/*_ _alloca.d
 * Copyright (C) 1990-2003 by Digital Mars, www.digitalmars.com
 * All Rights Reserved
 * Written by Walter Bright
 */

/+
#if DOS386
extern size_t _x386_break;
#else
extern size_t _pastdata;
#endif
+/

/*******************************************
 * Allocate data from the caller's stack frame.
 * This is a 'magic' function that needs help from the compiler to
 * work right, do not change its name, do not call it from other compilers.
 * Input:
 *	nbytes	number of bytes to allocate
 *	ECX	address of variable with # of bytes in locals
 *		This is adjusted upon return to reflect the additional
 *		size of the stack frame.
 * Returns:
 *	EAX	allocated data, null if stack overflows
 */

extern (C) void* __alloca(int nbytes)
{
    asm
    {
	naked			;
	mov	EDX,ECX		;
	mov	EAX,4[ESP]	; // get nbytes
	push	EBX		;
	push	EDI		;
	push	ESI		;
	add	EAX,3		;
	and	EAX,0xFFFFFFFC	; // round up to dword
	jnz	Abegin		;
	mov	EAX,4		; // allow zero bytes allocation, 0 rounded to dword is 4..
    Abegin:
	mov	ESI,EAX		; // ESI = nbytes
	neg	EAX		;
	add	EAX,ESP		; // EAX is now what the new ESP will be.
	jae	Aoverflow	;
    }
    version (Win32)
    {
    asm
    {
	// We need to be careful about the guard page
	// Thus, for every 4k page, touch it to cause the OS to load it in.
	mov	ECX,EAX		; // ECX is new location for stack
	mov	EBX,ESI		; // EBX is size to "grow" stack
    L1:
	test	[ECX+EBX],EBX	; // bring in page
	sub	EBX,0x1000	; // next 4K page down
	jae	L1		; // if more pages
	test	[ECX],EBX	; // bring in last page
    }
    }
    version (DOS386)
    {
    asm
    {
	// is ESP off bottom?
	cmp	EAX,_x386_break	;
	jbe	Aoverflow	;
    }
    }
    version (Unix)
    {
    asm
    {
	cmp	EAX,_pastdata	;
	jbe	Aoverflow	; // Unlikely - ~2 Gbytes under UNIX
    }
    }
    asm
    {
	// Copy down to [ESP] the temps on the stack.
	// The number of temps is (EBP - ESP - locals).
	mov	ECX,EBP		;
	sub	ECX,ESP		;
	sub	ECX,[EDX]	; // ECX = number of temps (bytes) to move.
	add	[EDX],ESI	; // adjust locals by nbytes for next call to alloca()
	mov	ESP,EAX		; // Set up new stack pointer.
	add	EAX,ECX		; // Return value = ESP + temps.
	mov	EDI,ESP		; // Destination of copy of temps.
	add	ESI,ESP		; // Source of copy.
	shr	ECX,2		; // ECX to count of dwords in temps
				  // Always at least 4 (nbytes, EIP, ESI,and EDI).
	rep			;
	movsd			;
	jmp	done		;

    Aoverflow:
	// Overflowed the stack.  Return null
	xor	EAX,EAX		;

    done:
	pop	ESI		;
	pop	EDI		;
	pop	EBX		;
	ret			;
    }
}