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

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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>
This commit is contained in:
Michael Brown
2024-11-26 12:53:01 +00:00
parent da6da6eb3b
commit 167a08f089
8 changed files with 140 additions and 85 deletions
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19
src/arch/arm64/include/bits/bigint.h
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@@ -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 =
@@ -54,6 +55,7 @@ bigint_add_raw ( const uint64_t *addend0, uint64_t *value0,
uint64_t discard_addend_i;
uint64_t discard_value_i;
unsigned int discard_size;
int carry;
__asm__ __volatile__ ( "cmn xzr, xzr\n\t" /* clear CF */
"\n1:\n\t"
@@ -68,9 +70,11 @@ bigint_add_raw ( const uint64_t *addend0, uint64_t *value0,
"=r" ( discard_size ),
"=r" ( discard_addend_i ),
"=r" ( discard_value_i ),
"=@cccs" ( carry ),
"+m" ( *value )
: "0" ( addend0 ), "1" ( value0 ), "2" ( size )
: "cc" );
: "0" ( addend0 ), "1" ( value0 ),
"2" ( size ) );
return carry;
}
/**
@@ -79,8 +83,9 @@ 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 =
@@ -90,6 +95,7 @@ bigint_subtract_raw ( const uint64_t *subtrahend0, uint64_t *value0,
uint64_t discard_subtrahend_i;
uint64_t discard_value_i;
unsigned int discard_size;
int borrow;
__asm__ __volatile__ ( "cmp xzr, xzr\n\t" /* set CF */
"\n1:\n\t"
@@ -104,10 +110,11 @@ bigint_subtract_raw ( const uint64_t *subtrahend0, uint64_t *value0,
"=r" ( discard_size ),
"=r" ( discard_subtrahend_i ),
"=r" ( discard_value_i ),
"=@cccc" ( borrow ),
"+m" ( *value )
: "0" ( subtrahend0 ), "1" ( value0 ),
"2" ( size )
: "cc" );
"2" ( size ) );
return borrow;
}
/**