Re: do we ever thread computed gotos?

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

On Thu, Oct 26, 2017 at 1:05 PM, Aldy Hernandez <aldyh@redhat.com> wrote:
> Howdy.
>
> In the backwards threader we attempt to thread paths that lead to a basic
> block ending in either a GIMPLE_COND, GIMPLE_SWITCH, or a GIMPLE_GOTO.  The
> latter doesn't make much sense, since we only handle constants.  What does a
> goto to a constant mean?  Does that ever happen?
>
> In tree-ssa-threadbackward.c, given a constant ARG, we attempt to find the
> taken edge out of a BB with find_taken_edge():
>
>   edge taken_edge = find_taken_edge (path[0], arg);
>
> But if we drill down into find_taken_edge, we can see that we don't even
> handle an ARG of a constant, only an ARGs of ADDR_EXPR or LABEL_EXPR:
>
>   if (computed_goto_p (stmt))
>     {
> ...
> ...
>       if (val
>           && (TREE_CODE (val) == ADDR_EXPR || TREE_CODE (val) == LABEL_EXPR)
>           && TREE_CODE (TREE_OPERAND (val, 0)) == LABEL_DECL)
>         return find_taken_edge_computed_goto (bb, TREE_OPERAND (val, 0));
>       return NULL;
>     }
>   gcc_unreachable ();
>
> I even tried bootstrapping and regtesting with:
>
> diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
> index 12bc80350f5..e80fba91ffd 100644
> --- a/gcc/tree-ssa-threadbackward.c
> +++ b/gcc/tree-ssa-threadbackward.c
> @@ -301,6 +301,8 @@ profitable_jump_thread_path (vec<basic_block> &path,
>
>       We have to know the outgoing edge to figure this out.  */
>    edge taken_edge = find_taken_edge (path[0], arg);
> +  if (taken_edge)
> +    gcc_assert (gimple_code (stmt) != GIMPLE_GOTO);
>
>
> ...and this was never triggered.
>
> Am I missing something fundamental here, or can we get rid of all the
> GIMPLE_GOTO handling in the backwards threader?

I think it is useful for threading in state machines implemented via
computed goto.

Richard.

> Thanks.
> Aldy