1 /* alloca.c -- allocate automatically reclaimed memory
2 (Mostly) portable public-domain implementation -- D A Gwyn
4 This implementation of the PWB library alloca function,
5 which is used to allocate space off the run-time stack so
6 that it is automatically reclaimed upon procedure exit,
7 was inspired by discussions with J. Q. Johnson of Cornell.
8 J.Otto Tennant <jot@cray.com> contributed the Cray support.
10 There are some preprocessor constants that can
11 be defined when compiling for your specific system, for
12 improved efficiency; however, the defaults should be okay.
14 The general concept of this implementation is to keep
15 track of all alloca-allocated blocks, and reclaim any
16 that are found to be deeper in the stack than the current
17 invocation. This heuristic does not reclaim storage as
18 soon as it becomes invalid, but it will do so eventually.
20 As a special case, alloca(0) reclaims storage without
21 allocating any. It is a good idea to use alloca(0) in
22 your main control loop, etc. to force garbage collection. */
33 # include "blockinput.h"
34 # define xalloc_die() memory_full ()
37 # define free EMACS_FREE
43 /* If compiling with GCC 2, this file's not needed. */
44 #if !defined (__GNUC__) || __GNUC__ < 2
46 /* If someone has defined alloca as a macro,
47 there must be some other way alloca is supposed to work. */
52 /* actually, only want this if static is defined as ""
53 -- this is for usg, in which emacs must undefine static
54 in order to make unexec workable
56 # ifndef STACK_DIRECTION
59 -- must know STACK_DIRECTION at compile-time
60 /* Using #error here is not wise since this file should work for
61 old and obscure compilers. */
62 # endif /* STACK_DIRECTION undefined */
66 /* If your stack is a linked list of frames, you have to
67 provide an "address metric" ADDRESS_FUNCTION macro. */
69 # if defined (CRAY) && defined (CRAY_STACKSEG_END)
71 # define ADDRESS_FUNCTION(arg) (char *) i00afunc (&(arg))
73 # define ADDRESS_FUNCTION(arg) &(arg)
76 /* Define STACK_DIRECTION if you know the direction of stack
77 growth for your system; otherwise it will be automatically
80 STACK_DIRECTION > 0 => grows toward higher addresses
81 STACK_DIRECTION < 0 => grows toward lower addresses
82 STACK_DIRECTION = 0 => direction of growth unknown */
84 # ifndef STACK_DIRECTION
85 # define STACK_DIRECTION 0 /* Direction unknown. */
88 # if STACK_DIRECTION != 0
90 # define STACK_DIR STACK_DIRECTION /* Known at compile-time. */
92 # else /* STACK_DIRECTION == 0; need run-time code. */
94 static int stack_dir; /* 1 or -1 once known. */
95 # define STACK_DIR stack_dir
98 find_stack_direction (void)
100 static char *addr = NULL; /* Address of first `dummy', once known. */
101 auto char dummy; /* To get stack address. */
104 { /* Initial entry. */
105 addr = ADDRESS_FUNCTION (dummy);
107 find_stack_direction (); /* Recurse once. */
112 if (ADDRESS_FUNCTION (dummy) > addr)
113 stack_dir = 1; /* Stack grew upward. */
115 stack_dir = -1; /* Stack grew downward. */
119 # endif /* STACK_DIRECTION == 0 */
121 /* An "alloca header" is used to:
122 (a) chain together all alloca'ed blocks;
123 (b) keep track of stack depth.
