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/* Intel Pentium-4 mpn_submul_1 -- Multiply a limb vector with a limb and
* subtract the result from a second limb vector.
*
* Copyright 2001, 2002 Free Software Foundation, Inc.
*
* This file is part of Libgcrypt.
*
* Libgcrypt is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as
* published by the Free Software Foundation; either version 2.1 of
* the License, or (at your option) any later version.
*
* Libgcrypt is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
*
* Note: This code is heavily based on the GNU MP Library.
* Actually it's the same code with only minor changes in the
* way the data is stored; this is to support the abstraction
* of an optional secure memory allocation which may be used
* to avoid revealing of sensitive data due to paging etc.
*/
#include "sysdep.h"
#include "asm-syntax.h"
/*******************
* mpi_limb_t
* _gcry_mpih_submul_1( mpi_ptr_t res_ptr, (sp + 4)
* mpi_ptr_t s1_ptr, (sp + 8)
* mpi_size_t s1_size, (sp + 12)
* mpi_limb_t s2_limb) (sp + 16)
*
* P4: 7 cycles/limb, unstable timing, at least on early Pentium4 silicon
* (stepping 10).
*
* This code is not particularly good at 7 c/l. The dependent chain is only
* 4 c/l and there's only 4 MMX unit instructions, so it's not clear why that
* speed isn't achieved.
*
* The arrangements made here to get a two instruction dependent chain are
* slightly subtle. In the loop the carry (or borrow rather) is a negative
* so that a paddq can be used to give a low limb ready to store, and a high
* limb ready to become the new carry after a psrlq.
*
* If the carry was a simple twos complement negative then the psrlq shift
* would need to bring in 0 bits or 1 bits according to whether the high was
* zero or non-zero, since a non-zero value would represent a negative
* needing sign extension. That wouldn't be particularly easy to arrange and
* certainly would add an instruction to the dependent chain, so instead an
* offset is applied so that the high limb will be 0xFFFFFFFF+c. With c in
* the range -0xFFFFFFFF to 0, the value 0xFFFFFFFF+c is in the range 0 to
* 0xFFFFFFFF and is therefore always positive and can always have 0 bits
* shifted in, which is what psrlq does.
*
* The extra 0xFFFFFFFF must be subtracted before c is used, but that can be
* done off the dependent chain. The total adjustment then is to add
* 0xFFFFFFFF00000000 to offset the new carry, and subtract
* 0x00000000FFFFFFFF to remove the offset from the current carry, for a net
* add of 0xFFFFFFFE00000001. In the code this is applied to the destination
* limb when fetched.
*
* It's also possible to view the 0xFFFFFFFF adjustment as a ones-complement
* negative, which is how it's undone for the return value, but that doesn't
* seem as clear.
*/
TEXT
ALIGN (4)
GLOBL C_SYMBOL_NAME(_gcry_mpih_submul_1)
C_SYMBOL_NAME(_gcry_mpih_submul_1:)
pxor %mm1, %mm1
.Lstart_1c:
movl 8(%esp), %eax
pcmpeqd %mm0, %mm0
movd 16(%esp), %mm7
pcmpeqd %mm6, %mm6
movl 4(%esp), %edx
psrlq $32, %mm0
movl 12(%esp), %ecx
psllq $32, %mm6
psubq %mm0, %mm6
psubq %mm1, %mm0
/*
C eax src, incrementing
C ebx
C ecx loop counter, decrementing
C edx dst, incrementing
C
C mm0 0xFFFFFFFF - borrow
C mm6 0xFFFFFFFE00000001
C mm7 multiplier
*/
.Lloop:
movd (%eax), %mm1
leal 4(%eax), %eax
movd (%edx), %mm2
paddq %mm6, %mm2
pmuludq %mm7, %mm1
psubq %mm1, %mm2
paddq %mm2, %mm0
subl $1, %ecx
movd %mm0, (%edx)
psrlq $32, %mm0
leal 4(%edx), %edx
jnz .Lloop
movd %mm0, %eax
notl %eax
emms
ret