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Subject: Re: [boost] [Review] Boost.Endian
From: Beman Dawes (bdawes_at_[hidden])
Date: 2011-09-08 07:31:14

On Wed, Sep 7, 2011 at 1:03 PM, john filo <filo_at_[hidden]> wrote:
> - What is your evaluation of the design?
> For the original scope (i.e. integers), very good.  However, I'd
> like to see additional support for float, double, and user defined
> types added.  Without it, this library isn't very useful to me
> personally since a large percentage of the values that I have to
> deal with are either float or double.

Several others have also expressed interest in FP, so I'll give it a
try. Issues:

* Because FP formats vary, just dealing with endianness doesn't ensure
* The endianness of FP and integer values differs on some platforms,
so we will have to build up a config file with separate entries for
each platform, and that will take time to mature.
* Ditto FP sizes.
* I'm only willing to provide conversion.hpp FP support. Providing
types that mimic FP types is far beyond my knowledge of how to deal
with floating point's notorious arithmetic issues.

> ----------------------------------------------------------------------
> - What is your evaluation of the documentation?
> Excellent.
> Is a tutorial section going to be added?  I'm not sure one is needed
> given the existing examples in the rest of the documentation, but the
> documentation's menu bar implies there will be one.

Yes, but it will be short and I won't have a chance to write it until
after the formal review.

> ----------------------------------------------------------------------
> - What is your evaluation of the implementation?
> Very good.
> At -O2, I was impressed with the performance relative to GCC's
> builtin byteswap routines.  When I first saw the implementation, I
> honestly didn't think it would perform very well given its complexity
> compared to a naive implementation (e.g. the version used in the
> endian::reorder functions).  I was pleasantly surprised.  I guess
> that's a testament to the level of optimization compilers are capable
> of these days.
> On an Intel Core2, the performance penalty of the uint32 endian<>
> template and the reorder() function relative to GCC-4.4 builtins
> (i.e. runtime divided by runtime of builtin) is as follows:
>     endian<>  reorder()
> ------------------------
> -O0    5.4       3.0
> -O2    2.3       6.5
> It seems like the reorder functions could be implemented in terms of
> the integer types for improved performance (and also maintainability).
> Something like:
> template<typename T>
> inline void reorder(T& x)
> {
>    x = *reinterpret_cast<
>        endian<endianness::nonnative, T, sizeof(T)/8> *>(&x);
> }
> template<typename T>
> inline void reorder(T source, T& target)
> {
>    target = *reinterpret_cast<
>        endian<endianness::nonnative, T, sizeof(T)/8> *>(&source);
> }
> Obviously the above requires extending the endianess enum with
> a "nonnative" value.
> Performance isn't that critical for me, but it does seem like
> providing platform specific implementations that take advantage of
> things like GCC's builtin byteswap functions should also be considered
> to get the performance up even further.
> Finally, for my last performance related comment, I was surprised to
> see that the native types had basically exact same performance as the
> nonnative types.  Couldn't the native types be specialized to
> effectively be a no-op?

There have been a lot of other suggestions about performance, and I'm
actively working on performance issues right now. Stay tuned.

> Would "byteswap" or maybe "swapbytes" be a better name than "reorder"?

I've tried a bunch of names, and am pretty happy with "reorder". But
"swapbytes" would also be a decent name.

> ----------------------------------------------------------------------
> - What is your evaluation of the potential usefulness of the library?
> For those who deal with non-native endian data, this library is
> extremely useful.  It automatically eliminates a whole class of common
> programming errors when dealing with such data.
> ----------------------------------------------------------------------
> - Did you try to use the library?  With what compiler?
> Yes; gcc-4.4.3 on Linux
> ----------------------------------------------------------------------
> - Are you knowledgeable about the problem domain?
> Yes.  I've used a similar home-grown library for the past 10 years,
> and have been looking forward this being added to Boost for a long
> time.
> ----------------------------------------------------------------------
>> - Do you think the library should be accepted as a Boost library?
> Absolutely.  I'd like to see support for float and double, but
> even without those additions, I still vote yes.

Thanks for the kind words,


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