Patches welcome :-).
On Fri, Sep 3, 2021, 18:42 Martin Sebor <mse...@gmail.com> wrote:
> On 9/3/21 8:41 AM, Aldy Hernandez via Gcc-patches wrote:
> > [Andrew, do you see any problem with using the minus relational code
> > here? It seems like everything matches up.]
> >
> > I've seen cases in the upcoming jump threader enhancements where we see
> > a difference of two pointers that are known to be equivalent, and yet we
> > fail to return 0 for the range. This is because we have no working
> > range-op entry for POINTER_DIFF_EXPR. The entry we currently have is
> > a mere placeholder to avoid ignoring POINTER_DIFF_EXPR's so
> > adjust_pointer_diff_expr() could get a whack at it here:
> >
> > // def = __builtin_memchr (arg, 0, sz)
> > // n = def - arg
> > //
> > // The range for N can be narrowed to [0, PTRDIFF_MAX - 1].
>
> For the two statements above a tighter bound is possible: [0, sz).
>
> With no range info, sz can be assumed to be in [0, N], where N
> is one less than the smaller of PTRDIFF_MAX and the size of arg.
> (With PTRDIFF_MAX being only hypothetical. max_object_size()
> should at some return the actual limit for the current target).
>
> The same (or even tighter) range can be derived from calls to
> other functions, including like mempcpy/stpcpy, or strchr, etc.
>
> Martin
>
> >
> > This patch adds the relational magic to range-op, which we can just
> > steal from the minus_expr code.
> >
> > Testing currently in progress. I will commit pending tests.
> >
> > gcc/ChangeLog:
> >
> > * range-op.cc (operator_minus::op1_op2_relation_effect): Abstract
> > out to...
> > (minus_op1_op2_relation_effect): ...here.
> > (class operator_pointer_diff): New.
> > (operator_pointer_diff::op1_op2_relation_effect): Call
> > minus_op1_op2_relation_effect.
> > (integral_table::integral_table): Add entry for POINTER_DIFF_EXPR.
> > ---
> > gcc/range-op.cc | 45 ++++++++++++++++++++++++++++++++++++++-------
> > 1 file changed, 38 insertions(+), 7 deletions(-)
> >
> > diff --git a/gcc/range-op.cc b/gcc/range-op.cc
> > index fee0e834c23..5e37133026d 100644
> > --- a/gcc/range-op.cc
> > +++ b/gcc/range-op.cc
> > @@ -1372,13 +1372,14 @@ operator_minus::wi_fold (irange &r, tree type,
> > }
> >
> > // Check to see if the relation REL between OP1 and OP2 has any effect
> on the
> > -// LHS of the expression. If so, apply it to LHS_RANGE.
> > +// LHS of the expression. If so, apply it to LHS_RANGE. This is a
> helper
> > +// function for both MINUS_EXPR and POINTER_DIFF_EXPR.
> >
> > -bool
> > -operator_minus::op1_op2_relation_effect (irange &lhs_range, tree type,
> > - const irange &op1_range
> ATTRIBUTE_UNUSED,
> > - const irange &op2_range
> ATTRIBUTE_UNUSED,
> > - relation_kind rel) const
> > +static bool
> > +minus_op1_op2_relation_effect (irange &lhs_range, tree type,
> > + const irange &op1_range ATTRIBUTE_UNUSED,
> > + const irange &op2_range ATTRIBUTE_UNUSED,
> > + relation_kind rel)
> > {
> > if (rel == VREL_NONE)
> > return false;
> > @@ -1440,6 +1441,16 @@ operator_minus::op1_op2_relation_effect (irange
> &lhs_range, tree type,
> > return true;
> > }
> >
> > +bool
> > +operator_minus::op1_op2_relation_effect (irange &lhs_range, tree type,
> > + const irange &op1_range,
> > + const irange &op2_range,
> > + relation_kind rel) const
> > +{
> > + return minus_op1_op2_relation_effect (lhs_range, type, op1_range,
> op2_range,
> > + rel);
> > +}
> > +
> > bool
> > operator_minus::op1_range (irange &r, tree type,
> > const irange &lhs,
> > @@ -1459,6 +1470,26 @@ operator_minus::op2_range (irange &r, tree type,
> > }
> >
> >
> > +class operator_pointer_diff : public range_operator
> > +{
> > + virtual bool op1_op2_relation_effect (irange &lhs_range,
> > + tree type,
> > + const irange &op1_range,
> > + const irange &op2_range,
> > + relation_kind rel) const;
> > +} op_pointer_diff;
> > +
> > +bool
> > +operator_pointer_diff::op1_op2_relation_effect (irange &lhs_range, tree
> type,
> > + const irange &op1_range,
> > + const irange &op2_range,
> > + relation_kind rel) const
> > +{
> > + return minus_op1_op2_relation_effect (lhs_range, type, op1_range,
> op2_range,
> > + rel);
> > +}
> > +
> > +
> > class operator_min : public range_operator
> > {
> > public:
> > @@ -4018,7 +4049,7 @@ integral_table::integral_table ()
> > set (OBJ_TYPE_REF, op_identity);
> > set (IMAGPART_EXPR, op_unknown);
> > set (REALPART_EXPR, op_unknown);
> > - set (POINTER_DIFF_EXPR, op_unknown);
> > + set (POINTER_DIFF_EXPR, op_pointer_diff);
> > set (ABS_EXPR, op_abs);
> > set (ABSU_EXPR, op_absu);
> > set (NEGATE_EXPR, op_negate);
> >
>
>
