// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0 OR ISC OR MIT-0

// ----------------------------------------------------------------------------
// Subtract modulo p_sm2, z := (x - y) mod p_sm2
// Inputs x[4], y[4]; output z[4]
//
//    extern void bignum_sub_sm2
//     (uint64_t z[static 4], uint64_t x[static 4], uint64_t y[static 4]);
//
// Standard x86-64 ABI: RDI = z, RSI = x, RDX = y
// Microsoft x64 ABI:   RCX = z, RDX = x, R8 = y
// ----------------------------------------------------------------------------

#include "_internal_s2n_bignum.h"


        S2N_BN_SYM_VISIBILITY_DIRECTIVE(bignum_sub_sm2)
        S2N_BN_SYM_PRIVACY_DIRECTIVE(bignum_sub_sm2)
        .text

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

#define d0 %rax
#define d1 %rcx
#define d2 %r8
#define d3 %r9

#define n1 %r10
#define n3 %rdx
#define c %r11

#define n1short %r10d



S2N_BN_SYMBOL(bignum_sub_sm2):
        _CET_ENDBR

#if WINDOWS_ABI
        pushq   %rdi
        pushq   %rsi
        movq    %rcx, %rdi
        movq    %rdx, %rsi
        movq    %r8, %rdx
#endif

// Load and subtract the two inputs as [d3;d2;d1;d0] = x - y (modulo 2^256)

        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

// Capture the carry, which indicates x < y, and create corresponding masked
// correction p_sm2' = [n3; c; n1; c] to add

        movq    $0xffffffff00000000, n1
        sbbq    c, c
        andq    c, n1
        movq    c, n3
        btr     $32, n3

// Do the corrective addition and copy to output

        addq    c, d0
        movq    d0, (z)
        adcq    n1, d1
        movq    d1, 8(z)
        adcq    c, d2
        movq    d2, 16(z)
        adcq    n3, d3
        movq    d3, 24(z)

#if WINDOWS_ABI
        popq   %rsi
        popq   %rdi
#endif
        ret

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