1 files changed, 18 insertions, 1 deletions

diff --git a/src/lib/libcrypto/bn/arch/riscv64/bn_arch.h b/src/lib/libcrypto/bn/arch/riscv64/bn_arch.h
index a8c50eb06d..e67de835cf 100644
--- a/src/lib/libcrypto/bn/arch/riscv64/bn_arch.h
+++ b/src/lib/libcrypto/bn/arch/riscv64/bn_arch.h
@@ -1,4 +1,4 @@
-/*      $OpenBSD: bn_arch.h,v 1.6 2023/07/09 10:36:53 jsing Exp $ */
+/*      $OpenBSD: bn_arch.h,v 1.7 2023/07/09 10:37:32 jsing Exp $ */
 /*
  * Copyright (c) 2023 Joel Sing <jsing@openbsd.org>
  *
@@ -63,6 +63,23 @@ bn_mulw(BN_ULONG a, BN_ULONG b, BN_ULONG *out_r1, BN_ULONG *out_r0)
         *out_r0 = r0;
 }
 
+#define HAVE_BN_SUBW
+
+static inline void
+bn_subw(BN_ULONG a, BN_ULONG b, BN_ULONG *out_borrow, BN_ULONG *out_r0)
+{
+        BN_ULONG borrow, r0;
+
+        __asm__ (
+            "sub   %[r0], %[a], %[b] \n"
+            "sltu  %[borrow], %[a], %[r0] \n"
+            : [borrow]"=r"(borrow), [r0]"=&r"(r0)
+            : [a]"r"(a), [b]"r"(b));
+
+        *out_borrow = borrow;
+        *out_r0 = r0;
+}
+
 #endif /* __GNUC__ */
 
 #endif