Add CPU feature detection for ADX on amd64.

Add detection of Multi-Precision Add-Carry Instruction Extensions on amd64.
s2n-bignum provides a number of fast multiplication routines that can
leverage these instructions.

ok tb@

1 files changed, 5 insertions, 1 deletions

diff --git a/src/lib/libcrypto/arch/amd64/crypto_cpu_caps.c b/src/lib/libcrypto/arch/amd64/crypto_cpu_caps.c
index 0bc440d34f..51a2da4616 100644
--- a/src/lib/libcrypto/arch/amd64/crypto_cpu_caps.c
+++ b/src/lib/libcrypto/arch/amd64/crypto_cpu_caps.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: crypto_cpu_caps.c,v 1.7 2025/07/22 09:18:02 jsing Exp $ */
+/*      $OpenBSD: crypto_cpu_caps.c,v 1.8 2025/08/14 15:11:01 jsing Exp $ */
 /*
  * Copyright (c) 2024 Joel Sing <jsing@openbsd.org>
  *
@@ -119,6 +119,10 @@ crypto_cpu_caps_init(void)
         if (max_cpuid >= 7) {
                 cpuid(7, NULL, &ebx, NULL, NULL);
 
+                /* Intel ADX feature bit - ebx[19]. */
+                if (((ebx >> 19) & 1) != 0)
+                        crypto_cpu_caps_amd64 |= CRYPTO_CPU_CAPS_AMD64_ADX;
+
                 /* Intel SHA extensions feature bit - ebx[29]. */
                 if (((ebx >> 29) & 1) != 0)
                         crypto_cpu_caps_amd64 |= CRYPTO_CPU_CAPS_AMD64_SHA;