;[]-----------------------------------------------------------------[]
;|   _ALLOCA.ASM -- allocate temporary stack space                   |
;[]-----------------------------------------------------------------[]

;
;       C/C++ Run Time Library - Version 6.0
; 
;       Copyright (c) 1992, 1993 by Borland International
;       All Rights Reserved.
; 

        include rules.asi
        include init.inc

;       Segments Definitions

Header@

; REGSTACK contains the number of bytes taken up in the stack
; by registers saved in the normal function prologue.  This includes
; space for EBX, ESI, and EDI.

REGSTACK   equ     12

; The variable _stkindex is initialized by _startup (startup.c) to contain
; the thread local storage index used to save each thread's stack base.

ExtSym@ _stkindex, dword, cdecl

;----------------------------------------------------------------------
; Name          _alloca - allocate temporary stack memory
;
; Usage         void *_alloca(size_t size);
;
; Prototype in  _malloc.h
;
; Description   _alloca allocates size bytes on the stack; the allocated
;               space is automatically freed up when the calling function
;               exits.
;
;               Because _alloca modifies the stack pointer, do not place calls
;               to _alloca in an expression that is an argument to a function.
;
;               If the calling function does not contain any references
;               to local variables in the stack, the stack will not
;               be restored correctly when the function exits, resulting
;               in a program crash.  To ensure that the stack is restored
;               correctly, use the following code in the calling function:
;
;                       char *p;
;                       char dummy[1];
;
;                       dummy[0] = 0;
;                       ...
;                       p = alloca(nbytes);
;
;               This is an internal function that corresponds to the
;               user-visible function alloca().  That function is
;               declared in malloc.h.
;
; Return value  Returns a pointer to the allocated stack area on success.
;               If the stack cannot be extended, a NULL pointer is returned.
;               The returned pointer should never be passed to free().
;
; Note          Compatible with Microsoft C and UNIX.  Not recommended.
;               Use malloc() instead.
;----------------------------------------------------------------------

Code_seg@

ExtFunc@ TlsGetValue, APIENTRY, 4  ; API function

Func@   _alloca, public, cdecl

        pop     edx             ; pop return address
        pop     eax             ; pop size parameter 
        add     eax,3           ; round size up to multiple of 4
        and     eax,not 3
        neg     eax             ; negate size
        add     eax,esp         ; add current stack pointer
        cmp     eax,esp         ; would new ESP wrap around?
        ja      bad             ; yes - return error

; Get the current thread's stack base.  The RTL stores this in the
; thread local storage slot indexed by _stkindex.

        push    eax             ; save new ESP
        push    edx             ; save return address
        push    _stkindex@      ; ecx = TlsGetValue(_stkindex)
        Call@   TlsGetValue
        mov     ecx, eax        ; put stack base in ECX
        pop     edx             ; recover return address
        pop     eax             ; recover new ESP
        add     ecx,REGSTACK    ; adjust base for saved register variables
        cmp     ecx,eax         ; would new ESP fall below stack base?
        ja      bad

; The current stack looks like this, from high to low address:
;       EBX saved by caller (assumed to be present)
;       ESI saved by caller (assumed to be present)
; ESP-> EDI saved by caller (assumed to be present)
;       ... empty space ...
; EAX -> new stack area

; We must probe the stack in 4K increments to force NT to un-guard the
; stack guard pages.  At this point, EAX contains the future value of ESP.

        mov     ecx, esp        ; get a copy of the current ESP
probeloop:
        sub     ecx, 4096       ; bump down to the next page
        cmp     ecx, eax        ; any more stack left to probe?
        jb      pushregs        ; no - we're done
        or      byte ptr [ecx], 0   ; yes - probe this page
        jmp     probeloop

; Before we can return, we need to push on the new stack area the registers
; saved by caller in the old stack area.  We need to copy at least 3 longwords
; (for EBX, ESI, and DI).  We can't tell if these registers were really
; saved, so we assume that they were, and allocate extra space for them.

pushregs:
        mov     ecx,esp         ; get a copy of old ESP into ECX
        mov     esp,eax         ; set up new ESP
        push    dword ptr [ecx+8]  ; push copies of saved reg variables
        push    dword ptr [ecx+4]
        push    dword ptr [ecx+0]
return:
    if PopParms@ eq 0
        sub     esp, 4          ; fake argument for caller to pop
    endif
        jmp     edx             ; return

; Come here if there isn't enough stack space.  Return a null pointer.

bad:
        xor     eax,eax         ; return NULL pointer
        jmp     return

EndFunc@ _alloca

Code_EndS@

        end
