Re: [PATCH 2/2] Enable elimination of zext/sext

[Kugan <kugan.vivekanandarajah@linaro.org>]

On 02/08/14 02:03, Kugan wrote:
>>>>  if (rhs_uns)
>>>>    return wi::ge_p (min, 0);  // if min >= 0 then range contains positive values
>>>>  else
>>>>    return wi::le_p (max, wi::max_value (TYPE_PRECISION (TREE_TYPE
>>>> (ssa)), SIGNED);  // if max <= signed-max-of-type then range doesn't
>>>> need sign-extension
>>>
>>> I think we will have to check that ssa has necessary sign/zero extension
>>> when assigned to lhs_type. If PROMOTE_MODE tells us that ssa's type will
>>> be interpreted differently, the value range of ssa also will have
>>> corresponding range.  In this cases, shouldn’t we have to check for
>>> upper and lower limit for both min and max?
>>
>> Hmm?  That's exactly what the check is testing...  we know that
>> min <= max thus if min >= 0 then max >= 0.
>>
>> zero_extension will never do anything on [0, INF]
>>
>> If max < MAX-SIGNED then sign-extension will not do anything.  Ok,
>> sign-extension will do sth for negative values still.  So rather
>>
>>   if (rhs_uns)
>>     return wi::geu_p (min, 0);
>>   else
>>     return wi::ges_p (min, 0) && wi::les_p (max, wi::max_value
>> (TYPE_PRECISION (TREE_TYPE (ssa)), SIGNED));
>>
>> ?

Looking at your comments again, I think we have to consider three things
here.

To be able assign to LHS (of lhs_uns and lhs_mode) without conversion of
RHS (tree SSA)

* If we ignore the mode changes (i.e. LHS_mode can be different in terms
of precision) and ignore PROMOTE_MODE and consider only the sign of LHS
and RHS
  if (lhs_uns)
   return wi::ge_p (min, 0, rhs_signop);  // if min >= 0 then range
contains positive values
 else
   if (rhs_uns)
     // if max <= signed-max-of-type then range doesn't need sign-extension
     return wi::le_p (max, wi::max_value (TYPE_PRECISION (TREE_TYPE
(ssa)), SIGNED);
   else
     return true;


* However, if we consider the PROMOTE_MODE might change the RHS sign
  if (lhs_uns)
    {
      return wi::ge_p (min, 0, rhs_signop);
    }
  else
    {
      signed_max = wide_int::from (TYPE_MAX_VALUE (lhs_type),
				   TYPE_PRECISION (TREE_TYPE (ssa)), rhs_signop);
      if (rhs_uns)
	/* If PROMOTE_MODE changed an RHS signed to unsigned and
	   SSA contains negative value range, we still have to do sign-extend.  */
	return wi::ge_p (min, 0, TYPE_SIGN (TREE_TYPE (ssa)))
	  && wi::le_p (max, signed_max, rhs_signop);
      else
	/* If PROMOTE_MODE changed an RHS unsigned to signed and SSA contains value
	   range more than signed-max-of-type, we still have to do sign-extend.  */
	return wi::le_p (max, signed_max, TYPE_SIGN (TREE_TYPE (ssa)));
    }

* If we also consider that LHS mode and RHS mode precision can be different
  if (lhs_uns)
    {
      unsigned_max = wide_int::from (TYPE_MAX_VALUE (lhs_type),
				     TYPE_PRECISION (TREE_TYPE (ssa)), rhs_signop);
      /* If min >= 0 then range contains positive values and doesnt need
	 zero-extension.  If max <= unsigned-max-of-type, then value fits type.  */
      return wi::ge_p (min, 0, rhs_signop)
	&& wi::le_p (max, unsigned_max, rhs_signop);
    }
  else
    {
      signed_max = wide_int::from (TYPE_MAX_VALUE (lhs_type),
				   TYPE_PRECISION (TREE_TYPE (ssa)), rhs_signop);
      signed_min = wide_int::from (TYPE_MIN_VALUE (lhs_type),
				   TYPE_PRECISION (TREE_TYPE (ssa)), rhs_signop);
      if (rhs_uns)
	/* If PROMOTE_MODE changed an RHS signed to unsigned and
	   SSA contains negative value range, we still have to do sign-extend.  */
	return wi::ge_p (min, 0, TYPE_SIGN (TREE_TYPE (ssa)))
	  && wi::le_p (max, signed_max, rhs_signop);
      else
	/* If PROMOTE_MODE changed an RHS unsigned to signed and SSA contains value
	   range more than signed-max-of-type, we still have to do sign-extend.  */
	return wi::le_p (max, signed_max, TYPE_SIGN (TREE_TYPE (ssa)))
	  && wi::ge_p (min, signed_min, rhs_signop);
    }
}


Since we can have PROMOTE_MODE changing the sign and LHS mode and RHS
mode precision can be different, the check should be the third one. Does
that make sense or am I still missing it?

Thanks again for your time,
Kugan