[PATCH] Fix PR82337 (SLSR and abnormal PHIs)

[Richard Biener]

On Wed, Sep 27, 2017 at 9:15 PM, Bill Schmidt
<wschmidt@linux.vnet.ibm.com> wrote:
> Hi,
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82337 reports a problem
> with SLSR performing an invalid optimization across an abnormal PHI.
> This is easy to avoid by ensuring that SSA names used in an abnormal
> PHI never appear as a basis or as a PHI basis in the candidate table.
> We won't optimize what we can't see...
>
> I've cleaned up the test case in the bug report so that it compiles
> without warnings and added it to the torture tests.  Bootstrapped and
> tested on powerpc64le-linux-gnu with no regressions.  Is this ok for
> trunk?  I would also like to backport it to all open releases after a
> short wait.

Ok for trunk and branches.

Thanks,
Richard.

> Thanks,
> Bill
>
>
> 2017-09-26  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>
>
>         PR tree-optimization/82337
>         * gimple-ssa-strength-reduction.c (find_phi_def): Don't record a
>         phi definition if the PHI result appears in an abnormal PHI.
>         (find_basis_for_base_expr): Don't record a basis if the LHS of the
>         basis appears in an abnormal PHI.
>
>
> Index: gcc/gimple-ssa-strength-reduction.c
> ===================================================================
> --- gcc/gimple-ssa-strength-reduction.c (revision 253232)
> +++ gcc/gimple-ssa-strength-reduction.c (working copy)
> @@ -488,7 +488,8 @@ find_phi_def (tree base)
>
>    c = base_cand_from_table (base);
>
> -  if (!c || c->kind != CAND_PHI)
> +  if (!c || c->kind != CAND_PHI
> +      || SSA_NAME_OCCURS_IN_ABNORMAL_PHI (gimple_phi_result (c->cand_stmt)))
>      return 0;
>
>    return c->cand_num;
> @@ -557,6 +558,11 @@ find_basis_for_base_expr (slsr_cand_t c, tree base
>                               gimple_bb (one_basis->cand_stmt)))
>         continue;
>
> +      tree lhs = gimple_assign_lhs (one_basis->cand_stmt);
> +      if (lhs && TREE_CODE (lhs) == SSA_NAME
> +         && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (lhs))
> +       continue;
> +
>        if (!basis || basis->cand_num < one_basis->cand_num)
>         basis = one_basis;
>      }
> Index: gcc/testsuite/gcc.c-torture/compile/pr82337.c
> ===================================================================
> --- gcc/testsuite/gcc.c-torture/compile/pr82337.c       (nonexistent)
> +++ gcc/testsuite/gcc.c-torture/compile/pr82337.c       (working copy)
> @@ -0,0 +1,25 @@
> +/* PR82337: SLSR needs to prevent abnormal SSA names from
> +   serving as a basis. */
> +char *a, *b, *c;
> +
> +struct d {
> +  short e;
> +  char f[];
> +};
> +
> +extern void j (void);
> +
> +void
> +g() {
> +  struct d *h;
> +  char *i;
> +  int d;
> +  do {
> +    i = h->f + d;
> +    20 ? j() : 0;
> +    i = c;
> +    if (__builtin_setjmp (h))
> +      b = h->f + d;
> +    d = (int)(*i);
> +  } while (a);
> +}
>