On Fri, Oct 25, 2024 at 4:39 PM Alexandre Oliva <ol...@adacore.com> wrote:
>
>
> Prepare for ifcombining noncontiguous blocks, adding (still unused)
> logic to the ifcombine profile updater to handle such cases.
>
>
> for gcc/ChangeLog
>
> * tree-ssa-ifcombine.cc (known_succ_p): New.
> (update_profile_after_ifcombine): Handle noncontiguous blocks.
> ---
> gcc/tree-ssa-ifcombine.cc | 109
> +++++++++++++++++++++++++++++++++++----------
> 1 file changed, 85 insertions(+), 24 deletions(-)
>
> diff --git a/gcc/tree-ssa-ifcombine.cc b/gcc/tree-ssa-ifcombine.cc
> index 6dcf5e6efe1de..b5b72be29bbf9 100644
> --- a/gcc/tree-ssa-ifcombine.cc
> +++ b/gcc/tree-ssa-ifcombine.cc
> @@ -49,6 +49,21 @@ along with GCC; see the file COPYING3. If not see
> false) >= 2)
> #endif
>
> +/* Return FALSE iff the COND_BB ends with a conditional whose result is not a
> + known constant. */
> +
> +static bool
> +known_succ_p (basic_block cond_bb)
> +{
> + gcond *cond = safe_dyn_cast <gcond *> (*gsi_last_bb (cond_bb));
> +
> + if (!cond)
> + return true;
> +
> + return (CONSTANT_CLASS_P (gimple_cond_lhs (cond))
> + && CONSTANT_CLASS_P (gimple_cond_rhs (cond)));
> +}
> +
It now occurs to me that you could use
find_taken_edge (cond_bb, NULL_TREE) != NULL
in place of known_succ_p.
> /* This pass combines COND_EXPRs to simplify control flow. It
> currently recognizes bit tests and comparisons in chains that
> represent logical and or logical or of two COND_EXPRs.
> @@ -356,14 +371,28 @@ recognize_bits_test (gcond *cond, tree *name, tree
> *bits, bool inv)
> }
>
>
> -/* Update profile after code in outer_cond_bb was adjusted so
> - outer_cond_bb has no condition. */
> +/* Update profile after code in either outer_cond_bb or inner_cond_bb was
> + adjusted so that it has no condition. */
>
> static void
> update_profile_after_ifcombine (basic_block inner_cond_bb,
> basic_block outer_cond_bb)
I would hope that Honza can take a look here - in absence OK once the rest is
approved.
Richard.
> {
> - edge outer_to_inner = find_edge (outer_cond_bb, inner_cond_bb);
> + /* In the following we assume that inner_cond_bb has single predecessor.
> */
> + gcc_assert (single_pred_p (inner_cond_bb));
> +
> + basic_block outer_to_inner_bb = inner_cond_bb;
> + profile_probability prob = profile_probability::always ();
> + for (;;)
> + {
> + basic_block parent = single_pred (outer_to_inner_bb);
> + prob *= find_edge (parent, outer_to_inner_bb)->probability;
> + if (parent == outer_cond_bb)
> + break;
> + outer_to_inner_bb = parent;
> + }
> +
> + edge outer_to_inner = find_edge (outer_cond_bb, outer_to_inner_bb);
> edge outer2 = (EDGE_SUCC (outer_cond_bb, 0) == outer_to_inner
> ? EDGE_SUCC (outer_cond_bb, 1)
> : EDGE_SUCC (outer_cond_bb, 0));
> @@ -374,29 +403,61 @@ update_profile_after_ifcombine (basic_block
> inner_cond_bb,
> std::swap (inner_taken, inner_not_taken);
> gcc_assert (inner_taken->dest == outer2->dest);
>
> - /* In the following we assume that inner_cond_bb has single predecessor.
> */
> - gcc_assert (single_pred_p (inner_cond_bb));
> -
> - /* Path outer_cond_bb->(outer2) needs to be merged into path
> - outer_cond_bb->(outer_to_inner)->inner_cond_bb->(inner_taken)
> - and probability of inner_not_taken updated. */
> -
> - inner_cond_bb->count = outer_cond_bb->count;
> + if (outer_to_inner_bb == inner_cond_bb
> + && known_succ_p (outer_cond_bb))
> + {
> + /* Path outer_cond_bb->(outer2) needs to be merged into path
> + outer_cond_bb->(outer_to_inner)->inner_cond_bb->(inner_taken)
> + and probability of inner_not_taken updated. */
> +
> + inner_cond_bb->count = outer_cond_bb->count;
> +
> + /* Handle special case where inner_taken probability is always. In this
> + case we know that the overall outcome will be always as well, but
> + combining probabilities will be conservative because it does not know
> + that outer2->probability is inverse of
> + outer_to_inner->probability. */
> + if (inner_taken->probability == profile_probability::always ())
> + ;
> + else
> + inner_taken->probability = outer2->probability
> + + outer_to_inner->probability * inner_taken->probability;
> + inner_not_taken->probability = profile_probability::always ()
> + - inner_taken->probability;
>
> - /* Handle special case where inner_taken probability is always. In this
> case
> - we know that the overall outcome will be always as well, but combining
> - probabilities will be conservative because it does not know that
> - outer2->probability is inverse of outer_to_inner->probability. */
> - if (inner_taken->probability == profile_probability::always ())
> - ;
> + outer_to_inner->probability = profile_probability::always ();
> + outer2->probability = profile_probability::never ();
> + }
> + else if (known_succ_p (inner_cond_bb))
> + {
> + /* Path inner_cond_bb->(inner_taken) needs to be merged into path
> + outer_cond_bb->(outer2). We've accumulated the probabilities from
> + outer_cond_bb->(outer)->...->inner_cond_bb in prob, so we have to
> + adjust that by inner_taken, and make inner unconditional. */
> +
> + prob *= inner_taken->probability;
> + outer2->probability += prob;
> + outer_to_inner->probability = profile_probability::always ()
> + - outer2->probability;
> +
> + inner_taken->probability = profile_probability::never ();
> + inner_not_taken->probability = profile_probability::always ();
> + }
> else
> - inner_taken->probability = outer2->probability +
> outer_to_inner->probability
> - * inner_taken->probability;
> - inner_not_taken->probability = profile_probability::always ()
> - - inner_taken->probability;
> -
> - outer_to_inner->probability = profile_probability::always ();
> - outer2->probability = profile_probability::never ();
> + {
> + /* We've moved part of the inner cond to outer, but we don't know the
> + probabilities for each part, so estimate the effects by moving half
> of
> + the odds of inner_taken to outer. */
> +
> + inner_taken->probability *= profile_probability::even ();
> + inner_not_taken->probability = profile_probability::always ()
> + - inner_taken->probability;
> +
> + prob *= inner_taken->probability;
> + outer2->probability += prob;
> + outer_to_inner->probability = profile_probability::always ()
> + - outer2->probability;
> + }
> }
>
> /* Replace the conditions in INNER_COND with COND.
>
> --
> Alexandre Oliva, happy hacker https://FSFLA.org/blogs/lxo/
> Free Software Activist GNU Toolchain Engineer
> More tolerance and less prejudice are key for inclusion and diversity
> Excluding neuro-others for not behaving ""normal"" is *not* inclusive
