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/* alloca.c -- allocate automatically reclaimed memory
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(Mostly) portable public-domain implementation -- D A Gwyn
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This implementation of the PWB library alloca function,
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which is used to allocate space off the run-time stack so
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that it is automatically reclaimed upon procedure exit,
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was inspired by discussions with J. Q. Johnson of Cornell.
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J.Otto Tennant <jot@cray.com> contributed the Cray support.
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There are some preprocessor constants that can
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be defined when compiling for your specific system, for
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improved efficiency; however, the defaults should be okay.
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The general concept of this implementation is to keep
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track of all alloca-allocated blocks, and reclaim any
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that are found to be deeper in the stack than the current
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invocation. This heuristic does not reclaim storage as
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soon as it becomes invalid, but it will do so eventually.
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As a special case, alloca(0) reclaims storage without
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allocating any. It is a good idea to use alloca(0) in
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your main control loop, etc. to force garbage collection. */
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/* If compiling with GCC 2, this file's not needed. */
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#if !defined (__GNUC__) || __GNUC__ < 2
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/* If someone has defined alloca as a macro,
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there must be some other way alloca is supposed to work. */
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/* If your stack is a linked list of frames, you have to
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provide an "address metric" ADDRESS_FUNCTION macro. */
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#if defined (CRAY) && defined (CRAY_STACKSEG_END)
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#define ADDRESS_FUNCTION(arg) (char *) i00afunc (&(arg))
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#define ADDRESS_FUNCTION(arg) &(arg)
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typedef void *pointer;
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typedef char *pointer;
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extern pointer malloc ();
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/* Define STACK_DIRECTION if you know the direction of stack
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growth for your system; otherwise it will be automatically
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STACK_DIRECTION > 0 => grows toward higher addresses
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STACK_DIRECTION < 0 => grows toward lower addresses
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STACK_DIRECTION = 0 => direction of growth unknown */
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#ifndef STACK_DIRECTION
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#define STACK_DIRECTION 0 /* Direction unknown. */
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#if STACK_DIRECTION != 0
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#define STACK_DIR STACK_DIRECTION /* Known at compile-time. */
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#else /* STACK_DIRECTION == 0; need run-time code. */
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static int stack_dir; /* 1 or -1 once known. */
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#define STACK_DIR stack_dir
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find_stack_direction ()
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static char *addr = NULL; /* Address of first `dummy', once known. */
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auto char dummy; /* To get stack address. */
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{ /* Initial entry. */
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addr = ADDRESS_FUNCTION (dummy);
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find_stack_direction (); /* Recurse once. */
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if (ADDRESS_FUNCTION (dummy) > addr)
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stack_dir = 1; /* Stack grew upward. */
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stack_dir = -1; /* Stack grew downward. */
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#endif /* STACK_DIRECTION == 0 */
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/* An "alloca header" is used to:
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(a) chain together all alloca'ed blocks;
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(b) keep track of stack depth.
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It is very important that sizeof(header) agree with malloc
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alignment chunk size. The following default should work okay. */
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#define ALIGN_SIZE sizeof(double)
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char align[ALIGN_SIZE]; /* To force sizeof(header). */
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union hdr *next; /* For chaining headers. */
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char *deep; /* For stack depth measure. */
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static header *last_alloca_header = NULL; /* -> last alloca header. */
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/* Return a pointer to at least SIZE bytes of storage,
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which will be automatically reclaimed upon exit from
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the procedure that called alloca. Originally, this space
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was supposed to be taken from the current stack frame of the
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caller, but that method cannot be made to work for some
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implementations of C, for example under Gould's UTX/32. */
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auto char probe; /* Probes stack depth: */
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register char *depth = ADDRESS_FUNCTION (probe);
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#if STACK_DIRECTION == 0
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if (STACK_DIR == 0) /* Unknown growth direction. */
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find_stack_direction ();
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/* Reclaim garbage, defined as all alloca'd storage that
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was allocated from deeper in the stack than currently. */
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register header *hp; /* Traverses linked list. */
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for (hp = last_alloca_header; hp != NULL;)
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if ((STACK_DIR > 0 && hp->h.deep > depth)
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|| (STACK_DIR < 0 && hp->h.deep < depth))
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register header *np = hp->h.next;
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free ((pointer) hp); /* Collect garbage. */
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hp = np; /* -> next header. */
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break; /* Rest are not deeper. */
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last_alloca_header = hp; /* -> last valid storage. */
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return NULL; /* No allocation required. */
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/* Allocate combined header + user data storage. */
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register pointer new = malloc (sizeof (header) + size);
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/* Address of header. */
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((header *) new)->h.next = last_alloca_header;
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((header *) new)->h.deep = depth;
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last_alloca_header = (header *) new;
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/* User storage begins just after header. */
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return (pointer) ((char *) new + sizeof (header));
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#if defined (CRAY) && defined (CRAY_STACKSEG_END)
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#ifdef DEBUG_I00AFUNC
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/* Stack structures for CRAY-1, CRAY X-MP, and CRAY Y-MP */
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struct stack_control_header
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long shgrow:32; /* Number of times stack has grown. */
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long shaseg:32; /* Size of increments to stack. */
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long shhwm:32; /* High water mark of stack. */
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long shsize:32; /* Current size of stack (all segments). */
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/* The stack segment linkage control information occurs at
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the high-address end of a stack segment. (The stack
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grows from low addresses to high addresses.) The initial
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part of the stack segment linkage control information is
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0200 (octal) words. This provides for register storage
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for the routine which overflows the stack. */
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struct stack_segment_linkage
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long ss[0200]; /* 0200 overflow words. */
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long sssize:32; /* Number of words in this segment. */
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long ssbase:32; /* Offset to stack base. */
