[4/6] Make the vectoriser do its own DCE

[Jeff Law]

On 08/28/2018 05:23 AM, Richard Sandiford wrote:
> The vectoriser normally leaves a later DCE pass to remove the scalar
> code, but we've accumulated various special cases for things that
> DCE can't handle, such as removing the scalar stores that have been
> replaced by vector stores, and the scalar calls to internal functions.
> (The latter must be removed for correctness, since no underlying scalar
> optabs exist for those calls.)
> 
> Now that vec_basic_block gives us an easy way of iterating over the
> original scalar code (ignoring any new code inserted by the vectoriser),
> it seems easier to do the DCE directly.  This involves marking the few
> cases in which the vector code needs part of the original scalar code
> to be kept around.
> 
> 
> 2018-08-28  Richard Sandiford  <richard.sandiford@arm.com>
> 
> gcc/
> 	* tree-vectorizer.h (_stmt_vec_info::used_by_vector_code_p): New
> 	member variable.
> 	(vect_mark_used_by_vector_code): Declare.
> 	(vect_remove_dead_scalar_stmts): Likewise.
> 	(vect_transform_stmt): Return void.
> 	(vect_remove_stores): Delete.
> 	* tree-vectorizer.c (vec_info::remove_stmt): Handle phis.
> 	* tree-vect-stmts.c (vect_mark_used_by_vector_code): New function.
> 	(vectorizable_call, vectorizable_simd_clone_call): Don't remove
> 	scalar calls here.
> 	(vectorizable_shift): Mark shift amounts in a vector-scalar shift
> 	as being used by the vector code.
> 	(vectorizable_load): Mark unhoisted scalar loads that feed a
> 	load-and-broadcast operation as being needed by the vector code.
> 	(vect_transform_stmt): Return void.
> 	(vect_remove_stores): Delete.
> 	(vect_maybe_remove_scalar_stmt): New function.
> 	(vect_remove_dead_scalar_stmts): Likewise.
> 	* tree-vect-slp.c (vect_slp_bb): Call vect_remove_dead_scalar_stmts.
> 	(vect_remove_slp_scalar_calls): Delete.
> 	(vect_schedule_slp): Don't call it.  Don't remove scalar stores here.
> 	* tree-vect-loop.c (vectorizable_reduction): Mark scalar phis that
> 	are retained by the vector code.
> 	(vectorizable_live_operation): Mark scalar live-out statements that
> 	are retained by the vector code.
> 	(vect_transform_loop_stmt): Remove seen_store argument.  Mark gconds
> 	in nested loops as being needed by the vector code.  Replace the
> 	outer loop's gcond with a dummy condition.
> 	(vect_transform_loop): Update calls accordingly.  Don't remove
> 	scalar stores or calls here; call vect_remove_dead_scalar_stmts
> 	instead.
> 	* tree-vect-loop-manip.c (vect_update_ivs_after_vectorizer): Don't
> 	remove PHIs that were classified as reductions but never actually
> 	vectorized.
Presumably you'll look at killing the actual DCE pass we're running as a
follow-up.

I'm hoping this will help the tendency of the vectorizer to leak
SSA_NAMEs.  Though it's late enough in the pipeline that leaking isn't
that bad.





> Index: gcc/tree-vectorizer.c
> ===================================================================
> --- gcc/tree-vectorizer.c	2018-08-28 12:05:11.466982927 +0100
> +++ gcc/tree-vectorizer.c	2018-08-28 12:05:14.014961439 +0100
> @@ -654,8 +654,13 @@ vec_info::remove_stmt (stmt_vec_info stm
>    set_vinfo_for_stmt (stmt_info->stmt, NULL);
>    gimple_stmt_iterator si = gsi_for_stmt (stmt_info->stmt);
>    unlink_stmt_vdef (stmt_info->stmt);
> -  gsi_remove (&si, true);
> -  release_defs (stmt_info->stmt);
> +  if (is_a <gphi *> (stmt_info->stmt))
> +    remove_phi_node (&si, true);
> +  else
> +    {
> +      gsi_remove (&si, true);
> +      release_defs (stmt_info->stmt);
> +    }
More than once I've wondered if we should factor this into its own
little function.  I'm pretty sure I've seen this style of code
elsewhere.  Your call whether or not to clean that up.

OK with or without creating a little helper for the code above.
jeff