[PR68083] stop ifcombine from moving uninitialized uses before their guards

On Oct 30, 2015, Alexandre Oliva <aoliva@redhat.com> wrote:

> Incidentally, bb_no_side_effects_p (inner_cond_bb) is called in all four
> tests in tree_ssa_ifcombine_bb_1, for each outer_cond_bb that
> tree_ssa_ifcombine_bb might choose.  Is there any reason to not factor
> it out to the test that checks whether the inner_cond_bb is indeed an
> if_then_else block, early in tree_ssa_ifcombine_bb, so as to
> short-circuit the whole thing when the inner block is not viable?

Like this...

Bail out early if the inner block has side effects or is otherwise not
eligible for ifcombine.

Regstrapped on x86_64-linux-gnu.  Ok to install?


for  gcc/ChangeLog

	* tree-ssa-ifcombine.c (tree_ssa_ifcombine_bb_1): Factor out
	bb_no_side_effects_p tests...
	(tree_ssa_ifcombine_bb): ... here.
---
 gcc/tree-ssa-ifcombine.c |   15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

On Fri, Oct 30, 2015 at 7:02 PM, Alexandre Oliva <aoliva@redhat.com> wrote:
> On Oct 30, 2015, Alexandre Oliva <aoliva@redhat.com> wrote:
>
>> Incidentally, bb_no_side_effects_p (inner_cond_bb) is called in all four
>> tests in tree_ssa_ifcombine_bb_1, for each outer_cond_bb that
>> tree_ssa_ifcombine_bb might choose.  Is there any reason to not factor
>> it out to the test that checks whether the inner_cond_bb is indeed an
>> if_then_else block, early in tree_ssa_ifcombine_bb, so as to
>> short-circuit the whole thing when the inner block is not viable?
>
> Like this...
>
> Bail out early if the inner block has side effects or is otherwise not
> eligible for ifcombine.
>
> Regstrapped on x86_64-linux-gnu.  Ok to install?

Ok.

Thanks,
Richard.

>
> for  gcc/ChangeLog
>
>         * tree-ssa-ifcombine.c (tree_ssa_ifcombine_bb_1): Factor out
>         bb_no_side_effects_p tests...
>         (tree_ssa_ifcombine_bb): ... here.
> ---
>  gcc/tree-ssa-ifcombine.c |   15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
>
> diff --git a/gcc/tree-ssa-ifcombine.c b/gcc/tree-ssa-ifcombine.c
> index 622dc6b..3b60968 100644
> --- a/gcc/tree-ssa-ifcombine.c
> +++ b/gcc/tree-ssa-ifcombine.c
> @@ -579,8 +579,7 @@ tree_ssa_ifcombine_bb_1 (basic_block inner_cond_bb, basic_block outer_cond_bb,
>       the inner cond_bb having no side-effects.  */
>    if (phi_pred_bb != else_bb
>        && recognize_if_then_else (outer_cond_bb, &inner_cond_bb, &else_bb)
> -      && same_phi_args_p (outer_cond_bb, phi_pred_bb, else_bb)
> -      && bb_no_side_effects_p (inner_cond_bb))
> +      && same_phi_args_p (outer_cond_bb, phi_pred_bb, else_bb))
>      {
>        /* We have
>            <outer_cond_bb>
> @@ -598,8 +597,7 @@ tree_ssa_ifcombine_bb_1 (basic_block inner_cond_bb, basic_block outer_cond_bb,
>    /* And a version where the outer condition is negated.  */
>    if (phi_pred_bb != else_bb
>        && recognize_if_then_else (outer_cond_bb, &else_bb, &inner_cond_bb)
> -      && same_phi_args_p (outer_cond_bb, phi_pred_bb, else_bb)
> -      && bb_no_side_effects_p (inner_cond_bb))
> +      && same_phi_args_p (outer_cond_bb, phi_pred_bb, else_bb))
>      {
>        /* We have
>            <outer_cond_bb>
> @@ -620,8 +618,7 @@ tree_ssa_ifcombine_bb_1 (basic_block inner_cond_bb, basic_block outer_cond_bb,
>       having no side-effects.  */
>    if (phi_pred_bb != then_bb
>        && recognize_if_then_else (outer_cond_bb, &then_bb, &inner_cond_bb)
> -      && same_phi_args_p (outer_cond_bb, phi_pred_bb, then_bb)
> -      && bb_no_side_effects_p (inner_cond_bb))
> +      && same_phi_args_p (outer_cond_bb, phi_pred_bb, then_bb))
>      {
>        /* We have
>            <outer_cond_bb>
> @@ -638,8 +635,7 @@ tree_ssa_ifcombine_bb_1 (basic_block inner_cond_bb, basic_block outer_cond_bb,
>    /* And a version where the outer condition is negated.  */
>    if (phi_pred_bb != then_bb
>        && recognize_if_then_else (outer_cond_bb, &inner_cond_bb, &then_bb)
> -      && same_phi_args_p (outer_cond_bb, phi_pred_bb, then_bb)
> -      && bb_no_side_effects_p (inner_cond_bb))
> +      && same_phi_args_p (outer_cond_bb, phi_pred_bb, then_bb))
>      {
>        /* We have
>            <outer_cond_bb>
> @@ -676,7 +672,8 @@ tree_ssa_ifcombine_bb (basic_block inner_cond_bb)
>         if (a && b)
>          ;
>       This requires a single predecessor of the inner cond_bb.  */
> -  if (single_pred_p (inner_cond_bb))
> +  if (single_pred_p (inner_cond_bb)
> +      && bb_no_side_effects_p (inner_cond_bb))
>      {
>        basic_block outer_cond_bb = single_pred (inner_cond_bb);
>
>
>
> --
> Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
> You must be the change you wish to see in the world. -- Gandhi
> Be Free! -- http://FSFLA.org/   FSF Latin America board member
> Free Software Evangelist|Red Hat Brasil GNU Toolchain Engineer