On Fri, 7 Aug 2026, Tobias Burnus wrote:
> This is for https://gcc.gnu.org/PR126664 where an invariant condition in
> a loop is estimated to have 0% and 100% edge probabilities. One can argue
> about the 0% vs. 1000% in the loop, but, in any case, after hoisting the
> condition, the probability is surely wrong as 41% and 59% are expected.
>
> The 0% leads to count 0, which prevents vectorization; with vectorization,
> the mentioned code in the PR runs, respectively, 9 or 40 times faster on
> an AMD GPU with OpenMP and OpenACC offloading. (cf. PR)
>
> * * *
>
> For the question about the 100% and 0% see Richard's comment in the PR
> in general and, additionally, his question whether the following is
> a bug - fix by:
> update_profile (epath, EDGE_SUCC (rd->dup_blocks[count], 0),
> - path_out_count, path_out_count);
> + path_in_count, path_out_count);
this didn't actually help ...
> * * *
>
> Back to the attached patch:
>
> The patch is based on the assumption that the probability can be
> different when moved outside of the loop - and just re-guesses it
> makes sense, which yields the expected 41% and 59% edge probabilities.
>
> Is the patch OK – or at least like the right approach?
unconditionally re-guessing seems overly conservative when the
unswitched condition is always executed in the loop.
- scale_loop_frequencies (loop, then_scale);
- scale_loop_frequencies (nloop, else_scale);
+ if (then_scale.initialized_p () && else_scale.initialized_p ())
+ {
+ scale_loop_frequencies (loop, then_scale);
+ scale_loop_frequencies (nloop, else_scale);
+ }
+ else
+ {
+ edge te = EDGE_SUCC (cond_bb, 0);
+ edge ee = EDGE_SUCC (cond_bb, 1);
+ scale_loop_frequencies (loop, te->probability);
+ scale_loop_frequencies (nloop, ee->probability);
+ }
doesn't the 2nd hunk always work? (are you sure of the
first/second edge going to the respective loops?)
> [At least for now, i.e. if it turned out that with other fixes,
> it is no longer needed, then it could be still reverted.]
>
>
> And: Any suggestion how to create a testcase for it?
>
> I could package the testcase from the PR and check in the 'unswitch'
> tree dump for the following:
>
> if (l_params$hphb_187 < 0.0)
> goto <bb 17>; [0.00%]
> else
> goto <bb 36>; [100.00%]
>
> But not with 0% and 100% but with a probability between 10% and
> 90% for either edge. - Any better idea?
>
> Tobias
>
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Jochen Jaser, Andrew McDonald, Abhinav Puri; (HRB 36809, AG Nuernberg)
