Yes. Condition to to switch two versioned loops is "true", the first two
arguments should be 100% and 0%.
It is different from normal loop split, we could not deduce exactly precise
probability for
condition-based loop split, since cfg inside loop2 would be changed.
(invar-branch is replaced
to "true", as shown in the comment on do_split_loop_on_cond). Any way, your way
of scaling
two loops' probabilities according to that of invar-branch, seems to be a
better heuristics than
original, which would give us more reasonable execution count, at least for
loop header bb.
Thanks,
Feng
________________________________________
From: Gcc-patches <gcc-patches-bounces+fxue=os.amperecomputing....@gcc.gnu.org>
on behalf of Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org>
Sent: Friday, August 6, 2021 7:46 PM
To: Xionghu Luo
Cc: seg...@kernel.crashing.org; wschm...@linux.ibm.com; li...@gcc.gnu.org;
gcc-patches@gcc.gnu.org; hubi...@ucw.cz; dje....@gmail.com
Subject: Re: [PATCH] Fix loop split incorrect count and probability
On Tue, 3 Aug 2021, Xionghu Luo wrote:
> loop split condition is moved between loop1 and loop2, the split bb's
> count and probability should also be duplicated instead of (100% vs INV),
> secondly, the original loop1 and loop2 count need be propotional from the
> original loop.
>
> Regression tested pass, OK for master?
>
> diff base/loop-cond-split-1.c.151t.lsplit
> patched/loop-cond-split-1.c.151t.lsplit:
> ...
> int prephitmp_16;
> int prephitmp_25;
>
> <bb 2> [local count: 118111600]:
> if (n_7(D) > 0)
> goto <bb 4>; [89.00%]
> else
> goto <bb 3>; [11.00%]
>
> <bb 3> [local count: 118111600]:
> return;
>
> <bb 4> [local count: 105119324]:
> pretmp_3 = ga;
>
> - <bb 5> [local count: 955630225]:
> + <bb 5> [local count: 315357973]:
> # i_13 = PHI <i_10(20), 0(4)>
> # prephitmp_12 = PHI <prephitmp_5(20), pretmp_3(4)>
> if (prephitmp_12 != 0)
> goto <bb 6>; [33.00%]
> else
> goto <bb 7>; [67.00%]
>
> - <bb 6> [local count: 315357972]:
> + <bb 6> [local count: 104068130]:
> _2 = do_something ();
> ga = _2;
>
> - <bb 7> [local count: 955630225]:
> + <bb 7> [local count: 315357973]:
> # prephitmp_5 = PHI <prephitmp_12(5), _2(6)>
> i_10 = inc (i_13);
> if (n_7(D) > i_10)
> goto <bb 21>; [89.00%]
> else
> goto <bb 11>; [11.00%]
>
> <bb 11> [local count: 105119324]:
> goto <bb 3>; [100.00%]
>
> - <bb 21> [local count: 850510901]:
> + <bb 21> [local count: 280668596]:
> if (prephitmp_12 != 0)
> - goto <bb 20>; [100.00%]
> + goto <bb 20>; [33.00%]
> else
> - goto <bb 19>; [INV]
> + goto <bb 19>; [67.00%]
>
> - <bb 20> [local count: 850510901]:
> + <bb 20> [local count: 280668596]:
> goto <bb 5>; [100.00%]
>
> - <bb 19> [count: 0]:
> + <bb 19> [local count: 70429947]:
> # i_23 = PHI <i_10(21)>
> # prephitmp_25 = PHI <prephitmp_5(21)>
>
> - <bb 12> [local count: 955630225]:
> + <bb 12> [local count: 640272252]:
> # i_15 = PHI <i_23(19), i_22(16)>
> # prephitmp_16 = PHI <prephitmp_25(19), prephitmp_16(16)>
> i_22 = inc (i_15);
> if (n_7(D) > i_22)
> goto <bb 16>; [89.00%]
> else
> goto <bb 11>; [11.00%]
>
> - <bb 16> [local count: 850510901]:
> + <bb 16> [local count: 569842305]:
> goto <bb 12>; [100.00%]
>
> }
>
> gcc/ChangeLog:
>
> * tree-ssa-loop-split.c (split_loop): Fix incorrect probability.
> (do_split_loop_on_cond): Likewise.
> ---
> gcc/tree-ssa-loop-split.c | 16 ++++++++--------
> 1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/gcc/tree-ssa-loop-split.c b/gcc/tree-ssa-loop-split.c
> index 3a09bbc39e5..8e5a7ded0f7 100644
> --- a/gcc/tree-ssa-loop-split.c
> +++ b/gcc/tree-ssa-loop-split.c
> @@ -583,10 +583,10 @@ split_loop (class loop *loop1)
> basic_block cond_bb;
>
> class loop *loop2 = loop_version (loop1, cond, &cond_bb,
> - profile_probability::always (),
> - profile_probability::always (),
> - profile_probability::always (),
> - profile_probability::always (),
> + true_edge->probability,
> + true_edge->probability.invert (),
> + true_edge->probability,
> + true_edge->probability.invert (),
> true);
there is no 'true_edge' variable at this point.
> gcc_assert (loop2);
>
> @@ -1486,10 +1486,10 @@ do_split_loop_on_cond (struct loop *loop1, edge
> invar_branch)
> initialize_original_copy_tables ();
>
> struct loop *loop2 = loop_version (loop1, boolean_true_node, NULL,
> - profile_probability::always (),
> - profile_probability::never (),
> - profile_probability::always (),
> - profile_probability::always (),
> + invar_branch->probability.invert (),
> + invar_branch->probability,
> + invar_branch->probability.invert (),
> + invar_branch->probability,
> true);
> if (!loop2)
> {
The patch introduction seems to talk about do_split_loop_on_cond only.
Since loop versioning inserts a condition with the passed probabilities
but in this case a 'boolean_true_node' condition the then and else
probabilities passed look correct. It's just the scaling arguments
that look wrong? This loop_version call should get a comment as to
why we are passing probabilities the way we do.
It does seem that scaling the loop by the invar_branch probability
is correct. Since this does similar things to unswitching, I see
that unswitching does
prob_true = edge_true->probability;
loop_version (loop, unshare_expr (cond),
NULL, prob_true,
prob_true.invert (),
prob_true, prob_true.invert (),
false);
which maybe suggests that your invar_branch based passing should
depend on 'true_invar'?
Also compared to unswitching the first loop is always entered,
so I wonder if the scaling is correct with respect to that
given unswitching where only ever one loop is entered?
Thanks,
Richard.
