[RFC] Optimize __arch_swab32 and __arch_swab16

[David Laight]

> > Which is a problem because the compiler could schedule
> > it be written back to real memory between the instructions.
> 
> It can? There is no memory here, just registers. Even if it
> is written to memory, how would that affect the register?

Although the function argument is passed in a register, the
compiler could generate a store-load sequence before and
after each __asm__() line.

> Assuming you are right, would rewriting it to
>   __asm__("rlwimi %0,%0,16,0x00ff0000\n\t"
> 	    "rlwinm %0,%0,24,0x0000ffff"
> 	    : "+r"(value));
> help?

Except that now you've stopped the compiler scheduling
another instruction between the two - probably forcing a
execution stall.

	David