Add option to avoid 64-bit multiplication
Motivation is similar to NO_UDBL_DIVISION.
The alternative implementation of 64-bit mult is straightforward and aims at
obvious correctness. Also, visual examination of the generate assembly show
that it's quite efficient with clang, armcc5 and arm-clang. However current
GCC generates fairly inefficient code for it.
I tried to rework the code in order to make GCC generate more efficient code.
Unfortunately the only way to do that is to get rid of 64-bit add and handle
the carry manually, but this causes other compilers to generate less efficient
code with branches, which is not acceptable from a side-channel point of view.
So let's keep the obvious code that works for most compilers and hope future
versions of GCC learn to manage registers in a sensible way in that context.
See https://bugs.launchpad.net/gcc-arm-embedded/+bug/1775263
diff --git a/scripts/config.pl b/scripts/config.pl
index 5bf2785..a89787a 100755
--- a/scripts/config.pl
+++ b/scripts/config.pl
@@ -95,6 +95,7 @@
MBEDTLS_ZLIB_SUPPORT
MBEDTLS_PKCS11_C
MBEDTLS_NO_UDBL_DIVISION
+MBEDTLS_NO_64BIT_MULTIPLICATION
_ALT\s*$
);
