Block or ones that have __attribute__((NSObject)) marked require helper functions.  In Objective-C objects require helper functions and in C++ stack based objects require helper functions. Variables that require helper functions use the form:

struct _block_byref_xxxx {
    void *isa;
    struct _block_byref_xxxx *forwarding;
    int flags;   //refcount;
    int size;
    // helper functions called via Block_copy() and Block_release()
    void (*byref_keep)(void  *dst, void *src);
    void (*byref_dispose)(void *);
    typeof(marked_variable) marked_variable;
};

The structure is initialized such that
 a) the forwarding pointer is set to the beginning of its enclosing structure,
 b) the size field is initialized to the total size of the enclosing structure,
 c) the flags field is set to either 0 if no helper functions are needed or (1<<25) if they are,
 d) the helper functions are initialized (if present)
 e) the variable itself is set to its initial value.
 f) the isa field is set to NULL

2.3.2 Access to __block variables from within its lexical scope.

In order to "move" the variable to the heap upon a copy_helper operation the compiler must rewrite access to such a variable to be indirect through the structures forwarding pointer.  For example:

  int __block i = 10;
  i = 11;

would be rewritten to be:

  struct _block_byref_i {
    void *isa;
    struct _block_byref_i *forwarding;
    int flags;   //refcount;
    int size;
    int captured_i;
  } i = { NULL, &i, 0, sizeof(struct _block_byref_i), 10 };

  i.forwarding->captured_i = 11;

In the case of a Block reference variable being marked __block the helper code generated must use the _Block_object_assign and _Block_object_dispose routines supplied by the runtime to make the copies.  For example:

   __block void (voidBlock)(void) = blockA;
   voidBlock = blockB;

would translate into

struct _block_byref_voidBlock {
    void *isa;
    struct _block_byref_voidBlock *forwarding;
    int flags;   //refcount;
    int size;
    void (*byref_keep)(struct _block_byref_voidBlock *dst, struct _block_byref_voidBlock *src);
    void (*byref_dispose)(struct _block_byref_voidBlock *);
    void (^captured_voidBlock)(void);
};

void _block_byref_keep_helper(struct _block_byref_voidBlock *dst, struct _block_byref_voidBlock *src) {
    //_Block_copy_assign(&dst->captured_voidBlock, src->captured_voidBlock, 0);
    _Block_object_assign(&dst->captured_voidBlock, src->captured_voidBlock, BLOCK_FIELD_IS_BLOCK | BLOCK_BYREF_CALLER);
}

void _block_byref_dispose_helper(struct _block_byref_voidBlock *param) {
    //_Block_destroy(param->captured_voidBlock, 0);
    _Block_object_dispose(param->captured_voidBlock, BLOCK_FIELD_IS_BLOCK | BLOCK_BYREF_CALLER)}

and
  struct _block_byref_voidBlock voidBlock = {( .forwarding=&voidBlock, .flags=(1<<25), .size=sizeof(struct _block_byref_voidBlock *),
      .byref_keep=_block_byref_keep_helper, .byref_dispose=_block_byref_dispose_helper,
      .captured_voidBlock=blockA )};

  voidBlock.forwarding->captured_voidBlock = blockB;
  

2.3.3 Importing __block variables into Blocks

A Block that uses a __block variable in its compound statement body must import the variable and emit copy_helper and dispose_helper helper functions that, in turn, call back into the runtime to actually copy or release the byref data block using the functions _Block_object_assign and _Block_object_dispose.

For example:

   int __block i = 2;
   functioncall(^{ i = 10; });

would translate to

struct _block_byref_i {
    void *isa;  // set to NULL
    struct _block_byref_voidBlock *forwarding;
    int flags;   //refcount;
    int size;
    void (*byref_keep)(struct _block_byref_i *dst, struct _block_byref_i *src);
    void (*byref_dispose)(struct _block_byref_i *);
    int captured_i;
};


struct __block_literal_5 {
    void *isa;
    int flags;
    int reserved; 
    void (*invoke)(struct __block_literal_5 *);
    struct __block_descriptor_5 *descriptor;
    struct _block_byref_i *i_holder;
};

void __block_invoke_5(struct __block_literal_5 *_block) {
   _block->forwarding->captured_i = 10;
}

void __block_copy_5(struct __block_literal_5 *dst, struct __block_literal_5 *src) {
     //_Block_byref_assign_copy(&dst->captured_i, src->captured_i);
     _Block_object_assign(&dst->captured_i, src->captured_i, BLOCK_FIELD_IS_BYREF | BLOCK_BYREF_CALLER);
}

