[PATCH] Saturate overflows return from SCEV in ranger.

[Richard Biener]

On Tue, Oct 20, 2020 at 5:21 PM Aldy Hernandez via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> bounds_of_var_in_loop is returning an overflowed int, which is causing
> us to create a range for which we can't compare the bounds causing
> an ICE in verify_range.
>
> Overflowed bounds cause compare_values() to return -2, which we
> don't handle in verify_range.
>
> We don't represent overflowed ranges in irange, so this patch just
> saturates any overflowed end-points to MIN or MAX.

I don't think TREE_OVERFLOW means what you think it means in the
context of bounds_of_var_in_loop - look at its bottom which does

  /* Even for valid range info, sometimes overflow flag will leak in.
     As GIMPLE IL should have no constants with TREE_OVERFLOW set, we
     drop them.  */
  if (TREE_OVERFLOW_P (*min))
    *min = drop_tree_overflow (*min);
  if (TREE_OVERFLOW_P (*max))
    *max = drop_tree_overflow (*max);

and the code explicitly checks for overflow, doing range adjustments
accordingly.

So not sure what you're trying to fix here.

Every use of TREE_OVERFLOW in the GIMPLE parts of the ME are a sign of a bug.

Richard.

> Pushed.
>
> gcc/ChangeLog:
>
>         PR 97501/tree-optimization
>         * gimple-range.cc (gimple_ranger::range_of_ssa_name_with_loop_info):
>         Saturate overflows returned from SCEV.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/pr97501.c: New test.
> ---
>  gcc/gimple-range.cc            |  4 ++--
>  gcc/testsuite/gcc.dg/pr97501.c | 14 ++++++++++++++
>  2 files changed, 16 insertions(+), 2 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/pr97501.c
>
> diff --git a/gcc/gimple-range.cc b/gcc/gimple-range.cc
> index e4864ba60f6..ed9609be68e 100644
> --- a/gcc/gimple-range.cc
> +++ b/gcc/gimple-range.cc
> @@ -1146,9 +1146,9 @@ gimple_ranger::range_of_ssa_name_with_loop_info (irange &r, tree name,
>        // ?? We could do better here.  Since MIN/MAX can only be an
>        // SSA, SSA +- INTEGER_CST, or INTEGER_CST, we could easily call
>        // the ranger and solve anything not an integer.
> -      if (TREE_CODE (min) != INTEGER_CST)
> +      if (TREE_CODE (min) != INTEGER_CST || TREE_OVERFLOW (min))
>         min = vrp_val_min (type);
> -      if (TREE_CODE (max) != INTEGER_CST)
> +      if (TREE_CODE (max) != INTEGER_CST || TREE_OVERFLOW (max))
>         max = vrp_val_max (type);
>        r.set (min, max);
>      }
> diff --git a/gcc/testsuite/gcc.dg/pr97501.c b/gcc/testsuite/gcc.dg/pr97501.c
> new file mode 100644
> index 00000000000..aedac83962d
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr97501.c
> @@ -0,0 +1,14 @@
> +// { dg-do compile }
> +// { dg-options "-O2" }
> +
> +static int c = 0;
> +
> +int main() {
> +  int b = 0;
> +  if (c) {
> +  for (;; b--)
> +    do
> +      b++;
> +    while (b);
> +  }
> +}
> --
> 2.26.2
>