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// 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, z := x - y
// Inputs x[m], y[n]; outputs function return (carry-out) and z[p]
//
// extern uint64_t bignum_sub
// (uint64_t p, uint64_t *z,
// uint64_t m, uint64_t *x, uint64_t n, uint64_t *y);
//
// Does the z := x - y operation, truncating modulo p words in general and
// returning a top borrow (0 or 1) in the p'th place, only subtracting input
// words below p (as well as m and n respectively) to get the diff and borrow.
//
// Standard x86-64 ABI: RDI = p, RSI = z, RDX = m, RCX = x, R8 = n, R9 = y, returns RAX
// Microsoft x64 ABI: RCX = p, RDX = z, R8 = m, R9 = x, [RSP+40] = n, [RSP+48] = y, returns RAX
// ----------------------------------------------------------------------------
#include "s2n_bignum_internal.h"
.intel_syntax noprefix
S2N_BN_SYM_VISIBILITY_DIRECTIVE(bignum_sub)
S2N_BN_SYM_PRIVACY_DIRECTIVE(bignum_sub)
.text
#define p rdi
#define z rsi
#define m rdx
#define x rcx
#define n r8
#define y r9
#define i r10
#define a rax
#define ashort eax
S2N_BN_SYMBOL(bignum_sub):
endbr64
#if WINDOWS_ABI
push rdi
push rsi
mov rdi, rcx
mov rsi, rdx
mov rdx, r8
mov rcx, r9
mov r8, [rsp+56]
mov r9, [rsp+64]
#endif
// Zero the main index counter for both branches
xor i, i
// First clamp the two input sizes m := min(p,m) and n := min(p,n) since
// we'll never need words past the p'th. Can now assume m <= p and n <= p.
// Then compare the modified m and n and branch accordingly
cmp p, m
cmovc m, p
cmp p, n
cmovc n, p
cmp m, n
jc ylonger
// The case where x is longer or of the same size (p >= m >= n)
sub p, m
sub m, n
inc m
test n, n
jz xtest
xmainloop:
mov a, [x+8*i]
sbb a, [y+8*i]
mov [z+8*i],a
inc i
dec n
jnz xmainloop
jmp xtest
xtoploop:
mov a, [x+8*i]
sbb a, 0
mov [z+8*i],a
inc i
xtest:
dec m
jnz xtoploop
sbb a, a
test p, p
jz tailskip
tailloop:
mov [z+8*i],a
inc i
dec p
jnz tailloop
tailskip:
neg a
#if WINDOWS_ABI
pop rsi
pop rdi
#endif
ret
// The case where y is longer (p >= n > m)
ylonger:
sub p, n
sub n, m
test m, m
jz ytoploop
ymainloop:
mov a, [x+8*i]
sbb a, [y+8*i]
mov [z+8*i],a
inc i
dec m
jnz ymainloop
ytoploop:
mov ashort, 0
sbb a, [y+8*i]
mov [z+8*i],a
inc i
dec n
jnz ytoploop
sbb a, a
test p, p
jnz tailloop
neg a
#if WINDOWS_ABI
pop rsi
pop rdi
#endif
ret
#if defined(__linux__) && defined(__ELF__)
.section .note.GNU-stack,"",%progbits
#endif
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