void __block_dispose_5(struct __block_literal_5 *src) {
     //_Block_byref_release(src->captured_i);
     _Block_object_dispose(src->captured_i, BLOCK_FIELD_IS_BYREF | BLOCK_BYREF_CALLER);
}

static struct __block_descriptor_5 {
    unsigned long int reserved;
    unsigned long int Block_size;
    void (*copy_helper)(struct __block_literal_5 *dst, struct __block_literal_5 *src);
    void (*dispose_helper)(struct __block_literal_5 *);
} __block_descriptor_5 = { 0, sizeof(struct __block_literal_5) __block_copy_5, __block_dispose_5 };

and

  struct _block_byref_i i = {( .forwarding=&i, .flags=0, .size=sizeof(struct _block_byref_i) )};
  struct __block_literal_5 _block_literal = {
	&_NSConcreteStackBlock,
	(1<<25)|(1<<29), <uninitialized>,
	__block_invoke_5,
	&__block_descriptor_5,
        2,
   };

2.3.4 Importing __attribute__((NSObject)) __block variables

A __block variable that is also marked __attribute__((NSObject)) should have byref_keep and byref_dispose helper functions that use _Block_object_assign and _Block_object_dispose.

2.3.5 __block escapes

Because Blocks referencing __block variables may have Block_copy() performed upon them the underlying storage for the variables may move to the heap.  In Objective-C Garbage Collection Only compilation environments the heap used is the garbage collected one and no further action is required.  Otherwise the compiler must issue a call to potentially release any heap storage for __block variables at all escapes or terminations of their scope.  The call should be:

    _Block_object_dispose(&_block_byref_xxx, BLOCK_FIELD_IS_BYREF);


2.3.6 Nesting

Blocks may contain Block literal expressions.  Any variables used within inner blocks are imported into all enclosing Block scopes even if the variables are not used.  This includes const imports as well as __block variables.

3. Objective C Extensions to Blocks

3.1 Importing Objects

Objects should be treated as __attribute__((NSObject)) variables; all copy_helper, dispose_helper, byref_keep, and byref_dispose helper functions should use _Block_object_assign and _Block_object_dispose.  There should be no code generated that uses -retain or -release methods.


3.2 Blocks as Objects

The compiler will treat Blocks as objects when synthesizing property setters and getters, will characterize them as objects when generating garbage collection strong and weak layout information in the same manner as objects, and will issue strong and weak write-barrier assignments in the same manner as objects.

3.3 __weak __block Support

Objective-C (and Objective-C++) support the __weak attribute on __block variables.  Under normal circumstances the compiler uses the Objective-C runtime helper support functions objc_assign_weak and objc_read_weak.  Both should continue to be used for all reads and writes of __weak __block variables:
	objc_read_weak(&block->byref_i->forwarding->i)

The __weak variable is stored in a _block_byref_xxxx structure and the Block has copy and dispose helpers for this structure that call:
	_Block_object_assign(&dest->_block_byref_i, src-> _block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_BYREF);
and
	_Block_object_dispose(src->_block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_BYREF);


In turn, the block_byref copy support helpers distinguish between whether the __block variable is a Block or not and should either call:
	_Block_object_assign(&dest->_block_byref_i, src->_block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_OBJECT | BLOCK_BYREF_CALLER);
for something declared as an object or
	_Block_object_assign(&dest->_block_byref_i, src->_block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_BLOCK | BLOCK_BYREF_CALLER);
for something declared as a Block.

A full example follows:


   __block __weak id obj = <initialization expression>;
   functioncall(^{ [obj somemessage]; });

would translate to

struct _block_byref_obj {
    void *isa;  // uninitialized
    struct _block_byref_obj *forwarding;
    int flags;   //refcount;
    int size;
    void (*byref_keep)(struct _block_byref_i *dst, struct _block_byref_i *src);
    void (*byref_dispose)(struct _block_byref_i *);
    id captured_obj;
};

void _block_byref_obj_keep(struct _block_byref_voidBlock *dst, struct _block_byref_voidBlock *src) {
    //_Block_copy_assign(&dst->captured_obj, src->captured_obj, 0);
    _Block_object_assign(&dst->captured_obj, src->captured_obj, BLOCK_FIELD_IS_OBJECT | BLOCK_FIELD_IS_WEAK | BLOCK_BYREF_CALLER);
}

