Re: [PATCH 1/2] tree-if-conv.c: fix ICE seen with -fno-tree-forwprop (PR tree-optimization/84178)

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, Mar 7, 2018 at 7:56 PM, David Malcolm <dmalcolm@redhat.com> wrote:
> PR tree-optimization/84178 reports a couple of source files that ICE inside
> ifcvt when compiled with -03 -fno-tree-forwprop (trunk and gcc 7).
>
> Both cases involve problems with ifcvt's per-BB gimplified predicates.
>
> Testcase 1 fails this assertion within release_bb_predicate during cleanup:
>
> 283           if (flag_checking)
> 284             for (gimple_stmt_iterator i = gsi_start (stmts);
> 285                  !gsi_end_p (i); gsi_next (&i))
> 286               gcc_assert (! gimple_use_ops (gsi_stmt (i)));
>
> The testcase contains a division in the loop, which leads to
> if_convertible_loop_p returning false (due to gimple_could_trap_p being true
> for the division).  This happens *after* the per-BB gimplified predicates
> have been created in predicate_bbs (loop).
> Hence tree_if_conversion bails out to "cleanup", but the gimplified predicates
> exist and make use of SSA names; for example this conjunction for two BB
> conditions:
>
>   _4 = h4.1_112 != 0;
>   _175 = (signed char) _117;
>   _176 = _175 >= 0;
>   _174 = _4 & _176;
>
> is using SSA names.
>
> This assertion was added in r236498 (aka c3deca2519d97c55876869c57cf11ae1e5c6cf8b):
>
>     2016-05-20  Richard Biener  <rguenther@suse.de>
>
>         * tree-if-conv.c (add_bb_predicate_gimplified_stmts): Use
>         gimple_seq_add_seq_without_update.
>         (release_bb_predicate): Assert we have no operands to free.
>         (if_convertible_loop_p_1): Calculate post dominators later.
>         Do not free BB predicates here.
>         (combine_blocks): Do not recompute BB predicates.
>         (version_loop_for_if_conversion): Save BB predicates around
>         loop versioning.
>
>         * gcc.dg/tree-ssa/ifc-cd.c: Adjust.
>
> The following patch fixes this by adding a call to gimple_seq_discard
> to release_bb_predicate.  It also updates the assertion, so that
> instead of asserting the stmts have no imm uses, instead assert that
> they weren't added to a bb before discarding them (otherwise discarding
> them would be a bug).  We know this is the case because
> insert_gimplified_predicates has:
>
>           /* Once the sequence is code generated, set it to NULL.  */
>           set_bb_predicate_gimplified_stmts (bb, NULL);
>
> but asserting it seems appropriate as a double-check.
>
> The patch doesn't address the 2nd issue within PR tree-optimization/84178.
>
> Successfully bootstrapped&regrtested on x86_64-pc-linux-gnu.
>
> OK for trunk?

OK.

Thanks,
Richard.

> gcc/ChangeLog:
>         PR tree-optimization/84178
>         * tree-if-conv.c (release_bb_predicate): Remove the
>         the assertion that the stmts have NULL use_ops.
>         Discard the statements, asserting that they haven't
>         yet been added to a BB.
>
> gcc/testsuite/ChangeLog:
>         PR tree-optimization/84178
>         * gcc.c-torture/compile/pr84178-1.c: New test.
> ---
>  gcc/testsuite/gcc.c-torture/compile/pr84178-1.c | 18 ++++++++++++++++++
>  gcc/tree-if-conv.c                              |  8 +++++---
>  2 files changed, 23 insertions(+), 3 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.c-torture/compile/pr84178-1.c
>
> diff --git a/gcc/testsuite/gcc.c-torture/compile/pr84178-1.c b/gcc/testsuite/gcc.c-torture/compile/pr84178-1.c
> new file mode 100644
> index 0000000..49f2c89
> --- /dev/null
> +++ b/gcc/testsuite/gcc.c-torture/compile/pr84178-1.c
> @@ -0,0 +1,18 @@
> +/* { dg-options "-fno-tree-forwprop" } */
> +
> +int zy, h4;
> +
> +void
> +r8 (long int mu, int *jr, int *fi, short int dv)
> +{
> +  do
> +    {
> +      int tx;
> +
> +      tx = !!h4 ? (zy / h4) : 1;
> +      mu = tx;
> +      *jr = (((unsigned char) mu > (254 >> dv)) ? 0 : (unsigned char) tx) + *fi;
> +    } while (*jr == 0);
> +
> +  r8 (mu, jr, fi, 1);
> +}
> diff --git a/gcc/tree-if-conv.c b/gcc/tree-if-conv.c
> index cac3fd7..5467f3f 100644
> --- a/gcc/tree-if-conv.c
> +++ b/gcc/tree-if-conv.c
> @@ -271,8 +271,7 @@ init_bb_predicate (basic_block bb)
>    set_bb_predicate (bb, boolean_true_node);
>  }
>
> -/* Release the SSA_NAMEs associated with the predicate of basic block BB,
> -   but don't actually free it.  */
> +/* Release the SSA_NAMEs associated with the predicate of basic block BB.  */
>
>  static inline void
>  release_bb_predicate (basic_block bb)
> @@ -280,11 +279,14 @@ release_bb_predicate (basic_block bb)
>    gimple_seq stmts = bb_predicate_gimplified_stmts (bb);
>    if (stmts)
>      {
> +      /* Ensure that these stmts haven't yet been added to a bb.  */
>        if (flag_checking)
>         for (gimple_stmt_iterator i = gsi_start (stmts);
>              !gsi_end_p (i); gsi_next (&i))
> -         gcc_assert (! gimple_use_ops (gsi_stmt (i)));
> +         gcc_assert (! gimple_bb (gsi_stmt (i)));
>
> +      /* Discard them.  */
> +      gimple_seq_discard (stmts);
>        set_bb_predicate_gimplified_stmts (bb, NULL);
>      }
>  }
> --
> 1.8.5.3
>