125 It is very important that sizeof(header) agree with malloc
126 alignment chunk size. The following default should work okay. */
129 # define ALIGN_SIZE sizeof(double)
134 char align[ALIGN_SIZE]; /* To force sizeof(header). */
137 union hdr *next; /* For chaining headers. */
138 char *deep; /* For stack depth measure. */
142 static header *last_alloca_header = NULL; /* -> last alloca header. */
144 /* Return a pointer to at least SIZE bytes of storage,
145 which will be automatically reclaimed upon exit from
146 the procedure that called alloca. Originally, this space
147 was supposed to be taken from the current stack frame of the
148 caller, but that method cannot be made to work for some
149 implementations of C, for example under Gould's UTX/32. */
154 auto char probe; /* Probes stack depth: */
155 register char *depth = ADDRESS_FUNCTION (probe);
157 # if STACK_DIRECTION == 0
158 if (STACK_DIR == 0) /* Unknown growth direction. */
159 find_stack_direction ();
162 /* Reclaim garbage, defined as all alloca'd storage that
163 was allocated from deeper in the stack than currently. */
166 register header *hp; /* Traverses linked list. */
172 for (hp = last_alloca_header; hp != NULL;)
173 if ((STACK_DIR > 0 && hp->h.deep > depth)
174 || (STACK_DIR < 0 && hp->h.deep < depth))
176 register header *np = hp->h.next;
178 free (hp); /* Collect garbage. */
180 hp = np; /* -> next header. */
183 break; /* Rest are not deeper. */
185 last_alloca_header = hp; /* -> last valid storage. */
193 return NULL; /* No allocation required. */
195 /* Allocate combined header + user data storage. */
198 /* Address of header. */
201 size_t combined_size = sizeof (header) + size;
202 if (combined_size < sizeof (header))
205 new = xmalloc (combined_size);
207 ((header *) new)->h.next = last_alloca_header;
208 ((header *) new)->h.deep = depth;
210 last_alloca_header = (header *) new;
212 /* User storage begins just after header. */
214 return (void *) ((char *) new + sizeof (header));
218 # if defined (CRAY) && defined (CRAY_STACKSEG_END)
220 # ifdef DEBUG_I00AFUNC
227 /* Stack structures for CRAY-1, CRAY X-MP, and CRAY Y-MP */
228 struct stack_control_header
230 long shgrow:32; /* Number of times stack has grown. */
231 long shaseg:32; /* Size of increments to stack. */
232 long shhwm:32; /* High water mark of stack. */
233 long shsize:32; /* Current size of stack (all segments). */
236 /* The stack segment linkage control information occurs at
237 the high-address end of a stack segment. (The stack
238 grows from low addresses to high addresses.) The initial
239 part of the stack segment linkage control information is
240 0200 (octal) words. This provides for register storage
241 for the routine which overflows the stack. */
243 struct stack_segment_linkage
245 long ss[0200]; /* 0200 overflow words. */
246 long sssize:32; /* Number of words in this segment. */
247 long ssbase:32; /* Offset to stack base. */
249 long sspseg:32; /* Offset to linkage control of previous
252 long sstcpt:32; /* Pointer to task common address block. */
253 long sscsnm; /* Private control structure number for
255 long ssusr1; /* Reserved for user. */
256 long ssusr2; /* Reserved for user. */
257 long sstpid; /* Process ID for pid based multi-tasking. */
258 long ssgvup; /* Pointer to multitasking thread giveup. */
259 long sscray[7]; /* Reserved for Cray Research. */
279 /* The following structure defines the vector of words
280 returned by the STKSTAT library routine. */
283 long now; /* Current total stack size. */
284 long maxc; /* Amount of contiguous space which would
285 be required to satisfy the maximum
286 stack demand to date. */
287 long high_water; /* Stack high-water mark. */
288 long overflows; /* Number of stack overflow ($STKOFEN) calls. */
289 long hits; /* Number of internal buffer hits. */
290 long extends; /* Number of block extensions. */
291 long stko_mallocs; /* Block allocations by $STKOFEN. */
292 long underflows; /* Number of stack underflow calls ($STKRETN). */
293 long stko_free; /* Number of deallocations by $STKRETN. */
294 long stkm_free; /* Number of deallocations by $STKMRET. */
295 long segments; /* Current number of stack segments. */
296 long maxs; /* Maximum number of stack segments so far. */
297 long pad_size; /* Stack pad size. */
298 long current_address; /* Current stack segment address. */
299 long current_size; /* Current stack segment size. This
300 number is actually corrupted by STKSTAT to
301 include the fifteen word trailer area. */
302 long initial_address; /* Address of initial segment. */
303 long initial_size; /* Size of initial segment. */
306 /* The following structure describes the data structure which trails
307 any stack segment. I think that the description in 'asdef' is
308 out of date. I only describe the parts that I am sure about. */
312 long this_address; /* Address of this block. */
313 long this_size; /* Size of this block (does not include
317 long link; /* Address of trailer block of previous
332 # endif /* not CRAY_STACK */
335 /* Determine a "stack measure" for an arbitrary ADDRESS.
