[crypto] Expose carry flag from big integer addition and subtraction

Expose the effective carry (or borrow) out flag from big integer
addition and subtraction, and use this to elide an explicit bit test
when performing x25519 reduction.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 20 insertions, 16 deletions

diff --git a/src/arch/loong64/include/bits/bigint.h b/src/arch/loong64/include/bits/bigint.h
index 234d8dfa7..0222354df 100644
--- a/src/arch/loong64/include/bits/bigint.h
+++ b/src/arch/loong64/include/bits/bigint.h
@@ -43,8 +43,9 @@ bigint_init_raw ( uint64_t *value0, unsigned int size,
  * @v addend0		Element 0 of big integer to add
  * @v value0		Element 0 of big integer to be added to
  * @v size		Number of elements
+ * @ret carry		Carry out
  */
-static inline __attribute__ (( always_inline )) void
+static inline __attribute__ (( always_inline )) int
 bigint_add_raw ( const uint64_t *addend0, uint64_t *value0,
 		 unsigned int size ) {
 	bigint_t ( size ) __attribute__ (( may_alias )) *value =
@@ -53,20 +54,20 @@ bigint_add_raw ( const uint64_t *addend0, uint64_t *value0,
 	uint64_t *discard_value;
 	uint64_t discard_addend_i;
 	uint64_t discard_value_i;
-	uint64_t discard_carry;
 	uint64_t discard_temp;
 	unsigned int discard_size;
+	uint64_t carry;
 
 	__asm__ __volatile__ ( "\n1:\n\t"
 			       /* Load addend[i] and value[i] */
 			       "ld.d %3, %0, 0\n\t"
 			       "ld.d %4, %1, 0\n\t"
 			       /* Add carry flag and addend */
-			       "add.d %4, %4, %5\n\t"
-			       "sltu %6, %4, %5\n\t"
+			       "add.d %4, %4, %6\n\t"
+			       "sltu %5, %4, %6\n\t"
 			       "add.d %4, %4, %3\n\t"
-			       "sltu %5, %4, %3\n\t"
-			       "or %5, %5, %6\n\t"
+			       "sltu %6, %4, %3\n\t"
+			       "or %6, %5, %6\n\t"
 			       /* Store value[i] */
 			       "st.d %4, %1, 0\n\t"
 			       /* Loop */
@@ -79,11 +80,12 @@ bigint_add_raw ( const uint64_t *addend0, uint64_t *value0,
 				 "=r" ( discard_size ),
 				 "=r" ( discard_addend_i ),
 				 "=r" ( discard_value_i ),
-				 "=r" ( discard_carry ),
 				 "=r" ( discard_temp ),
+				 "=r" ( carry ),
 				 "+m" ( *value )
 			       : "0" ( addend0 ), "1" ( value0 ),
-				 "2" ( size ), "5" ( 0 ) );
+				 "2" ( size ), "6" ( 0 ) );
+	return carry;
 }
 
 /**
@@ -92,8 +94,9 @@ bigint_add_raw ( const uint64_t *addend0, uint64_t *value0,
  * @v subtrahend0	Element 0 of big integer to subtract
  * @v value0		Element 0 of big integer to be subtracted from
  * @v size		Number of elements
+ * @ret borrow		Borrow out
  */
-static inline __attribute__ (( always_inline )) void
+static inline __attribute__ (( always_inline )) int
 bigint_subtract_raw ( const uint64_t *subtrahend0, uint64_t *value0,
 		      unsigned int size ) {
 	bigint_t ( size ) __attribute__ (( may_alias )) *value =
@@ -102,20 +105,20 @@ bigint_subtract_raw ( const uint64_t *subtrahend0, uint64_t *value0,
 	uint64_t *discard_value;
 	uint64_t discard_subtrahend_i;
 	uint64_t discard_value_i;
-	uint64_t discard_carry;
 	uint64_t discard_temp;
 	unsigned int discard_size;
+	uint64_t borrow;
 
 	__asm__ __volatile__ ( "\n1:\n\t"
 			       /* Load subtrahend[i] and value[i] */
 			       "ld.d %3, %0, 0\n\t"
 			       "ld.d %4, %1, 0\n\t"
 			       /* Subtract carry flag and subtrahend */
-			       "sltu %6, %4, %5\n\t"
-			       "sub.d %4, %4, %5\n\t"
-			       "sltu %5, %4, %3\n\t"
+			       "sltu %5, %4, %6\n\t"
+			       "sub.d %4, %4, %6\n\t"
+			       "sltu %6, %4, %3\n\t"
 			       "sub.d %4, %4, %3\n\t"
-			       "or %5, %5, %6\n\t"
+			       "or %6, %5, %6\n\t"
 			       /* Store value[i] */
 			       "st.d %4, %1, 0\n\t"
 			       /* Loop */
@@ -128,11 +131,12 @@ bigint_subtract_raw ( const uint64_t *subtrahend0, uint64_t *value0,
 				 "=r" ( discard_size ),
 				 "=r" ( discard_subtrahend_i ),
 				 "=r" ( discard_value_i ),
-				 "=r" ( discard_carry ),
 				 "=r" ( discard_temp ),
+				 "=r" ( borrow ),
 				 "+m" ( *value )
 			       : "0" ( subtrahend0 ), "1" ( value0 ),
-				 "2" ( size ), "5" ( 0 ) );
+				 "2" ( size ), "6" ( 0 ) );
+	return borrow;
 }
 
 /**