Re: [avr-gcc-list] Re: AVR byte swap optimization

[Shaun Jackman]

On 12/18/06, David VanHorn <address@hidden> wrote:
> > * If the swap function is called on a constant, the compiler cannot
> > remove the inline assembler. In general, any inline assembler tends to
> > handcuff the optimizer to some degree.
>
> I'm new to C, but why would you want to swap a constant?
> CAN you swap a constant?  Seems an inconstant constant :)

Quite! My usual case for this is when the constant is defined in host
order, but I'm comparing against a network order value. For example,

        if (incoming == htons(EXPECTED))

Here, incoming is a 16-bit network order value and htons (a.k.a.
byteswap_16) is taking the 16-bit host order constant EXPECTED. If
htons is implemented as a mathematical expression, the compiler can
optimise it away to nothing.

Cheers,
Shaun