On Wed, 23 Aug 2023, Jan Hubicka wrote:
> > We seem to peel one iteration for no good reason. The loop is
> > a do-while loop already. The key is we see the first iteration
> > exit condition is known not taken and then:
> Hi,
> this is patch fixing wrong return value in should_duplicate_loop_header_p.
> Doing so uncovered suboptimal decisions on some jump threading testcases
> where we chose to stop duplicating just before basic block that has zero
> cost and duplicating so would be always a win.
>
> This is because the heuristics trying to chose right point to duplicate
> all winning blocks and to get loop to be do_while did not account
> zero_cost blocks in all cases. The patch simplifies the logic by
> simply remembering zero cost blocks and handling them last after
> the right stopping point is chosen.
>
> Bootstrapped/regtested x86_64-linux, OK?
OK.
Thanks,
Richard.
> gcc/ChangeLog:
>
> * tree-ssa-loop-ch.cc (enum ch_decision): Fix comment.
> (should_duplicate_loop_header_p): Fix return value for static exits.
> (ch_base::copy_headers): Improve handling of ch_possible_zero_cost.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/tree-ssa/copy-headers-9.c: Update template.
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/copy-headers-9.c
> b/gcc/testsuite/gcc.dg/tree-ssa/copy-headers-9.c
> index b49d1fc9576..11ee29458a2 100644
> --- a/gcc/testsuite/gcc.dg/tree-ssa/copy-headers-9.c
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/copy-headers-9.c
> @@ -13,7 +13,6 @@ void test (int m, int n)
> }
> while (i<10);
> }
> -/* { dg-final { scan-tree-dump-times "Duplicating bb . is a win" 2 "ch2" } }
> */
> -/* { dg-final { scan-tree-dump-times "Duplicating bb . is a win. it has
> zero" 1 "ch2" } } */
> +/* { dg-final { scan-tree-dump-times "Duplicating bb . is a win" 1 "ch2" } }
> */
> /* { dg-final { scan-tree-dump-times "Will duplicate bb" 2 "ch2" } } */
> /* { dg-final { scan-tree-dump "is now do-while loop" "ch2" } } */
> diff --git a/gcc/tree-ssa-loop-ch.cc b/gcc/tree-ssa-loop-ch.cc
> index 6cdb87a762f..461416e4086 100644
> --- a/gcc/tree-ssa-loop-ch.cc
> +++ b/gcc/tree-ssa-loop-ch.cc
> @@ -176,7 +176,7 @@ enum ch_decision
> ch_impossible,
> /* We can copy it if it enables wins. */
> ch_possible,
> - /* We can "cop" it if it enables wins and doing
> + /* We can "copy" it if it enables wins and doing
> so will introduce no new code. */
> ch_possible_zero_cost,
> /* We want to copy. */
> @@ -464,7 +464,7 @@ should_duplicate_loop_header_p (basic_block header, class
> loop *loop,
> TODO: Even if duplication costs some size we may opt to do so in case
> exit probability is significant enough (do partial peeling). */
> if (static_exit)
> - return code_size_cost ? ch_possible_zero_cost : ch_win;
> + return !code_size_cost ? ch_possible_zero_cost : ch_possible;
>
> /* We was not able to prove that conditional will be eliminated. */
> int insns = estimate_num_insns (last, &eni_size_weights);
> @@ -824,6 +824,7 @@ ch_base::copy_headers (function *fun)
> int last_win_nheaders = 0;
> bool last_win_invariant_exit = false;
> ch_decision ret;
> + auto_vec <ch_decision, 32> decision;
> hash_set <edge> *invariant_exits = new hash_set <edge>;
> hash_set <edge> *static_exits = new hash_set <edge>;
> while ((ret = should_duplicate_loop_header_p (header, loop, ranger,
> @@ -833,6 +834,7 @@ ch_base::copy_headers (function *fun)
> != ch_impossible)
> {
> nheaders++;
> + decision.safe_push (ret);
> if (ret >= ch_win)
> {
> last_win_nheaders = nheaders;
> @@ -841,20 +843,6 @@ ch_base::copy_headers (function *fun)
> fprintf (dump_file, " Duplicating bb %i is a win\n",
> header->index);
> }
> - /* Duplicate BB if has zero cost but be sure it will not
> - imply duplication of other BBs. */
> - else if (ret == ch_possible_zero_cost
> - && (last_win_nheaders == nheaders - 1
> - || (last_win_nheaders == nheaders - 2
> - && last_win_invariant_exit)))
> - {
> - last_win_nheaders = nheaders;
> - last_win_invariant_exit = false;
> - if (dump_file && (dump_flags & TDF_DETAILS))
> - fprintf (dump_file,
> - " Duplicating bb %i is a win; it has zero cost\n",
> - header->index);
> - }
> else
> if (dump_file && (dump_flags & TDF_DETAILS))
> fprintf (dump_file, " May duplicate bb %i\n", header->index);
> @@ -884,6 +872,16 @@ ch_base::copy_headers (function *fun)
> fprintf (dump_file,
> " Duplicating header BB to obtain do-while loop\n");
> }
> + /* "Duplicate" all BBs with zero cost following last basic blocks we
> + decided to copy. */
> + while (last_win_nheaders < (int)decision.length ()
> + && decision[last_win_nheaders] == ch_possible_zero_cost)
> + {
> + if (dump_file && (dump_flags & TDF_DETAILS))
> + fprintf (dump_file,
> + " Duplicating extra bb is a win; it has zero cost\n");
> + last_win_nheaders++;
> + }
>
> if (last_win_nheaders)
> candidates.safe_push ({loop, last_win_nheaders,
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
