/*
 * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License").
 * You may not use this file except in compliance with the License.
 * A copy of the License is located at
 *
 *  http://aws.amazon.com/apache2.0
 *
 * or in the "LICENSE" file accompanying this file. This file is distributed
 * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
 * express or implied. See the License for the specific language governing
 * permissions and limitations under the License.
 */

// ----------------------------------------------------------------------------
// Subtract modulo p_521, z := (x - y) mod p_521
// Inputs x[9], y[9]; output z[9]
//
//    extern void bignum_sub_p521
//     (uint64_t z[static 9], uint64_t x[static 9], uint64_t y[static 9]);
//
// Standard x86-64 ABI: RDI = z, RSI = x, RDX = y
// ----------------------------------------------------------------------------


        .globl  bignum_sub_p521
        .globl  _bignum_sub_p521
        .text

#define z %rdi
#define x %rsi
#define y %rdx

#define d0 %rax
#define d1 %rcx
#define d2 %r8
#define d3 %r9
#define d4 %r10
#define d5 %r11
#define d6 %r12
#define d7 %rbx

// Re-use one input pointer as other variable once safe to do so

#define d8 %rsi



bignum_sub_p521:
_bignum_sub_p521:

// Save more registers to play with

        pushq   %rbx
        pushq   %r12

// First just subtract the numbers as [d8;d7;d6;d5;d4;d3;d2;d1;d0] = x - y

        movq    (x), d0
        subq    (y), d0
        movq    8(x), d1
        sbbq    8(y), d1
        movq    16(x), d2
        sbbq    16(y), d2
        movq    24(x), d3
        sbbq    24(y), d3
        movq    32(x), d4
        sbbq    32(y), d4
        movq    40(x), d5
        sbbq    40(y), d5
        movq    48(x), d6
        sbbq    48(y), d6
        movq    56(x), d7
        sbbq    56(y), d7
        movq    64(x), d8
        sbbq    64(y), d8

// Now if x < y we want (x - y) + p_521 == (x - y) - 1 (mod 2^521)
// Otherwise we just want the existing x - y result. So subtract
// 1 iff the initial subtraction carried, then mask to 521 bits.
// Write back the results as generated.

        sbbq    $0, d0
        movq    d0, (z)
        sbbq    $0, d1
        movq    d1, 8(z)
        sbbq    $0, d2
        movq    d2, 16(z)
        sbbq    $0, d3
        movq    d3, 24(z)
        sbbq    $0, d4
        movq    d4, 32(z)
        sbbq    $0, d5
        movq    d5, 40(z)
        sbbq    $0, d6
        movq    d6, 48(z)
        sbbq    $0, d7
        movq    d7, 56(z)
        sbbq    $0, d8
        andq    $0x1FF, d8
        movq    d8, 64(z)

// Restore registers and return

        popq    %r12
        popq    %rbx

        ret

#if defined(__linux__) && defined(__ELF__)
.section .note.GNU-stack,"",%progbits
#endif
