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Optimize mpi_bigendian_to_host() for speed and size

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Use GCC / Clang builtins for byte swapping.
This commit is contained in:
Hanno Becker 2018-11-08 11:53:49 +00:00
parent 8116ef7c81
commit f872007782
1 changed files with 55 additions and 27 deletions
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82
library/bignum.c
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@ -718,18 +718,59 @@ cleanup:
/* Convert a big-endian byte array aligned to the size of mbedtls_mpi_uint
* into the storage form used by mbedtls_mpi. */
static mbedtls_mpi_uint mpi_uint_bigendian_to_host_c( mbedtls_mpi_uint x )
{
uint8_t i;
mbedtls_mpi_uint tmp = 0;
/* This works regardless of the endianness. */
for( i = 0; i < ciL; i++, x >>= 8 )
tmp |= ( x & 0xFF ) << ( ( ciL - 1 - i ) << 3 );
return( tmp );
}
static mbedtls_mpi_uint mpi_uint_bigendian_to_host( mbedtls_mpi_uint x )
{
#if defined(__BYTE_ORDER__)
/* Nothing to do on bigendian systems. */
#if ( __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ )
return( x );
#endif /* __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ */
#if ( __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ )
/* For GCC and Clang, have builtins for byte swapping. */
#if( defined(__GNUC__) && __GNUC_PREREQ(4,3) )
#define have_bswap
#elif defined(__clang__) && \
defined(__has_builtin) && \
__has_builtin(__builtin_bswap32) && \
__has_builtin(__builtin_bswap64)
#define have_bswap
#endif
#if defined(have_bswap)
/* The compiler is hopefully able to statically evaluate this! */
switch( sizeof(mbedtls_mpi_uint) )
{
case 4:
return( __builtin_bswap32(x) );
case 8:
return( __builtin_bswap64(x) );
}
#endif
#endif /* __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ */
#endif /* __BYTE_ORDER__ */
/* Fall back to C-based reordering if we don't know the byte order
* or we couldn't use a compiler-specific builtin. */
return( mpi_uint_bigendian_to_host_c( x ) );
}
static void mpi_bigendian_to_host( mbedtls_mpi_uint * const p, size_t limbs )
{
size_t i;
unsigned char *cur_byte_left;
unsigned char *cur_byte_right;
mbedtls_mpi_uint *cur_limb_left;
mbedtls_mpi_uint *cur_limb_right;
mbedtls_mpi_uint tmp_left, tmp_right;
if( limbs == 0 )
return;
@ -742,30 +783,17 @@ static void mpi_bigendian_to_host( mbedtls_mpi_uint * const p, size_t limbs )
* than the right index (it's not a problem if limbs is odd and the
* indices coincide in the last iteration).
*/
for( cur_limb_left = p, cur_limb_right = p + ( limbs - 1 );
cur_limb_left <= cur_limb_right;
cur_limb_left++, cur_limb_right-- )
{
cur_byte_left = (unsigned char*) cur_limb_left;
cur_byte_right = (unsigned char*) cur_limb_right;
tmp_left = 0;
tmp_right = 0;
for( i = 0; i < ciL; i++ )
{
tmp_left |= ( (mbedtls_mpi_uint) *cur_byte_left++ )
<< ( ( ciL - 1 - i ) << 3 );
tmp_right |= ( (mbedtls_mpi_uint) *cur_byte_right++ )
<< ( ( ciL - 1 - i ) << 3 );
}
*cur_limb_right = tmp_left;
*cur_limb_left = tmp_right;
mbedtls_mpi_uint tmp;
/* Note that if cur_limb_left == cur_limb_right,
* this code effectively swaps the bytes only once. */
tmp = mpi_uint_bigendian_to_host( *cur_limb_left );
*cur_limb_left = mpi_uint_bigendian_to_host( *cur_limb_right );
*cur_limb_right = tmp;
}
return;
}
/*