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// $OpenBSD: bignum_modsub.S,v 1.4 2025/08/12 10:23:40 jsing Exp $
//
// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
// ----------------------------------------------------------------------------
// Subtract modulo m, z := (x - y) mod m, assuming x and y reduced
// Inputs x[k], y[k], m[k]; output z[k]
//
// extern void bignum_modsub(uint64_t k, uint64_t *z, const uint64_t *x,
// const uint64_t *y, const uint64_t *m);
//
// Standard x86-64 ABI: RDI = k, RSI = z, RDX = x, RCX = y, R8 = m
// Microsoft x64 ABI: RCX = k, RDX = z, R8 = x, R9 = y, [RSP+40] = m
// ----------------------------------------------------------------------------
#include "s2n_bignum_internal.h"
.intel_syntax noprefix
S2N_BN_SYM_VISIBILITY_DIRECTIVE(bignum_modsub)
S2N_BN_SYM_PRIVACY_DIRECTIVE(bignum_modsub)
.text
#define k rdi
#define z rsi
#define x rdx
#define y rcx
#define m r8
#define i r9
#define j r10
#define a rax
#define c r11
S2N_BN_SYMBOL(bignum_modsub):
_CET_ENDBR
#if WINDOWS_ABI
push rdi
push rsi
mov rdi, rcx
mov rsi, rdx
mov rdx, r8
mov rcx, r9
mov r8, [rsp+56]
#endif
// If k = 0 do nothing
test k, k
jz bignum_modsub_end
// Subtract z := x - y and record a mask for the carry x - y < 0
xor c, c
mov j, k
xor i, i
bignum_modsub_subloop:
mov a, [x+8*i]
sbb a, [y+8*i]
mov [z+8*i], a
inc i
dec j
jnz bignum_modsub_subloop
sbb c, c
// Now do a masked addition z := z + [c] * m
xor i, i
bignum_modsub_addloop:
mov a, [m+8*i]
and a, c
neg j
adc [z+8*i], a
sbb j, j
inc i
cmp i, k
jc bignum_modsub_addloop
bignum_modsub_end:
#if WINDOWS_ABI
pop rsi
pop rdi
#endif
ret
#if defined(__linux__) && defined(__ELF__)
.section .note.GNU-stack,"",%progbits
#endif
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