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long sspseg:32; /* Offset to linkage control of previous
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long sstcpt:32; /* Pointer to task common address block. */
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long sscsnm; /* Private control structure number for
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long ssusr1; /* Reserved for user. */
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long ssusr2; /* Reserved for user. */
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long sstpid; /* Process ID for pid based multi-tasking. */
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long ssgvup; /* Pointer to multitasking thread giveup. */
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long sscray[7]; /* Reserved for Cray Research. */
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/* The following structure defines the vector of words
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returned by the STKSTAT library routine. */
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long now; /* Current total stack size. */
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long maxc; /* Amount of contiguous space which would
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be required to satisfy the maximum
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stack demand to date. */
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long high_water; /* Stack high-water mark. */
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long overflows; /* Number of stack overflow ($STKOFEN) calls. */
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long hits; /* Number of internal buffer hits. */
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long extends; /* Number of block extensions. */
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long stko_mallocs; /* Block allocations by $STKOFEN. */
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long underflows; /* Number of stack underflow calls ($STKRETN). */
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long stko_free; /* Number of deallocations by $STKRETN. */
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long stkm_free; /* Number of deallocations by $STKMRET. */
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long segments; /* Current number of stack segments. */
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long maxs; /* Maximum number of stack segments so far. */
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long pad_size; /* Stack pad size. */
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long current_address; /* Current stack segment address. */
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long current_size; /* Current stack segment size. This
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number is actually corrupted by STKSTAT to
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include the fifteen word trailer area. */
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long initial_address; /* Address of initial segment. */
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long initial_size; /* Size of initial segment. */
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/* The following structure describes the data structure which trails
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any stack segment. I think that the description in 'asdef' is
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out of date. I only describe the parts that I am sure about. */
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long this_address; /* Address of this block. */
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long this_size; /* Size of this block (does not include
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long link; /* Address of trailer block of previous
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#endif /* not CRAY_STACK */
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/* Determine a "stack measure" for an arbitrary ADDRESS.
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I doubt that "lint" will like this much. */
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struct stk_stat status;
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struct stk_trailer *trailer;
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/* We want to iterate through all of the segments. The first
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step is to get the stack status structure. We could do this
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more quickly and more directly, perhaps, by referencing the
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$LM00 common block, but I know that this works. */
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/* Set up the iteration. */
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trailer = (struct stk_trailer *) (status.current_address
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+ status.current_size
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/* There must be at least one stack segment. Therefore it is
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a fatal error if "trailer" is null. */
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/* Discard segments that do not contain our argument address. */
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block = (long *) trailer->this_address;
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size = trailer->this_size;
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if (block == 0 || size == 0)
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trailer = (struct stk_trailer *) trailer->link;
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if ((block <= address) && (address < (block + size)))
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/* Set the result to the offset in this segment and add the sizes
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of all predecessor segments. */
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result = address - block;
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if (trailer->this_size <= 0)
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result += trailer->this_size;
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trailer = (struct stk_trailer *) trailer->link;
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while (trailer != 0);
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/* We are done. Note that if you present a bogus address (one
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not in any segment), you will get a different number back, formed
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from subtracting the address of the first block. This is probably
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not what you want. */
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#else /* not CRAY2 */
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/* Stack address function for a CRAY-1, CRAY X-MP, or CRAY Y-MP.
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Determine the number of the cell within the stack,
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given the address of the cell. The purpose of this
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routine is to linearize, in some sense, stack addresses
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long size, pseg, this_segment, stack;
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struct stack_segment_linkage *ssptr;
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/* Register B67 contains the address of the end of the
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current stack segment. If you (as a subprogram) store
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your registers on the stack and find that you are past
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the contents of B67, you have overflowed the segment.
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B67 also points to the stack segment linkage control
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area, which is what we are really interested in. */
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stkl = CRAY_STACKSEG_END ();
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ssptr = (struct stack_segment_linkage *) stkl;
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/* If one subtracts 'size' from the end of the segment,
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one has the address of the first word of the segment.
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If this is not the first segment, 'pseg' will be
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pseg = ssptr->sspseg;
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size = ssptr->sssize;
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this_segment = stkl - size;
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/* It is possible that calling this routine itself caused
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a stack overflow. Discard stack segments which do not
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contain the target address. */
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while (!(this_segment <= address && address <= stkl))
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#ifdef DEBUG_I00AFUNC
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fprintf (stderr, "%011o %011o %011o\n", this_segment, address, stkl);
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ssptr = (struct stack_segment_linkage *) stkl;
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size = ssptr->sssize;
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pseg = ssptr->sspseg;
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this_segment = stkl - size;
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result = address - this_segment;
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/* If you subtract pseg from the current end of the stack,
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you get the address of the previous stack segment's end.
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This seems a little convoluted to me, but I'll bet you save
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a cycle somewhere. */
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#ifdef DEBUG_I00AFUNC
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fprintf (stderr, "%011o %011o\n", pseg, size);
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ssptr = (struct stack_segment_linkage *) stkl;
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size = ssptr->sssize;
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pseg = ssptr->sspseg;
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#endif /* not CRAY2 */
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#endif /* no alloca */
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#endif /* not GCC version 2 */
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alloca.c
