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

[Paolo Bonzini <bonzini@gnu.org>]

On 07/30/2010 02:39 PM, Richard Guenther wrote:
>>> +     case LSHIFT_EXPR:
>>> +     case RSHIFT_EXPR:
>>> +       if (r2val.lattice_val == CONSTANT
>>> + 	&&  double_int_zero_p (r2val.bit_lattice_val))
>>
>> Even if some bits are varying, the result will have at least r2val.value&
>> ~r2val.bit_lattice_val bits shifted in.  So you can do something like
>>
>>        if (r2val.lattice_val != CONSTANT)
>>          break;
>>        if (double_int_zero_p (r2val.bit_lattice_val)) {
>>          in_mask = r1val.bit_lattice_val;
>>          in_value = r1val.value;
>>        } else {
>>          in_mask = double_int_minus_one;
>>          in_value = double_int_minus_one;
>>        }
>>
>>        shift = r2val.value&  ~r2val.bit_lattice_val;
>>        if (SHIFT_COUNT_TRUNCATED)
>>          shift&= GETMODE_BITSIZE (TYPE_MODE (type)) - 1;
>>
>>        val.lattice_val = CONSTANT;
>>        val.bit_lattice_val
>> 	= double_int_lshift (in_mask, shift,
>> 			     TYPE_PRECISION (type), false);
>>        val.value = double_int_lshift (in_value, shift,
>> 				     TYPE_PRECISION (type), false);
>
> Hmm.  I don't quite understand.  Are you saying that only the
> lower bits of r2val are important if SHIFT_COUNT_TRUNCATED?
> At least we need to know the sign of the shift amount
> to be able to tell if we're shifting in zeros from the right
> or ones from the left.

Well, yes. :)

I wonder if using negative shift counts is just a useless complication, 
since that's what got me confused.  If you just make "shift" a positive 
shift count and test the tree code rather than the sign of shift, the 
code I gave above to handle SHIFT_COUNT_TRUNCATED magically starts to work.

>>> + 	  else if (shift<  0)
>>> + 	    {
>>> + 	      val.lattice_val = CONSTANT;
>>> + 	      val.bit_lattice_val
>>> + 		= double_int_rshift (r1val.bit_lattice_val,
>>> + 				     -shift, TYPE_PRECISION (type), true);
>>> + 	      val.value = double_int_rshift (r1val.value, shift,
>>> + 					     TYPE_PRECISION (type), true);
>>> + 	    }
>>
>> Here shifted in bits are varying for signed types (unless the sign bit is
>> constant, but that's not handled here either).
>
> That should be handled fine by using an arithmetic shift for both
> the lattice and the value.

Aha, right.

> So for varying lattice "sign" bit we
> shift in varying.  Hm, but indeed for unsigned constants we shift
> in the sign bit - fixed to
>
>            else if (shift<  0)
>              {
>                val.bit_lattice_val
>                  = double_int_rshift (r1val.bit_lattice_val,
>                                       -shift, TYPE_PRECISION (type), true);

This should be !uns too, since for unsigned values shifted-in bits are 
constant.  But it's much more clever than I thought: if the topmost bit 
is constant, the newly computed lattice is also constant.  Very nice.

This unfortunately wouldn't work if r1val.bit_lattice_val is changed to 
in_mask as in my example above.  To handle that you could use

    msb_constant = uns || !double_bit_set_p (r1val.bit_lattice_val,
                                             TYPE_PRECISION (type) - 1)

and then you pass !msb_constant to this right shift.  I'll let you 
decide whether it's overkill, but in any case the current clever code 
needs documentation.

Just remember to add testcases covering shifting of unsigned values, 
positive signed values, and negative signed values.  Also file a missing 
optimization bug for e.g. (x << (y | 8)) & 255 if you don't include my 
suggestions.

>>> + 	  unsigned r1ctz = 0, r2ctz = 0;
>>> + 	  while (r1ctz<  HOST_BITS_PER_WIDE_INT
>>> + 		&&  !(r1val.bit_lattice_val.low&  (1<<  r1ctz))
>>> + 		&&  !(r1val.value.low&  (1<<  r1ctz)))
>>> + 	    r1ctz++;
>>> + 	  while (r2ctz<  HOST_BITS_PER_WIDE_INT
>>> + 		&&  !(r2val.bit_lattice_val.low&  (1<<  r2ctz))
>>> + 		&&  !(r2val.value.low&  (1<<  r2ctz)))
>>> + 	    r2ctz++;
>>
>> This is just a ctz on (v | m), no?  This makes it easier to track high bits as
>> well.
>
> Yes.  Probably worth adding double_int_ctz.

Agreed.

Paolo