remove unaligned loads and stores on x86

[arvidn]

It invokes undefined behavior regardless of the compiler back-end. more details in #21

[pdimov]

Instead of removing the #ifdefs altogether, why not use memcpy there?

[arvidn]

You mean as an optimisation of unrolled_byte_loops when no endian conversion is necessary?
I think such optimisation is orthogonal to the current special-case, since it should:

1. be applied based on the host endianness (not specific architecture)
2. be applied for big-endian types on big endian hosts

[pdimov]

I mean as a replacement for the current casts, as this would minimize the patch both in lines and in spirit.

[pdimov]

1. be applied based on the host endianness (not specific architecture)

It's not clear whether it will be an optimization. It's possible for the unrolled_byte_loops to be more efficient if memcpy is not an intrinsic but compiles to a function call. We'll have to check on godbolt.org.

For x86 I think that all major compilers recognize memcpy on 4/8 bytes.

[pdimov]

Here's a testbed: https://godbolt.org/g/MfP3Hc

[pdimov]

At least for MSVC, removing the special case notably degrades the code.

[arvidn]

I can put in memcpy there instead. It's tempting to depend on endian-macros instead of architecture macros though, and to use memcpy for the big endian case (on big endian machines).

It seems like a pretty safe assumption that memcpy is more efficient than a hand-written copy loop, in general. It's likely an easier optimisation to make.

[pdimov]

As tested on godbolt, all x86 compilers (a) benefit significantly from the special case and (b) generate the same code with the current one and with memcpy.

ARM however generates a function call to memcpy so I'm not sure that the special case should be applied there.

Looks like the safest choice is to keep the current #ifdefs and just replace the cast inside with memcpy.

[pdimov]

Test added in 62802fe. Fixed in e93f6a2.