void _block_byref_obj_dispose(struct _block_byref_voidBlock *param) {
    //_Block_destroy(param->captured_obj, 0);
    _Block_object_dispose(param->captured_obj, BLOCK_FIELD_IS_OBJECT | BLOCK_FIELD_IS_WEAK | BLOCK_BYREF_CALLER);
};

for the block byref part and

struct __block_literal_5 {
    void *isa;
    int flags;
    int reserved; 
    void (*invoke)(struct __block_literal_5 *);
    struct __block_descriptor_5 *descriptor;
    struct _block_byref_obj *byref_obj;
};

void __block_invoke_5(struct __block_literal_5 *_block) {
   [objc_read_weak(&_block->byref_obj->forwarding->captured_obj) somemessage];
}

void __block_copy_5(struct __block_literal_5 *dst, struct __block_literal_5 *src) {
     //_Block_byref_assign_copy(&dst->byref_obj, src->byref_obj);
     _Block_object_assign(&dst->byref_obj, src->byref_obj, BLOCK_FIELD_IS_BYREF | BLOCK_FIELD_IS_WEAK);
}

void __block_dispose_5(struct __block_literal_5 *src) {
     //_Block_byref_release(src->byref_obj);
     _Block_object_dispose(src->byref_obj, BLOCK_FIELD_IS_BYREF | BLOCK_FIELD_IS_WEAK);
}

static struct __block_descriptor_5 {
    unsigned long int reserved;
    unsigned long int Block_size;
    void (*copy_helper)(struct __block_literal_5 *dst, struct __block_literal_5 *src);
    void (*dispose_helper)(struct __block_literal_5 *);
} __block_descriptor_5 = { 0, sizeof(struct __block_literal_5), __block_copy_5, __block_dispose_5 };

and within the compound statement:

  struct _block_byref_obj obj = {( .forwarding=&obj, .flags=(1<<25), .size=sizeof(struct _block_byref_obj),
				.byref_keep=_block_byref_obj_keep, .byref_dispose=_block_byref_obj_dispose,
				.captured_obj = <initialization expression> )};

  struct __block_literal_5 _block_literal = {
	&_NSConcreteStackBlock,
	(1<<25)|(1<<29), <uninitialized>,
	__block_invoke_5,
	&__block_descriptor_5,
        &obj,		// a reference to the on-stack structure containing "captured_obj"
   };


   functioncall(_block_literal->invoke(&_block_literal));


4.0 C++ Support

Within a block stack based C++ objects are copied into const copies using the copy constructor.  It is an error if a stack based C++ object is used within a block if it does not have a copy constructor.  In addition both copy and destroy helper routines must be synthesized for the block to support the Block_copy() operation, and the flags work marked with the (1<<26) bit in addition to the (1<<25) bit.  The copy helper should call the constructor using appropriate offsets of the variable within the supplied stack based block source and heap based destination for all const constructed copies, and similarly should call the destructor in the destroy routine.

As an example, suppose a C++ class FOO existed with a copy constructor.  Within a code block a stack version of a FOO object is declared and used within a Block literal expression:

{
    FOO foo;
    void (^block)(void) = ^{ printf("%d\n", foo.value()); };
}

The compiler would synthesize

struct __block_literal_10 {
    void *isa;
    int flags;
    int reserved; 
    void (*invoke)(struct __block_literal_10 *);
    struct __block_descriptor_10 *descriptor;
    const FOO foo;
};

void __block_invoke_10(struct __block_literal_10 *_block) {
   printf("%d\n", _block->foo.value());
}

void __block_literal_10(struct __block_literal_10 *dst, struct __block_literal_10 *src) {
     FOO_ctor(&dst->foo, &src->foo);
}

void __block_dispose_10(struct __block_literal_10 *src) {
     FOO_dtor(&src->foo);
}

static struct __block_descriptor_10 {
    unsigned long int reserved;
    unsigned long int Block_size;
    void (*copy_helper)(struct __block_literal_10 *dst, struct __block_literal_10 *src);
    void (*dispose_helper)(struct __block_literal_10 *);
} __block_descriptor_10 = { 0, sizeof(struct __block_literal_10), __block_copy_10, __block_dispose_10 };

and the code would be:
{
  FOO foo;
  comp_ctor(&foo); // default constructor
  struct __block_literal_10 _block_literal = {
	&_NSConcreteStackBlock,
	(1<<25)|(1<<26)|(1<<29), <uninitialized>,
	__block_invoke_10,
	&__block_descriptor_10,
   };
   comp_ctor(&_block_literal->foo, &foo);  // const copy into stack version
   struct __block_literal_10 &block = &_block_literal;  // assign literal to block variable
   block->invoke(block);	// invoke block
   comp_dtor(&_block_literal->foo); // destroy stack version of const block copy
   comp_dtor(&foo); // destroy original version
}


