Re: strcmp is too heavy for its everyday usage...

[Richard Henderson <rth at twiddle dot net>]

On Fri, Jan 09, 2004 at 10:11:45AM +0500, Denis Zaitsev wrote:
>     (r= a[0] - b[0]) &&
>     (r= a[1] - b[1]) &&
>     (r= a[2] - b[2]) &&
>     (r= a[3] - b[3]);
>     return r;
> }
> 
> It's a typical inline code for compare 4-byte of mem.  When it is
> used, say, in such a context
> 
>         s(a,b) ? A() : B();
> 
> GCC discards the value of r perfectly, leaving the only code needed
> for compare bytes for eq/neq.  But GCC doesn't merge the 4 byte
> comparing into single word comparing.  And, as I understand, it will
> never do that, as it's not asked to.  Or this kind of optimization is
> assumed ok for compiler, but just still unimplemented?

Certainly it's ok if it converts.

However, on most targets you'd have to know that a and b are aligned.
Worse, even for targets like x86 that support unaligned loads you have
to know for certain that neither a[3] nor b[3] could possibly segv
when a[0] and b[0] won't.  That condition is trivial when a and b are
aligned, but otherwise...

Adding compensation code to deal with the extra conditions that must
be satisfied will probably negate whatever you're hoping to gain here.



r~