[PATCH][RFC] Bit CCP and pointer alignment propagation

[Jay Foad]

> > > + typedef struct bit_prop_value_d {
> > > +   /* Lattice value.  */
> > > +   unsigned lattice_val;
> > > +
> > > +   /* Bit lattice value.  Zero for constant, one for varying.  */
> > > +   double_int bit_lattice_val;
> >
> > I always think of this member as a mask, why not call it so?  At runtime
> > for a value X that has MASK and VALUE in the lattice this holds:
> > (X & ~MASK) == VALUE .  Actually I'm not sure you ensure this as you
> > sometimes only adjust the mask, but not value, so you we might know only
> > this: (X & ~MASK) == (VALUE & ~MASK).
>
> That's true, varying value bits have undefined content.

If you define a canonical form where varying value bits have to be zero:

  gcc_assert((foo.value & foo.bit_lattice_val) == 0);

then you can use non-canonical values in the (bit_lattice_val, value)
pair to represent different lattice values. For example:

lattice_val   : represented by (bit_lattice_val, value)
-------------------------------------------------------
UNINITIALIZED : (-1, -2)
UNDEFINED     : (-1, -1)
CONSTANT      : any (blv, v) where v & blv == 0
VARYING       : (-1, 1)

With this scheme (v & blv) + 2 gives you the lattice value as you have
it now.  Then you don't need the lattice_val field in struct
bit_prop_value_d, which should make them pack together much better.
Dunno whether it's worth the obfuscation, though.

Jay.