C++ objects stored in __block storage start out on the stack in a block_byref data structure as do other variables.  Such objects (if not const objects) must support a regular copy constructor.  The block_byref data structure will have copy and destroy helper routines synthesized by the compiler.  The copy helper will have code created to perform the copy constructor based on the initial stack block_byref data structure, and will also set the (1<<26) bit in addition to the (1<<25) bit.  The destroy helper will have code to do the destructor on the object stored within the supplied block_byref heap data structure.  For example,

    __block FOO blockStorageFoo;

requires the normal constructor for the embedded blockStorageFoo object

    FOO_ctor(& _block_byref_blockStorageFoo->blockStorageFoo);

and at scope termination the destructor:

    FOO_dtor(& _block_byref_blockStorageFoo->blockStorageFoo);

Note that the forwarding indirection is NOT used.

The compiler would need to generate (if used from a block literal) the following copy/dispose helpers:

void _block_byref_obj_keep(struct _block_byref_blockStorageFoo *dst, struct _block_byref_blockStorageFoo *src) {
     FOO_ctor(&dst->blockStorageFoo, &src->blockStorageFoo);
}

void _block_byref_obj_dispose(struct _block_byref_blockStorageFoo *src) {
     FOO_dtor(&src->blockStorageFoo);
}

for the appropriately named constructor and destructor for the class/struct FOO.

To support member variable and function access the compiler will synthesize a const pointer to a block version of the "this" pointer.

5.0 Runtime Helper Functions

The runtime helper functions are described in /usr/local/include/Block_private.h.  To summarize their use, a block requires copy/dispose helpers if it imports any block variables, __block storage variables, __attribute__((NSObject)) variables, or C++ const copied objects with constructor/destructors.  The (1<<26) bit is set and functions are generated.

The block copy helper function should, for each of the variables of the type mentioned above, call
     _Block_object_assign(&dst->target, src->target, BLOCK_FIELD_<appropo>);
in the copy helper and
    _Block_object_dispose(->target, BLOCK_FIELD_<appropo>);
in the dispose helper where
      <appropo> is

enum {
    BLOCK_FIELD_IS_OBJECT   =  3,  // id, NSObject, __attribute__((NSObject)), block, ...
    BLOCK_FIELD_IS_BLOCK    =  7,  // a block variable
    BLOCK_FIELD_IS_BYREF    =  8,  // the on stack structure holding the __block variable

    BLOCK_FIELD_IS_WEAK     = 16,  // declared __weak

    BLOCK_BYREF_CALLER      = 128, // called from byref copy/dispose helpers
};

and of course the CTORs/DTORs for const copied C++ objects.

The block_byref data structure similarly requires copy/dispose helpers for block variables, __attribute__((NSObject)) variables, or C++ const copied objects with constructor/destructors, and again the (1<<26) bit is set and functions are generated in the same manner.

Under ObjC we allow __weak as an attribute on __block variables, and this causes the addition of BLOCK_FIELD_IS_WEAK orred onto the BLOCK_FIELD_IS_BYREF flag when copying the block_byref structure in the block copy helper, and onto the BLOCK_FIELD_<appropo> field within the block_byref copy/dispose helper calls.

The prototypes, and summary, of the helper functions are

/* Certain field types require runtime assistance when being copied to the heap.  The following function is used
   to copy fields of types: blocks, pointers to byref structures, and objects (including __attribute__((NSObject)) pointers.
   BLOCK_FIELD_IS_WEAK is orthogonal to the other choices which are mutually exclusive.
   Only in a Block copy helper will one see BLOCK_FIELD_IS_BYREF.
 */
void _Block_object_assign(void *destAddr, const void *object, const int flags);

/* Similarly a compiler generated dispose helper needs to call back for each field of the byref data structure.
   (Currently the implementation only packs one field into the byref structure but in principle there could be more).
   The same flags used in the copy helper should be used for each call generated to this function:
 */
void _Block_object_dispose(const void *object, const int flags);