RFD: hookizing BITS_PER_UNIT in tree optimizers / frontends

[Nathan Froyd]

On Wed, Nov 24, 2010 at 02:48:01PM +0000, Pedro Alves wrote:
> On Wednesday 24 November 2010 13:45:40, Joern Rennecke wrote:
> > Quoting Pedro Alves <pedro@codesourcery.com>:
> > Also, these separate hooks for common operations can make the code more
> > readable, particularly in the bits_in_units_ceil case.
> > I.e.
> >                  foo_var = ((bitsize + targetm.bits_per_unit () - 1)
> >                             / targetm.bits_per_unit ());
> > vs.
> >                  foo_var = targetm.bits_in_units_ceil (bitsize);
> > 
> 
> bits_in_units_ceil could well be a macro or helper function
> implemented on top of targetm.bits_per_unit (which itself could
> be a data field instead of a function call), that only accessed
> bits_per_unit once.  It could even be implemented as a helper
> macro / function today, on top of BITS_PER_UNIT.

I think adding the functions as inline functions somewhere and using
them in the appropriate places would be a reasonable standalone
cleanup.  It'd be easy to move towards something more general later.
Writing:

  int bits = ...;
  ... (X + bits - 1)/ bits;

  also generates ever-so-slightly smaller code than:

  ... (X + BITS_PER_UNIT - 1) / BITS_PER_UNIT;

on targets where BITS_PER_UNIT is not constant.

I personally am not a fan of the X_in_Y naming, though; I think X_to_Y
is a little clearer.

-Nathan