On Thu, Oct 26, 2017 at 1:05 PM, Aldy Hernandez <al...@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
