Re: Use of C99 int types

[Renate Meijer <kleuske@xs4all.nl>]

On Apr 3, 2005, at 2:30 PM, Dag Arne Osvik wrote:

> Stephen Rothwell wrote:
>
> > On Sun, 03 Apr 2005 13:55:39 +0200 Dag Arne Osvik <da@osvik.no> wrote:
> >
> > > I've been working on a new DES implementation for Linux, and ran into
> > > the problem of how to get access to C99 types like uint_fast32_t for
> > > internal (not interface) use.  In my tests, key setup on Athlon 64 
> > > slows
> > > down by 40% when using u32 instead of uint_fast32_t.
> >
> > If you look in stdint.h you may find that uint_fast32_t is actually
> > 64 bits on Athlon 64 ... so does it help if you use u64?
>
> Yes, but wouldn't it be much better to avoid code like the following, 
> which may also be wrong (in terms of speed)?
>
> #ifdef CONFIG_64BIT  // or maybe CONFIG_X86_64?
>  #define fast_u32 u64
> #else
>  #define fast_u32 u32
> #endif

Isn't it better to use a general integer type, reflecting the cpu's 
native register-size and let the compiler sort it out? Restrict all 
uses of explicit width types to where it's *really* needed, that is, in 
drivers, network-code, etc. I firmly oppose any definition of "#define 
fast_u32 u64". This kind of definitions will only create needless 
confusion.

I wonder how much other code is suffering from this kind of overly 
explicit typing. It's much easier to make assumptions about integer 
size unwittingly than it is to avoid them. I used to assume (for 
instance) that sizeof(int) == sizeof(long) == sizeof(void *) at one 
point in my career. Fortunately, reality soon asserted itself again.

Regards,

Renate Meijer.

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