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mbedtls_mpi_div_mpi: directly grow T1 to its useful size

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T1 is set to a 2-limb value. The first operation that takes it as
input is mbedtls_mpi_mul_int, which makes it grow to 3 limbs. Later it
is shifted left, which causes it to grow again. Set its size to the
final size from the start. This saves two calls to calloc(), at the
expense of a slowdown in some operations involving T1 as input since
it now has more leading zeros.

Setting T1 to 3 limbs initially instead of 2 saves about 6% of the
calloc() calls in test_suite_ecp and does not incur a performance
penalty. Setting T1 to A->n + 2 limbs instead of 2 saves about 20% of
the calloc calls and does not cause a measurable performance
difference on my Linux PC.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2020-07-24 00:12:59 +02:00
parent cd0dbf36b6
commit 2536aa709b
1 changed files with 1 additions and 1 deletions
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2
library/bignum.c
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@ -1830,7 +1830,7 @@ int mbedtls_mpi_div_mpi( mbedtls_mpi *Q, mbedtls_mpi *R, const mbedtls_mpi *A,
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &Z, A->n + 2 ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_lset( &Z, 0 ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &T1, 2 ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &T1, A->n + 2 ) );
k = mbedtls_mpi_bitlen( &Y ) % biL;
if( k < biL - 1 )