Simo Sorce <s...@redhat.com> writes: > On Wed, 2019-03-20 at 06:14 +0100, Niels Möller wrote: >> And another possible trick for big-endian is to do an "opposite-endian" >> left shift as >> >> ((x & 0x7f7f7f7f7f7f7f7f) << 1) | ((x & 0x8080808080808080) >> 15) >> where this bit is the carry out ^ > > This would allow us to avoid copies at the cost of more complicated code. > > Which do you prefer? using endian.h where available? Or having two > separate codepaths depending on the endianess of the machine ?
If it matters for performance, use the fastest variant. Using separate implementations of xts_shift, with #if:s depending on endianness and compiler support, is fine. I'd expect the opposite-endian shift to be more efficient when bswap is particularly slow, and implemented in terms of shifting and masking. A bit difficult to determine, though. Neither existence of endian.h macros or __builtin_bswap64 implies that the byte swapping is cheap. Are there any interesting platforms these days that lack an efficient bswap instruction? And are big-endian? Does mips have a bswap instruction? Regards, /Niels -- Niels Möller. PGP-encrypted email is preferred. Keyid 368C6677. Internet email is subject to wholesale government surveillance. _______________________________________________ nettle-bugs mailing list nettle-bugs@lists.lysator.liu.se http://lists.lysator.liu.se/mailman/listinfo/nettle-bugs