336 I doubt that "lint" will like this much. */
339 i00afunc (long *address)
341 struct stk_stat status;
342 struct stk_trailer *trailer;
346 /* We want to iterate through all of the segments. The first
347 step is to get the stack status structure. We could do this
348 more quickly and more directly, perhaps, by referencing the
349 $LM00 common block, but I know that this works. */
353 /* Set up the iteration. */
355 trailer = (struct stk_trailer *) (status.current_address
356 + status.current_size
359 /* There must be at least one stack segment. Therefore it is
360 a fatal error if "trailer" is null. */
365 /* Discard segments that do not contain our argument address. */
369 block = (long *) trailer->this_address;
370 size = trailer->this_size;
371 if (block == 0 || size == 0)
373 trailer = (struct stk_trailer *) trailer->link;
374 if ((block <= address) && (address < (block + size)))
378 /* Set the result to the offset in this segment and add the sizes
379 of all predecessor segments. */
381 result = address - block;
390 if (trailer->this_size <= 0)
392 result += trailer->this_size;
393 trailer = (struct stk_trailer *) trailer->link;
395 while (trailer != 0);
397 /* We are done. Note that if you present a bogus address (one
398 not in any segment), you will get a different number back, formed
399 from subtracting the address of the first block. This is probably
400 not what you want. */
405 # else /* not CRAY2 */
406 /* Stack address function for a CRAY-1, CRAY X-MP, or CRAY Y-MP.
407 Determine the number of the cell within the stack,
408 given the address of the cell. The purpose of this
409 routine is to linearize, in some sense, stack addresses
413 i00afunc (long address)
417 long size, pseg, this_segment, stack;
420 struct stack_segment_linkage *ssptr;
422 /* Register B67 contains the address of the end of the
423 current stack segment. If you (as a subprogram) store
424 your registers on the stack and find that you are past
425 the contents of B67, you have overflowed the segment.
427 B67 also points to the stack segment linkage control
428 area, which is what we are really interested in. */
430 stkl = CRAY_STACKSEG_END ();
431 ssptr = (struct stack_segment_linkage *) stkl;
433 /* If one subtracts 'size' from the end of the segment,
434 one has the address of the first word of the segment.
436 If this is not the first segment, 'pseg' will be
439 pseg = ssptr->sspseg;
440 size = ssptr->sssize;
442 this_segment = stkl - size;
444 /* It is possible that calling this routine itself caused
445 a stack overflow. Discard stack segments which do not
446 contain the target address. */
448 while (!(this_segment <= address && address <= stkl))
450 # ifdef DEBUG_I00AFUNC
451 fprintf (stderr, "%011o %011o %011o\n", this_segment, address, stkl);
456 ssptr = (struct stack_segment_linkage *) stkl;
457 size = ssptr->sssize;
458 pseg = ssptr->sspseg;
459 this_segment = stkl - size;
462 result = address - this_segment;
464 /* If you subtract pseg from the current end of the stack,
465 you get the address of the previous stack segment's end.
466 This seems a little convoluted to me, but I'll bet you save
467 a cycle somewhere. */
471 # ifdef DEBUG_I00AFUNC
472 fprintf (stderr, "%011o %011o\n", pseg, size);
475 ssptr = (struct stack_segment_linkage *) stkl;
476 size = ssptr->sssize;
477 pseg = ssptr->sspseg;
483 # endif /* not CRAY2 */
486 # endif /* no alloca */
487 #endif /* not GCC version 2 */