Re: [boost] Boost.En­dian comments

Subject: Re: [boost] Boost.En­dian comments
From: Beman Dawes (bdawes_at_[hidden])
Date: 2011-09-07 13:15:15

On Mon, Sep 5, 2011 at 2:56 PM, tymofey <tymofey_at_[hidden]> wrote:
> This is not a full review, but rather a few comments on the implementation of reorder(x) functions.
> What you are doing is casting pointers around and assigning individual bytes, which might not really effective:
>
> inline void reorder(int64_t source, int64_t& target)
> {
>    const char* s (reinterpret_cast<const char*>(&source));
>    char * t (reinterpret_cast<char*>(&target) + sizeof(target) - 1);
>    *t = *s;
>    *--t = *++s;
>    *--t = *++s;
>    *--t = *++s;
>    *--t = *++s;
>    *--t = *++s;
>    *--t = *++s;
>    *--t = *++s;
> }
>
> it does eight increments, eight decrements, one addition, one substraction and eight assignments.
> maybe something like:
>
> inline void reorder(int64_t source, int64_t& target)
> {
>    target = ((source << 0x38) & 0xFF00000000000000)
>           | ((source << 0x28) & 0x00FF000000000000)
>           | ((source << 0x18) & 0x0000FF0000000000)
>           | ((source << 0x08) & 0x000000FF00000000)
>           | ((source >> 0x08) & 0x00000000FF000000)
>           | ((source >> 0x18) & 0x0000000000FF0000)
>           | ((source >> 0x28) & 0x000000000000FF00)
>           | ((source >> 0x38) & 0x00000000000000FF);
> }
>
> would be more efficient?

Yes, but if I read the standard correctly, it is relying on undefined behavior.

Both the C and C++ standards say:

The value of E1 << E2 is E1 left-shifted E2 bit positions; vacated
bits are zero-filled. If E1 has an unsigned type, the value of the
result is E1 × 2 [to the power] E2, reduced modulo one more than the
maximum value representable in the result type. Otherwise, if E1 has a
signed type and non-negative value, and E1×2 [to the power] E2 is
representable in the result type, then that is the resulting value;
otherwise, the behavior is undefined.

In practice I don't think this is a problem, but Boost code generally
doesn't rely on code that has undefined behavior. Perhaps it would be
OK to #ifdef on the architecture, and use shift-and-mask
implementations only on architectures that don't do something weird
(like cause an interrupt) in the case covered by the last sentence.

--Beman

--Beman