Re: [boost] Suggest: detail functions for full built-in arithmetic

From: Guillaume Melquiond (guillaume.melquiond_at_[hidden])
Date: 2007-09-09 02:33:48

Le samedi 08 septembre 2007 à 23:10 -0400, Daryle Walker a écrit :
> Maybe we should add a header to our detail (and/or pending) folder 
> encapsulating the full versions of built-in integer arithmetic 
> operations, that most(?) processors have, but neither C nor C++ let us 
> access.  I'm talking about functions like:
> 
> std::pair<unsigned, bool>  full_add( unsigned a1, unsigned a2 );
> 
> The processor has a "carry" flag whenever an integer addition overflows, 
> but we can't access it (without assembly-language hacks).

Is it really worth it? Nowadays, compilers are smart enough. They
generate better code when details are not obfuscated by hand-crafted
assembly code. I tested the following code with GCC:

        #include <algorithm>
        
        std::pair<unsigned, bool> full_add(unsigned a, unsigned b)
        { return std::make_pair(a + b, a + b < a); }
        
        bool no_overflow(unsigned a, unsigned b)
        { return !full_add(a, b).second; }

The no_overflow function is a bit dumb, but it is here to check that the
compiler was able to perform optimizations. This is the generated code
on x86-64:

        _Z11no_overflowjj:
                addl    %edi, %esi
                setae   %al
                ret

This is the optimal assembly code. And there is no way to obtain this
code if you use an assembly hack in the full_add function.

Best regards,

Guillaume