tree 47e3fbc5b3bb657f34e40dbee51516455372d846
parent d10e8fae9e30cac60297b1e1834002db183429e5
author Gilles Peskine <Gilles.Peskine@arm.com> 1595459002 +0200
committer Gilles Peskine <Gilles.Peskine@arm.com> 1607377885 +0100

Avoid reallocating during subtraction

mbedtls_mpi_sub_abs systematically allocated a new mpi when the result
was aliased with the right operand (i.e. X = A - X). This aliasing
very commonly happens during ECP operations. Rewrite the function to
allocate only if the result might not fit otherwise.

This costs a few bytes of code size in bignum.o, and might make
mbedtls_mpi_sub_abs very very slightly slower when no reallocation is
done. However, there is a substantial performance gain in ECP
operations with Montgomery curves (10-20% on my PC).

test_suite_ecp drops from 1422794 to 1271506 calls to calloc().

This commit also fixes a bug whereby mbedtls_mpi_sub_abs would leak
memory when X == B (so TB was in use) and the result was negative.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>