external/bsd/pcc/dist/pcc-libs/libpcc/_alloca.c

   1 /*      $Id: _alloca.c,v 1.1.1.2 2009/09/04 00:27:36 gmcgarry Exp $     */
   2 /*
   3  * This explanation of _alloca() comes from Chris Giese, posted to
   4  * alt.os.dev:
   5  *
   6  * By default, Windows reserves 1 meg of virtual memory for the stack.
   7  * No page of stack memory is actually allocated (commited) until the
   8  * page is accessed.  This is demand-allocation.  The page beyond the
   9  * top of the stack is the guard page.  If this page is accessed,
  10  * memory will be allocated for it, and the guard page moved downward
  11  * by 4K (one page).  Thus, the stack can grow beyond the initial 1 meg.
  12  * Windows will not, however, let you grow the stack by accessing
  13  * discontiguous pages of memory.  Going beyond the guard page causes
  14  * an exception.  Stack-probing code prevents this.
  15  */
  16
  17 #ifndef __MSC__
  18
  19 asm(    "       .text\n"
  20         "       .globl __alloca\n"
  21         "__alloca:\n"
  22 #ifdef __i386__
  23         "       pop %edx\n"
  24         "       pop %eax\n"
  25         "       add $3,%eax\n"
  26         "       and $-4,%eax\n"
  27         "1:     cmp $4096,%eax\n"
  28         "       jge 2f\n"
  29         "       sub %eax,%esp\n"
  30         "       test %eax,(%esp)\n"
  31         "       mov %esp,%eax\n"
  32         "       push %edx\n"
  33         "       push %edx\n"
  34         "       ret\n"
  35         "2:     sub $4096,%esp\n"
  36         "       sub $4096,%eax\n"
  37         "       test %eax,(%esp)\n"
  38         "       jmp 1b\n"
  39 #endif
  40 );
  41
  42 #endif