From da6da6eb3b83fe92002e9c8e245933498ba19a48 Mon Sep 17 00:00:00 2001
From: Michael Brown <mcb30@ipxe.org>
Date: Thu, 7 Nov 2024 14:43:56 +0000
Subject: [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>
---
 src/crypto/bigint.c       |  5 ++---
 src/crypto/x25519.c       |  3 +--
 src/include/ipxe/bigint.h | 27 +++++++++++++++++++++++++++
 3 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/src/crypto/bigint.c b/src/crypto/bigint.c
index 3f5db8517..35701a1d0 100644
--- 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 d58f7168c..19f9a2c02 100644
--- 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 f7900e0e6..a85761815 100644
--- 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,
-- 
cgit