[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, 10 insertions, 8 deletions

diff --git a/src/include/ipxe/bigint.h b/src/include/ipxe/bigint.h
index a85761815..2a0a200c5 100644
--- a/src/include/ipxe/bigint.h
+++ b/src/include/ipxe/bigint.h
@@ -70,23 +70,25 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  * @v addend		Big integer to add
  * @v value		Big integer to be added to
+ * @ret carry		Carry out
  */
-#define bigint_add( addend, value ) do {				\
+#define bigint_add( addend, value ) ( {					\
 	unsigned int size = bigint_size (addend);			\
 	bigint_add_raw ( (addend)->element, (value)->element, size );	\
-	} while ( 0 )
+	} )
 
 /**
  * Subtract big integers
  *
  * @v subtrahend	Big integer to subtract
  * @v value		Big integer to be subtracted from
+ * @ret borrow		Borrow out
  */
-#define bigint_subtract( subtrahend, value ) do {			\
+#define bigint_subtract( subtrahend, value ) ( {			\
 	unsigned int size = bigint_size (subtrahend);			\
 	bigint_subtract_raw ( (subtrahend)->element, (value)->element,	\
 			      size );					\
-	} while ( 0 )
+	} )
 
 /**
  * Shift big integer left
@@ -389,10 +391,10 @@ void bigint_init_raw ( bigint_element_t *value0, unsigned int size,
 		       const void *data, size_t len );
 void bigint_done_raw ( const bigint_element_t *value0, unsigned int size,
 		       void *out, size_t len );
-void bigint_add_raw ( const bigint_element_t *addend0,
-		      bigint_element_t *value0, unsigned int size );
-void bigint_subtract_raw ( const bigint_element_t *subtrahend0,
-			   bigint_element_t *value0, unsigned int size );
+int bigint_add_raw ( const bigint_element_t *addend0,
+		     bigint_element_t *value0, unsigned int size );
+int bigint_subtract_raw ( const bigint_element_t *subtrahend0,
+			  bigint_element_t *value0, unsigned int size );
 void bigint_shl_raw ( bigint_element_t *value0, unsigned int size );
 void bigint_shr_raw ( bigint_element_t *value0, unsigned int size );
 int bigint_is_zero_raw ( const bigint_element_t *value0, unsigned int size );