From: Chris Duncan <chris@codecow.com>
Date: Wed, 30 Sep 2026 15:30:58 +0000 (-0700)
Subject: Add field element doubling template.
X-Git-Url: https://git.codecow.com/?a=commitdiff_plain;h=948bcaf141071aa765fb634c123159faac2b6fb8;p=nano25519.git

Add field element doubling template.
---

diff --git a/scripts/fe.gen.mjs b/scripts/fe.gen.mjs
index 3f62a70..5bd688b 100644
--- a/scripts/fe.gen.mjs
+++ b/scripts/fe.gen.mjs
@@ -32,6 +32,7 @@ export function fe_add (h, f, g) {
  * @param {string} h Variable name of FieldElement difference destination
  * @param {string} f Variable name of FieldElement minuend source
  * @param {string} g Variable name of FieldElement subtrahend source
+ * @returns {string} Three vector subtraction commands
  */
 export function fe_sub (h, f, g) {
 	return `
@@ -42,6 +43,22 @@ export function fe_sub (h, f, g) {
 `
 }
 
+/**
+ * Add a FieldElement to itself and store the sum.
+ *
+ * @param {string} h Variable name of FieldElement sum destination
+ * @param {string} f Variable name of FieldElement summand source
+ * @returns {string} Three vector doubling commands
+ */
+export function fe_dbl (h, f) {
+	return `
+	// ${h} = 2*${f}
+	${h}0 = v128.shl<i32>(${f}0, 1)
+	${h}1 = v128.shl<i32>(${f}1, 1)
+	${h}2 = v128.shl<i32>(${f}2, 1)
+`
+}
+
 export const FE = `//! SPDX-FileCopyrightText: 2026 Chris Duncan <chris@codecow.com>
 //! SPDX-License-Identifier: GPL-3.0-or-later
 /**
@@ -172,9 +189,19 @@ export function fe_copy (h: FieldElement, f: FieldElement): void {
 export function fe_dbl (h: FieldElement, f: FieldElement): void {
 	const h_ptr = changetype<usize>(h)
 	const f_ptr = changetype<usize>(f)
-	v128.store(h_ptr, v128.shl<i32>(v128.load(f_ptr, 0), 1), 0)
-	v128.store(h_ptr, v128.shl<i32>(v128.load(f_ptr, 16), 1), 16)
-	v128.store_lane<i64>(h_ptr, v128.shl<i32>(v128.load(f_ptr, 32), 1), 0, 32)
+
+	let h0: v128, h1: v128, h2: v128
+
+	let f0 = v128.load(f_ptr, 0)
+	let f1 = v128.load(f_ptr, 16)
+	let f2 = v128.load(f_ptr, 32)
+
+	${fe_dbl('h', 'f')}
+
+	// store h
+	v128.store(h_ptr, h0, 0)
+	v128.store(h_ptr, h1, 16)
+	v128.store_lane<u64>(h_ptr, h2, 0, 32)
 }
 
 /**
diff --git a/src/assembly/ed25519/fe.ts b/src/assembly/ed25519/fe.ts
index fc46ae7..93e443a 100644
--- a/src/assembly/ed25519/fe.ts
+++ b/src/assembly/ed25519/fe.ts
@@ -131,9 +131,22 @@ export function fe_copy (h: FieldElement, f: FieldElement): void {
 export function fe_dbl (h: FieldElement, f: FieldElement): void {
 	const h_ptr = changetype<usize>(h)
 	const f_ptr = changetype<usize>(f)
-	v128.store(h_ptr, v128.shl<i32>(v128.load(f_ptr, 0), 1), 0)
-	v128.store(h_ptr, v128.shl<i32>(v128.load(f_ptr, 16), 1), 16)
-	v128.store_lane<i64>(h_ptr, v128.shl<i32>(v128.load(f_ptr, 32), 1), 0, 32)
+
+	let h0: v128, h1: v128, h2: v128
+
+	let f0 = v128.load(f_ptr, 0)
+	let f1 = v128.load(f_ptr, 16)
+	let f2 = v128.load(f_ptr, 32)
+
+	// h = 2*f
+	h0 = v128.shl<i32>(f0, 1)
+	h1 = v128.shl<i32>(f1, 1)
+	h2 = v128.shl<i32>(f2, 1)
+
+	// store h
+	v128.store(h_ptr, h0, 0)
+	v128.store(h_ptr, h1, 16)
+	v128.store_lane<u64>(h_ptr, h2, 0, 32)
 }
 
 /**