[PATCH 2/3] crypto: ecc - Optimize vli additive operations using compiler builtins

[Ahmad Fatoum <a.fatoum@barebox.org>]

This is an adoption of the corresponding Linux commit:

| commit 9deef9c5b8fcc702e69252bef46735d8b0d1f70c
| Author:     Fabian Blatter <fabianblatter09@gmail.com>
| AuthorDate: Sun Jun 7 13:24:35 2026 +0200
|
|     crypto: ecc - Optimize vli additive operations using compiler builtins
|
|     Replace the software carry flag emulation with compiler builtins.
|
|     Even the newest compilers struggle with taking advantage of the
|     hardware carry flag. Compiler builtins allow the compiler to
|     much more easily achieve this while still remaining constant-time.
|
|     This yields an approximately 6-7% performance improvement
|     on the ecc_gen_privkey, ecc_make_pub_key and crypto_ecdh_shared_secret
|     functions on x86_64 on all curve sizes.
|
|     Additionally, the code becomes much more readable.
|
|     Signed-off-by: Fabian Blatter <fabianblatter09@gmail.com>
|     Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
|     Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

We take this mainly to stay in sync with Linux, so the Origin-URL can
follow along with the next fix.

Signed-off-by: Ahmad Fatoum <a.fatoum@barebox.org>
---
 crypto/ecc.c | 100 +++++++++++++++++++++++++++++++--------------------
 1 file changed, 61 insertions(+), 39 deletions(-)

diff --git a/crypto/ecc.c b/crypto/ecc.c
index 67faae0a68f9..01cb1ae88ead 100644
--- a/crypto/ecc.c
+++ b/crypto/ecc.c
@@ -1,4 +1,4 @@
-// SPDX-Comment: Origin-URL: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/crypto/ecc.c?id=27b536a2ec8e2f85a0380c2d13c9ecbc7aaab406
+// SPDX-Comment: Origin-URL: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/crypto/ecc.c?id=9deef9c5b8fcc702e69252bef46735d8b0d1f70c
 /*
  * Copyright (c) 2013, 2014 Kenneth MacKay. All rights reserved.
  * Copyright (c) 2019 Vitaly Chikunov <vt@altlinux.org>
@@ -267,6 +267,48 @@ static void vli_rshift1(u64 *vli, unsigned int ndigits)
 	}
 }
 
+#ifdef __has_builtin
+#if __has_builtin(__builtin_addcll)
+#define USE_BUILTIN_ADDC
+#endif
+#endif
+
+/* Computes result = left + right + carry_in and updates carry_out */
+static inline void add_carry(u64 left, u64 right, u64 *result, u64 carry_in,
+			     u64 *carry_out)
+{
+#ifdef USE_BUILTIN_ADDC
+	*result = __builtin_addcll(left, right, carry_in, carry_out);
+#else
+	u64 sum1, sum2;
+	u64 c1 = __builtin_uaddll_overflow(left, right, &sum1);
+	u64 c2 = __builtin_uaddll_overflow(sum1, carry_in, &sum2);
+	*result = sum2;
+	*carry_out = c1 | c2;
+#endif
+}
+
+#ifdef __has_builtin
+#if __has_builtin(__builtin_subcll)
+#define USE_BUILTIN_SUBC
+#endif
+#endif
+
+/* Computes result = left - right - borrow_in and updates borrow_out */
+static inline void sub_borrow(u64 left, u64 right, u64 *result, u64 borrow_in,
+			      u64 *borrow_out)
+{
+#ifdef USE_BUILTIN_SUBC
+	*result = __builtin_subcll(left, right, borrow_in, borrow_out);
+#else
+	u64 diff1, diff2;
+	u64 b1 = __builtin_usubll_overflow(left, right, &diff1);
+	u64 b2 = __builtin_usubll_overflow(diff1, borrow_in, &diff2);
+	*result = diff2;
+	*borrow_out = b1 | b2;
+#endif
+}
+
 /* Computes result = left + right, returning carry. Can modify in place. */
 static u64 vli_add(u64 *result, const u64 *left, const u64 *right,
 		   unsigned int ndigits)
@@ -274,15 +316,8 @@ static u64 vli_add(u64 *result, const u64 *left, const u64 *right,
 	u64 carry = 0;
 	int i;
 
-	for (i = 0; i < ndigits; i++) {
-		u64 sum;
-
-		sum = left[i] + right[i] + carry;
-		if (sum != left[i])
-			carry = (sum < left[i]);
-
-		result[i] = sum;
-	}
+	for (i = 0; i < ndigits; i++)
+		add_carry(left[i], right[i], &result[i], carry, &carry);
 
 	return carry;
 }
@@ -291,40 +326,29 @@ static u64 vli_add(u64 *result, const u64 *left, const u64 *right,
 static u64 vli_uadd(u64 *result, const u64 *left, u64 right,
 		    unsigned int ndigits)
 {
-	u64 carry = right;
+	u64 carry;
 	int i;
 
-	for (i = 0; i < ndigits; i++) {
-		u64 sum;
+	if (ndigits == 0)
+		return right;
 
-		sum = left[i] + carry;
-		if (sum != left[i])
-			carry = (sum < left[i]);
-		else
-			carry = !!carry;
+	carry = __builtin_uaddll_overflow(left[0], right, &result[0]);
 
-		result[i] = sum;
-	}
+	for (i = 1; i < ndigits; i++)
+		carry = __builtin_uaddll_overflow(left[i], carry, &result[i]);
 
 	return carry;
 }
 
 /* Computes result = left - right, returning borrow. Can modify in place. */
 u64 vli_sub(u64 *result, const u64 *left, const u64 *right,
-		   unsigned int ndigits)
+	    unsigned int ndigits)
 {
 	u64 borrow = 0;
 	int i;
 
-	for (i = 0; i < ndigits; i++) {
-		u64 diff;
-
-		diff = left[i] - right[i] - borrow;
-		if (diff != left[i])
-			borrow = (diff > left[i]);
-
-		result[i] = diff;
-	}
+	for (i = 0; i < ndigits; i++)
+		sub_borrow(left[i], right[i], &result[i], borrow, &borrow);
 
 	return borrow;
 }
@@ -332,20 +356,18 @@ EXPORT_SYMBOL(vli_sub);
 
 /* Computes result = left - right, returning borrow. Can modify in place. */
 static u64 vli_usub(u64 *result, const u64 *left, u64 right,
-	     unsigned int ndigits)
+		    unsigned int ndigits)
 {
-	u64 borrow = right;
+	u64 borrow;
 	int i;
 
-	for (i = 0; i < ndigits; i++) {
-		u64 diff;
+	if (ndigits == 0)
+		return right;
 
-		diff = left[i] - borrow;
-		if (diff != left[i])
-			borrow = (diff > left[i]);
+	borrow = __builtin_usubll_overflow(left[0], right, &result[0]);
 
-		result[i] = diff;
-	}
+	for (i = 1; i < ndigits; i++)
+		borrow = __builtin_usubll_overflow(left[i], borrow, &result[i]);
 
 	return borrow;
 }
-- 
2.47.3