From f492676a550f5689b8e85f7fc43e4e6152061e39 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Pawe=C5=82=20Bylica?= <pawel@hepcolgum.band>
Date: Mon, 10 Aug 2026 21:01:27 +0200
Subject: [PATCH] crypto: Use the generic ecc::dbl for G2 point doubling
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bn254::dbl duplicated the generic a=0 Jacobian doubling already in ecc::dbl.
Both cost 18 Fq multiplications, but bn254::dbl reached the z coordinate via
(y+z)² − y² − z², needing a z² that has no other use there, where ecc::dbl
takes 2yz directly: same multiplication count, about a dozen fewer modular
additions per doubling, and a G2 subgroup check does 63 doublings per pair.
lin_func_and_dbl keeps its own copy of the formula, since it needs z² for the
line coefficients anyway.

Cuts about 0.3% off the ECPAIRING instruction count.
---
 lib/evmone_precompiles/ecc.hpp                |  2 +-
 .../pairing/bn254/utils.hpp                   | 27 -------------------
 2 files changed, 1 insertion(+), 28 deletions(-)

diff --git a/lib/evmone_precompiles/ecc.hpp b/lib/evmone_precompiles/ecc.hpp
index e71d2b6241..968b91c925 100644
--- a/lib/evmone_precompiles/ecc.hpp
+++ b/lib/evmone_precompiles/ecc.hpp
@@ -371,7 +371,7 @@ ProjPoint<Curve> add(const ProjPoint<Curve>& p, const AffinePoint<Curve>& q) noe
 }
 
 template <typename Curve>
-ProjPoint<Curve> dbl(const ProjPoint<Curve>& p) noexcept
+constexpr ProjPoint<Curve> dbl(const ProjPoint<Curve>& p) noexcept
 {
     const auto& [x1, y1, z1] = p;
 
diff --git a/lib/evmone_precompiles/pairing/bn254/utils.hpp b/lib/evmone_precompiles/pairing/bn254/utils.hpp
index 0c5f8ced1e..3179ba5408 100644
--- a/lib/evmone_precompiles/pairing/bn254/utils.hpp
+++ b/lib/evmone_precompiles/pairing/bn254/utils.hpp
@@ -192,33 +192,6 @@ constexpr ecc::ProjPoint<E2> add(
     return {X3, Y3, Z3};
 }
 
-/// Computes `Q + Q` in Jacobian coordinates.
-constexpr ecc::ProjPoint<E2> dbl(const ecc::ProjPoint<E2>& Q) noexcept
-{
-    const auto& x = Q.x;
-    const auto& y = Q.y;
-    const auto& z = Q.z;
-
-    const auto y_squared = y * y;
-    const auto x_squared = x * x;
-    const auto z_squared = z * z;
-    const auto y_4 = y_squared * y_squared;
-    const auto _4y_4 = y_4 + y_4 + y_4 + y_4;
-
-    const auto R = y_squared + y_squared;
-    const auto A = (x + R);
-    const auto S = A * A - x_squared - _4y_4;  // 2xR = (x+R)^2 - x^2 - R^2
-    const auto M = x_squared + x_squared + x_squared;
-
-    const auto N = y + z;
-
-    const auto Xp = M * M - (S + S);
-    const auto Yp = M * (S - Xp) - (_4y_4 + _4y_4);
-    const auto Zp = N * N - y_squared - z_squared;  // 2yz = (y+z)^2 - y^2 - z^2
-
-    return {Xp, Yp, Zp};
-}
-
 /// Computes `N` doubles of the point `a` in Jacobian coordinates.
 template <int N>
 constexpr ecc::ProjPoint<E2> n_dbl(const ecc::ProjPoint<E2>& a) noexcept