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crypto: Use local accumulator t[] in mul_amm_256 to avoid aliasing #1477

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crypto: Use local accumulator t[] in mul_amm_256 to avoid aliasing #1477

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41 changes: 23 additions & 18 deletions lib/evmone_precompiles/mulmod.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -10,40 +10,45 @@ void mul_amm_256(std::span<uint64_t, 4> r, std::span<const uint64_t, 4> x,
std::span<const uint64_t, 4> y, std::span<const uint64_t, 4> mod, uint64_t mod_inv) noexcept
{
static constexpr size_t N = 4;
const auto r_lo = r.subspan<0, 3>();
const auto r_hi = r.subspan<1>();

// Local accumulator t[] avoids aliasing penalties when r overlaps x or y.
std::array<uint64_t, N> t; // NOLINT(*-pro-type-member-init)
const auto t_lo = std::span{t}.subspan<0, N - 1>();
const auto t_hi = std::span{t}.subspan<1>();
const auto mod_hi = mod.subspan<1>();

// First iteration: r is uninitialized, so use mul instead of addmul.
bool r_carry = false;
// First iteration: t is uninitialized, so use mul instead of addmul.
bool t_carry = false;
{
const auto c1 = mul(r, x, y[0]);
const auto c1 = mul(t, x, y[0]);

const auto m = r[0] * mod_inv;
const auto c2 = (umul(mod[0], m) + r[0])[1];
const auto m = t[0] * mod_inv;
const auto c2 = (umul(mod[0], m) + t[0])[1];

const auto c3 = addmul(r_lo, r_hi, mod_hi, m, c2);
std::tie(r[N - 1], r_carry) = addc(c1, c3);
const auto c3 = addmul(t_lo, t_hi, mod_hi, m, c2);
std::tie(t[N - 1], t_carry) = addc(c1, c3);
}

// Remaining 3 iterations.
#pragma GCC unroll N - 1
for (size_t i = 1; i != N; ++i)
{
const auto c1 = addmul(r, r, x, y[i]);
const auto [sum1, d1] = addc(c1, uint64_t{r_carry});
const auto c1 = addmul(t, t, x, y[i]);
const auto [sum1, d1] = addc(c1, uint64_t{t_carry});

const auto m = r[0] * mod_inv;
const auto c2 = (umul(mod[0], m) + r[0])[1];
const auto m = t[0] * mod_inv;
const auto c2 = (umul(mod[0], m) + t[0])[1];

const auto c3 = addmul(r_lo, r_hi, mod_hi, m, c2);
const auto c3 = addmul(t_lo, t_hi, mod_hi, m, c2);
const auto [sum2, d2] = addc(sum1, c3);
r[N - 1] = sum2;
t[N - 1] = sum2;
assert(!(d1 && d2));
r_carry = d1 || d2;
t_carry = d1 || d2;
}

if (r_carry)
sub(r, mod);
if (t_carry)
sub(t, mod);

std::ranges::copy(t, r.begin());
}
} // namespace evmone::crypto