Re: [PATCH] [PR68097] Try to avoid recursing for floats in tree_*_nonnegative_warnv_p.

[Richard Biener <richard.guenther@gmail.com>]

On Sat, Nov 12, 2022 at 7:30 PM Aldy Hernandez <aldyh@redhat.com> wrote:
>
> It irks me that a PR named "we should track ranges for floating-point
> hasn't been closed in this release.  This is an attempt to do just
> that.
>
> As mentioned in the PR, even though we track ranges for floats, it has
> been suggested that avoiding recursing through SSA defs in
> gimple_assign_nonnegative_warnv_p is also a goal.  We can do this with
> various ranger components without the need for a heavy handed approach
> (i.e. a full ranger).
>
> I have implemented two versions of known_float_sign_p() that answer
> the question whether we definitely know the sign for an operation or a
> tree expression.
>
> Both versions use get_global_range_query, which is a wrapper to query
> global ranges.  This means, that no caching or propagation is done.
> In the case of an SSA, we just return the global range for it (think
> SSA_NAME_RANGE_INFO).  In the case of a tree code with operands, we
> also use get_global_range_query to resolve the operands, and then call
> into range-ops, which is our lowest level component.  There is no
> ranger or gori involved.  All we're doing is resolving the operation
> with the ranges passed.
>
> This is enough to avoid recursing in the case where we definitely know
> the sign of a range.  Otherwise, we still recurse.
>
> Note that instead of get_global_range_query(), we could use
> get_range_query() which uses a ranger (if active in a pass), or
> get_global_range_query if not.  This would allow passes that have an
> active ranger (with enable_ranger) to use a full ranger.  These passes
> are currently, VRP, loop unswitching, DOM, loop versioning, etc.  If
> no ranger is active, get_range_query defaults to global ranges, so
> there's no additional penalty.
>
> Would this be acceptable, at least enough to close (or rename the PR ;-))?

I think the checks would belong to the gimple_stmt_nonnegative_warnv_p function
only (that's the SSA name entry from the fold-const.cc ones)?

I also notice the use of 'bool' for the "sign".  That's not really
descriptive.  We
have SIGNED and UNSIGNED (aka enum signop), not sure if that's the
perfect match vs. NEGATIVE and NONNEGATIVE.  Maybe the functions
name is just bad and they should be known_float_negative_p?

>         PR tree-optimization/68097
>
> gcc/ChangeLog:
>
>         * fold-const.cc (known_float_sign_p): New.
>         (tree_unary_nonnegative_warnv_p): Call known_float_sign_p.
>         (tree_binary_nonnegative_warnv_p): Same.
>         (tree_single_nonnegative_warnv_p): Same.
> ---
>  gcc/fold-const.cc | 51 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 51 insertions(+)
>
> diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
> index b89cac91cae..bd74cfca996 100644
> --- a/gcc/fold-const.cc
> +++ b/gcc/fold-const.cc
> @@ -14577,6 +14577,44 @@ tree_simple_nonnegative_warnv_p (enum tree_code code, tree type)
>    return false;
>  }
>
> +/* Return true if T is of type floating point and has a known sign.
> +   If so, set the sign in SIGN.  */
> +
> +static bool
> +known_float_sign_p (bool &sign, tree t)
> +{
> +  if (!frange::supports_p (TREE_TYPE (t)))
> +    return false;
> +
> +  frange r;
> +  return (get_global_range_query ()->range_of_expr (r, t)
> +         && r.signbit_p (sign));
> +}
> +
> +/* Return true if TYPE is a floating-point type and (CODE OP0 OP1) has
> +   a known sign.  If so, set the sign in SIGN.  */
> +
> +static bool
> +known_float_sign_p (bool &sign, enum tree_code code, tree type, tree op0,
> +                   tree op1 = NULL_TREE)
> +{
> +  if (!frange::supports_p (type))
> +    return false;
> +
> +  range_op_handler handler (code, type);
> +  if (handler)
> +    {
> +      frange res, r0, r1;
> +      get_global_range_query ()->range_of_expr (r0, op0);
> +      if (op1)
> +       get_global_range_query ()->range_of_expr (r1, op1);
> +      else
> +       r1.set_varying (type);
> +      return handler.fold_range (res, type, r0, r1) && res.signbit_p (sign);
> +    }
> +  return false;
> +}
> +
>  /* Return true if (CODE OP0) is known to be non-negative.  If the return
>     value is based on the assumption that signed overflow is undefined,
>     set *STRICT_OVERFLOW_P to true; otherwise, don't change
> @@ -14589,6 +14627,10 @@ tree_unary_nonnegative_warnv_p (enum tree_code code, tree type, tree op0,
>    if (TYPE_UNSIGNED (type))
>      return true;
>
> +  bool sign;
> +  if (known_float_sign_p (sign, code, type, op0))
> +    return !sign;
> +
>    switch (code)
>      {
>      case ABS_EXPR:
> @@ -14656,6 +14698,10 @@ tree_binary_nonnegative_warnv_p (enum tree_code code, tree type, tree op0,
>    if (TYPE_UNSIGNED (type))
>      return true;
>
> +  bool sign;
> +  if (known_float_sign_p (sign, code, type, op0, op1))
> +    return !sign;
> +
>    switch (code)
>      {
>      case POINTER_PLUS_EXPR:
> @@ -14778,6 +14824,8 @@ tree_binary_nonnegative_warnv_p (enum tree_code code, tree type, tree op0,
>  bool
>  tree_single_nonnegative_warnv_p (tree t, bool *strict_overflow_p, int depth)
>  {
> +  bool sign;
> +
>    if (TYPE_UNSIGNED (TREE_TYPE (t)))
>      return true;
>
> @@ -14796,6 +14844,9 @@ tree_single_nonnegative_warnv_p (tree t, bool *strict_overflow_p, int depth)
>        return RECURSE (TREE_OPERAND (t, 1)) && RECURSE (TREE_OPERAND (t, 2));
>
>      case SSA_NAME:
> +      if (known_float_sign_p (sign, t))
> +       return !sign;
> +
>        /* Limit the depth of recursion to avoid quadratic behavior.
>          This is expected to catch almost all occurrences in practice.
>          If this code misses important cases that unbounded recursion
> --
> 2.38.1
>