Constrain valid arguments to BIT_FIELD_REF

[Robert Dewar]

Alexandre Oliva wrote:
> On Mar  9, 2008, Richard Guenther <rguenther@suse.de> wrote:
> 
>> On Sun, 9 Mar 2008, Alexandre Oliva wrote:
>>> AM33/2.0 and H8SX come to mind, although it's been a while since I
>>> dealt with the memory bit-field operations of these two ports to have
>>> the details handy.
> 
>> Ok, I would expect it a size benefit at most.
> 
> I wouldn't think so.  Turning a single hardware-optimized instruction
> into a series of load, shift, mask, combine, store is unlikely to
> benefit just size.  Especially considering that these optimized
> instructions were introduced in revisions of the processor.
> 
> And then, I've failed to see a compelling reason to justify such a
> change, that would have the effect of disabling this optimization,
> even if it was "just" a size optimization.

Anyway, size and performance are always related because of icache
effects.