Re: [Boost-users] How to find: best integer type for processor

Subject: Re: [Boost-users] How to find: best integer type for processor
From: Cory Nelson (phrosty_at_[hidden])
Date: 2008-10-30 11:41:00

On Thu, Oct 30, 2008 at 8:24 AM, Daryle Walker <darylew_at_[hidden]> wrote:
> Is there a macro and/or typedef that will tell me bit-length used for a
> processor and/or the best integer type for said processor?  The "int" type
> is supposed to be that type, and it was in the 16- and 32-bit eras.  (I
> learned C programming with the former in the... early 1990s!)  However, the
> powers-that-be decided, for "backwards compatibility," to freeze "int" and
> "long" at 32 bits and use a new type, "long long," for 64 bits, which should
> be the optimized integer type for 64-bit processors.  (If they kept to the
> plan, "int" would be 64 bits, "long" either 64 or 128, "short" up to 32, and
> the new type will be "short short" at 16.)  I want to know the best type so,
> when I build multi-integer arrays for various purposes, I don't pick an
> element type that causes a slow down, whether to focus on a fraction of a
> register (if the chosen type is too small) to handle two registers at once
> (if the chosen type is too large).

Most things don't need 64-bit ints, so it would just be wasting memory
to use them if 32-bit ints are as fast or faster.  This is why "int"
stays 32-bit on most x64 ABIs.

If you want the largest native integer, though not 100% portable to
weird archs, this should work in x86/x64 and most others:

typedef boost::int_t<sizeof(void*) * 8>::fast fastint_t;

-- 
Cory Nelson
http://www.int64.org