[PATCH, RFC] PR middle-end/55299, contract bitnot through ASR and rotations

[Mikhail Maltsev <maltsevm at gmail dot com>]

Hi.

The attached patch adds new match-and-simplify patterns, which fold
~((~a) >> b) into (a >> b) for arithmetic shifts (i.e. when A is signed)
and perform similar folds for rotations. It also fixes PR
tree-optimization/54579 (because we already fold (-a - 1) into ~a).

A couple of questions:
1. Should we limit folding to this special case or rather introduce some
canonical order of bitnot and shifts (when they are commutative)? In the
latter case, which order is better: bitnot as shift/rotate operand or
vise-versa?

2. I noticed that some rotation patterns are folded on tree, while other
are folded rather late (during second forward propagation). For example
on LP64:

#define INT_BITS  (sizeof (int) * 8)

unsigned int
rol(unsigned int a, unsigned int b)
{
  return a << b | a >> (INT_BITS - b);
}

INT_BITS has type unsigned long, so b and (INT_BITS - b) have different
types and tree folding fails (if I change int to long, everything is
OK). Should this be addressed somehow?

3. Do the new patterns require any special handling of nop-conversions?


-- 
Regards,
    Mikhail Maltsev

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