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Subject: Re: What even happened to <net> and byte swapping?
From: Edward Catmur (ecatmur_at_[hidden])
Date: 2021-02-26 09:08:02


On Fri, Feb 26, 2021 at 1:13 PM Matthew Woehlke via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> So... in other words, yes, you're taking the "deal with one byte at a
> time" approach. *Maybe* that *interface* is a candidate for
> standardization, *iff* it is able to operate on values in-place. The
> reason I say that is because it will (ahem: "conditionally", for all the
> pedants that replied) byte-swap under the hood, as any other
> implementation is almost certainly going to perform much more poorly.
>

By in-place, you mean *modifying* an integral *lvalue*? That sounds highly
bug-prone; what if you lose track of whether a particular variable has been
converted from format to host endian?

(FWIW, I've found the safest technique is to have (library) types whose
object representation is (potentially) byte-swapped, and convert on load
and store; in C++, that means conversion operator and converting
constructor. But while it'd be nice to have that in the library, it's more
important to get the lower level facility first.)

Also, a pure function has better performance, not worse, because it leaves
its result in a prvalue. Note that bswap operates on registers, so an
in-place operation would be 3 instructions (load, bswap, store) while a
pure function is at most 2 instructions (mov, bswap) and no memory access.

You're quibbling over semantics. Your implementation does still
> byte-swap, in that it takes input bytes and (effectively) copies them in
> a different order. Your approach also appears to currently have the
> limitation of requiring a second copy of the data, which is potentially
> inefficient if the data is a mix of endian-dependent and endian-agnostic
> data, though I expect that could be relaxed.

A second copy of the data in register, which is practically free, and can
be elided if you don't access the format-endian data after converting it to
host-endian.

Also, if you aren't using
> actual byte-swap intrinsics under the hood, you are most likely leaving
> performance on the table. (Granted, your approach is more portable, but
> a vendor implementation would be expected to use intrinsics.)
>

gcc's byteswap intrinsics are exposed as pure functions. Also, what
compiler is incapable of recognizing a hand-rolled swap and optimizing it
to the bswap instruction?

Note that I mention "byte swapping" because that's what the operation
> actually *does* (except when it's a no-op), not because I'm suggesting
> an API that *unconditionally* swaps. The original proposal was to
> standardize the ntohX / htonX functions, and that is clearly a better
> approach, although "best" would be more like hto{le,be} and {le,be}toh
> (with no 'X' needed because this is C++ and we have overloads).
>

ntohX / htonX are pure functions.

> Overloads taking `span` might be nice, also.
>

How can you convert a span of bytes without knowing the layout of the
fields within it? Say you have a range of 10 bytes that is {int64, uint32,
4x int8, int16}? And what if there's IEEE floats in there?



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