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Subtraction optimization

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This commit is contained in:
Tomasz Drwięga 2016-03-07 19:03:29 +01:00
parent c5840be1cb
commit 76865694ce
1 changed files with 16 additions and 3 deletions
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19
util/bigint/src/uint.rs
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@ -166,9 +166,22 @@ macro_rules! uint_overflowing_add {
#[cfg(not(all(asm_available, target_arch="x86_64")))]
macro_rules! uint_overflowing_sub {
($name:ident, $n_words: expr, $self_expr: expr, $other: expr) => ({
let res = overflowing!((!$other).overflowing_add(From::from(1u64)));
let res = overflowing!($self_expr.overflowing_add(res));
(res, $self_expr < $other)
let $name(ref me) = $self_expr;
let $name(ref you) = $other;
let mut ret = [0u64; $n_words];
let mut carry = 0u64;
for i in 0..$n_words {
let (res1, overflow1) = me[i].overflowing_sub(you[i]);
let (res2, overflow2) = res1.overflowing_sub(carry);
ret[i] = res2;
carry = overflow1 as u64 + overflow2 as u64;
}
($name(ret), carry > 0)
})
}