Re: [boost] [boost::endian] Request for comments/interest

Subject: Re: [boost] [boost::endian] Request for comments/interest
From: Terry Golubiewski (tjgolubi_at_[hidden])
Date: 2010-06-03 09:54:47

> That is surprising.  swap() should have no overhead compared to the endian
> object types.  Then again, it's very early code, and I've done no 
> optimization
> yet.  It could be something as simply as an "inline" missing.
>
>> Both approaches are equally efficient for the same-endian case.
>
> Do you mean endian object and swap_in_place or endian object and swap?

I didn't see a missing inline.

I think I'm beginning to understand what is going on.
The type-based endian approach suffers in the swap-in-place case, because 
its designed to be an efficient copier.
My implementation of type-based endian requires a copy to a temporary, which 
must be in memory (not a register), before rewriting the data to the same 
location in converted form.
std::swap() must do this too, but it can use a register for the temporary. 
This (I think) explains the performance advantage swapping shows for 
in-place conversion.
However, the swap approach suffers when doing a copy, because it has to read 
the data, then write it (in-place), and then read it again, before writing 
it to the destination, if you don't want to modify the original data.

terry