Re: [m68k 13/13] restrict bitfield operations with dynamic parameter

[Jeffrey Law <law at redhat dot com>]

On Tue, 2007-01-30 at 12:26 +0100, zippel@linux-m68k.org wrote:
> Hi,
> 
> When combine creates a bitfield instruction it assumes simple shift
> operations, but it doesn't work quite that way on m68k. If the bitfield
> crosses the register boundary it wraps around, thus it actually is a
> rotate operation.
> 
> This example demonstrates where it goes wrong (reduced from emacs):
> 
> #define XINT(v) (((int)(v) << 4) >> 4)
> #define XUINT(v) (((unsigned)(v) << 4) >> 4)
> 
> int f(int v, int s)
> {
> 	return (XUINT(v) >> -XINT(s)) & 0x0fffffff;
> }
> 
> gcc takes the shift and the mask and generates a bitfield op, but the
> real bitfield instruction wraps around at the register end and doesn't
> produce the expected result.
> 
> I guess it would be quite some work to teach combine about this and I
> don't know how these behave on other ports, so the simple solution
> below just disables dynamic offsets and widths for register arguments
> and while I'm at it I also cleaned up the predicates here as well.
Yea, I don't think we should teach combine about this behavior :-)

> 
> 
> 
> 2007-01-30  Roman Zippel <zippel@linux-m68k.org>
> 
> 	* config/m68k/m68k.md (extv,extzv,insv): disable dynamic
> 	parameter for register bitfield operations, general predicates
> 	cleanup
OK.

Jeff