From 3908552fb93cc692f01e6418181ddf2c74f210cd Mon Sep 17 00:00:00 2001 From: Sean Bowe Date: Tue, 15 Aug 2017 12:53:22 -0600 Subject: [PATCH] Introduce stable arithmetic to avoid nightly/unstable features for some platforms. --- Cargo.toml | 2 + src/lib.rs | 129 ++++++++++++++++++++++++++++++++++++++++++----------- 2 files changed, 106 insertions(+), 25 deletions(-) diff --git a/Cargo.toml b/Cargo.toml index 9cb5e3d27..d13ce4b6c 100644 --- a/Cargo.toml +++ b/Cargo.toml @@ -17,3 +17,5 @@ clippy = { version = "0.0.151", optional = true } [features] unstable-wnaf = [] unstable-features = ["unstable-wnaf"] +u128-support = [] +default = ["u128-support"] diff --git a/src/lib.rs b/src/lib.rs index 37f511102..ba745fa80 100644 --- a/src/lib.rs +++ b/src/lib.rs @@ -604,35 +604,114 @@ fn test_bit_iterator() { assert!(a.next().is_none()); } -/// Calculate a - b - borrow, returning the result and modifying -/// the borrow value. -#[inline(always)] -pub(crate) fn sbb(a: u64, b: u64, borrow: &mut u64) -> u64 { - let tmp = (1u128 << 64) + (a as u128) - (b as u128) - (*borrow as u128); +use self::arith::*; - *borrow = if tmp >> 64 == 0 { 1 } else { 0 }; +#[cfg(feature = "u128-support")] +mod arith { - tmp as u64 + /// Calculate a - b - borrow, returning the result and modifying + /// the borrow value. + #[inline(always)] + pub(crate) fn sbb(a: u64, b: u64, borrow: &mut u64) -> u64 { + let tmp = (1u128 << 64) + (a as u128) - (b as u128) - (*borrow as u128); + + *borrow = if tmp >> 64 == 0 { 1 } else { 0 }; + + tmp as u64 + } + + /// Calculate a + b + carry, returning the sum and modifying the + /// carry value. + #[inline(always)] + pub(crate) fn adc(a: u64, b: u64, carry: &mut u64) -> u64 { + let tmp = (a as u128) + (b as u128) + (*carry as u128); + + *carry = (tmp >> 64) as u64; + + tmp as u64 + } + + /// Calculate a + (b * c) + carry, returning the least significant digit + /// and setting carry to the most significant digit. + #[inline(always)] + pub(crate) fn mac_with_carry(a: u64, b: u64, c: u64, carry: &mut u64) -> u64 { + let tmp = (a as u128) + (b as u128) * (c as u128) + (*carry as u128); + + *carry = (tmp >> 64) as u64; + + tmp as u64 + } } -/// Calculate a + b + carry, returning the sum and modifying the -/// carry value. -#[inline(always)] -pub(crate) fn adc(a: u64, b: u64, carry: &mut u64) -> u64 { - let tmp = (a as u128) + (b as u128) + (*carry as u128); +#[cfg(not(feature = "u128-support"))] +mod arith { + #[inline(always)] + fn split_u64(i: u64) -> (u64, u64) { + (i >> 32, i & 0xFFFFFFFF) + } - *carry = (tmp >> 64) as u64; + #[inline(always)] + fn combine_u64(hi: u64, lo: u64) -> u64 { + (hi << 32) | lo + } - tmp as u64 -} - -/// Calculate a + (b * c) + carry, returning the least significant digit -/// and setting carry to the most significant digit. -#[inline(always)] -pub(crate) fn mac_with_carry(a: u64, b: u64, c: u64, carry: &mut u64) -> u64 { - let tmp = (a as u128) + (b as u128) * (c as u128) + (*carry as u128); - - *carry = (tmp >> 64) as u64; - - tmp as u64 + #[inline(always)] + pub(crate) fn sbb(a: u64, b: u64, borrow: &mut u64) -> u64 { + let (a_hi, a_lo) = split_u64(a); + let (b_hi, b_lo) = split_u64(b); + let (b, r0) = split_u64((1 << 32) + a_lo - b_lo - *borrow); + let (b, r1) = split_u64((1 << 32) + a_hi - b_hi - ((b == 0) as u64)); + + *borrow = (b == 0) as u64; + + combine_u64(r1, r0) + } + + #[inline(always)] + pub(crate) fn adc(a: u64, b: u64, carry: &mut u64) -> u64 { + let (a_hi, a_lo) = split_u64(a); + let (b_hi, b_lo) = split_u64(b); + let (carry_hi, carry_lo) = split_u64(*carry); + + let (t, r0) = split_u64(a_lo + b_lo + carry_lo); + let (t, r1) = split_u64(t + a_hi + b_hi + carry_hi); + + *carry = t; + + combine_u64(r1, r0) + } + + #[inline(always)] + pub(crate) fn mac_with_carry(a: u64, b: u64, c: u64, carry: &mut u64) -> u64 { + /* + [ b_hi | b_lo ] + [ c_hi | c_lo ] * + ------------------------------------------- + [ b_lo * c_lo ] <-- w + [ b_hi * c_lo ] <-- x + [ b_lo * c_hi ] <-- y + [ b_hi * c_lo ] <-- z + [ a_hi | a_lo ] + [ C_hi | C_lo ] + */ + + let (a_hi, a_lo) = split_u64(a); + let (b_hi, b_lo) = split_u64(b); + let (c_hi, c_lo) = split_u64(c); + let (carry_hi, carry_lo) = split_u64(*carry); + + let (w_hi, w_lo) = split_u64(b_lo * c_lo); + let (x_hi, x_lo) = split_u64(b_hi * c_lo); + let (y_hi, y_lo) = split_u64(b_lo * c_hi); + let (z_hi, z_lo) = split_u64(b_hi * c_hi); + + let (t, r0) = split_u64(w_lo + a_lo + carry_lo); + let (t, r1) = split_u64(t + w_hi + x_lo + y_lo + a_hi + carry_hi); + let (t, r2) = split_u64(t + x_hi + y_hi + z_lo); + let (_, r3) = split_u64(t + z_hi); + + *carry = combine_u64(r3, r2); + + combine_u64(r1, r0) + } }