[crypto] Add bigint_msb_is_set() to clarify code

Add a dedicated bigint_msb_is_set() to reduce the amount of open
coding required in the common case of testing the sign of a two's
complement big integer.

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

diff --git a/src/crypto/bigint.c b/src/crypto/bigint.c
index 3f5db8517bfffce5ba75816c32cf256c52bf7cde..35701a1d063a28ee6901e39fb60d69cec03bd38f 100644 (file)
--- a/src/crypto/bigint.c
+++ b/src/crypto/bigint.c
@@ -162,14 +162,13 @@ void bigint_reduce_raw ( const bigint_element_t *minuend0,
                bigint_t ( minuend_size ) modulus;
        } *temp = tmp;
        const unsigned int width = ( 8 * sizeof ( bigint_element_t ) );
-       const bigint_element_t msb_mask = ( 1UL << ( width - 1 ) );
        bigint_element_t *element;
        unsigned int minuend_max;
        unsigned int modulus_max;
        unsigned int subshift;
-       bigint_element_t msb;
        int offset;
        int shift;
+       int msb;
        int i;
 
        /* Start profiling */
@@ -289,7 +288,7 @@ void bigint_reduce_raw ( const bigint_element_t *minuend0,
                } else {
                        bigint_subtract ( &temp->modulus, &temp->minuend );
                }
-               msb = ( temp->minuend.element[ minuend_size - 1 ] & msb_mask );
+               msb = bigint_msb_is_set ( &temp->minuend );
                if ( shift > 0 )
                        bigint_shr ( &temp->modulus );
        }

diff --git a/src/crypto/x25519.c b/src/crypto/x25519.c
index d58f7168c37629d4e23429fb2983dba3320a000c..19f9a2c0213bfaeb3145572887f6ead4ad71a64b 100644 (file)
--- a/src/crypto/x25519.c
+++ b/src/crypto/x25519.c
@@ -563,7 +563,6 @@ void x25519_invert ( const union x25519_oct258 *invertend,
  * @v value            Big integer to be subtracted from, if possible
  */
 static void x25519_reduce_by ( const x25519_t *subtrahend, x25519_t *value ) {
-       unsigned int max_bit = ( ( 8 * sizeof ( *value ) ) - 1 );
        x25519_t tmp;
 
        /* Conditionally subtract subtrahend
@@ -573,7 +572,7 @@ static void x25519_reduce_by ( const x25519_t *subtrahend, x25519_t *value ) {
         */
        bigint_copy ( value, &tmp );
        bigint_subtract ( subtrahend, value );
-       bigint_swap ( value, &tmp, bigint_bit_is_set ( value, max_bit ) );
+       bigint_swap ( value, &tmp, bigint_msb_is_set ( value ) );
 }
 
 /**

diff --git a/src/include/ipxe/bigint.h b/src/include/ipxe/bigint.h
index f7900e0e6450dd2587c0e4ddd15b5755ee376c07..a85761815c86b791e79a82b628069cef2fc2f315 100644 (file)
--- a/src/include/ipxe/bigint.h
+++ b/src/include/ipxe/bigint.h
@@ -142,6 +142,16 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
        unsigned int size = bigint_size (value);                        \
        bigint_bit_is_set_raw ( (value)->element, size, bit ); } )
 
+/**
+ * Test if most significant bit is set in big integer
+ *
+ * @v value            Big integer
+ * @ret is_set         Most significant bit is set
+ */
+#define bigint_msb_is_set( value ) ( {                                 \
+       unsigned int size = bigint_size (value);                        \
+       bigint_msb_is_set_raw ( (value)->element, size ); } )
+
 /**
  * Find highest bit set in big integer
  *
@@ -358,6 +368,23 @@ bigint_bit_is_set_raw ( const bigint_element_t *value0, unsigned int size,
        return ( !! ( value->element[index] & ( 1UL << subindex ) ) );
 }
 
+/**
+ * Test if most significant bit is set in big integer
+ *
+ * @v value0           Element 0 of big integer
+ * @v size             Number of elements
+ * @ret is_set         Most significant bit is set
+ */
+static inline __attribute__ (( always_inline )) int
+bigint_msb_is_set_raw ( const bigint_element_t *value0, unsigned int size ) {
+       const bigint_t ( size ) __attribute__ (( may_alias )) *value =
+               ( ( const void * ) value0 );
+       unsigned int index = ( size - 1 );
+       unsigned int subindex = ( ( 8 * sizeof ( value->element[0] ) ) - 1 );
+
+       return ( !! ( value->element[index] & ( 1UL << subindex ) ) );
+}
+
 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,