On Wed, Feb 10, 2021 at 4:16 PM Tamar Christina via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi Honza and Martin,
>
> As explained in PR98782 the problem is that the behavior of IRA is quite
> tightly
> bound to the frequencies that IPA puts out. With the change introduced in
> g:1118a3ff9d3ad6a64bba25dc01e7703325e23d92 some, but not all edge predictions
> changed. This introduces a local problem which is impossible to fix using
> global flags or using any hooks to IRA.
>
> I propose this new parameter ipa-cp-allow-multi-edge-costing as a temporary
> stop
> gap as something that is safe for stage4.
>
> This allows us to opt in to allow the double edge costing, and buys us some
> time
> to properly look at RA in GCC 12 to see if there's a proper solution for this
> problem.
>
> Using --param ipa-cp-allow-multi-edge-costing=1 allows us to regain all the
> loses
> compared to GCC 10, but also thanks to Martin's patches we get a tiny extra
> gain
> as well.
>
> Would this be an acceptable stop-gap measure for the both of you?
With the --param defaulted to off this is solely a knob to get better benchmark
numbers? Sorry, but GCC isn't a benchmark compiler.
Btw, even such kind of --params deserve documentation. I guess if it
then states "Use this to speedup SPEC CPU 123.xyz" then the switch becomes
disallowed by SPEC rules even for PEAK, no?
Richard.
> Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
>
> Ok for master?
>
> Thanks,
> Tamar
>
> gcc/ChangeLog:
>
> PR rtl-optimization/98782
> * params.opt (ipa-cp-allow-multi-edge-costing): New.
> * predict.c (maybe_predict_edge): Use it.
>
> --- inline copy of patch --
> diff --git a/gcc/params.opt b/gcc/params.opt
> index
> dff8a331f82e44c1b90e5b9f88ffc61e84f03d2d..f54eba2836f5d3a9b66489e3766f3d45eeaf5952
> 100644
> --- a/gcc/params.opt
> +++ b/gcc/params.opt
> @@ -277,6 +277,10 @@ The size of translation unit that IPA-CP pass considers
> large.
> Common Joined UInteger Var(param_ipa_cp_value_list_size) Init(8) Param
> Optimization
> Maximum size of a list of values associated with each parameter for
> interprocedural constant propagation.
>
> +-param=ipa-cp-allow-multi-edge-costing=
> +Common Joined UInteger Var(param_ipa_cp_allow_multi_edge_costing) Init(0)
> IntegerRange(0, 1) Param Optimization
> +Allow double prediction to happen during IPA edge analysis.
> +
> -param=ipa-jump-function-lookups=
> Common Joined UInteger Var(param_ipa_jump_function_lookups) Init(8) Param
> Optimization
> Maximum number of statements visited during jump function offset discovery.
> diff --git a/gcc/predict.c b/gcc/predict.c
> index
> d0a8e5f8e04fc3d1ec770602589299b0a30b3b59..c15dd92310dfe77bedc50c027d000833c0a92315
> 100644
> --- a/gcc/predict.c
> +++ b/gcc/predict.c
> @@ -3171,16 +3171,22 @@ maybe_predict_edge (edge e, enum br_predictor pred,
> enum prediction taken)
> {
> if (edge_predicted_by_p (e, pred, taken))
> return;
> - if (pred == PRED_LOOP_GUARD
> - && edge_predicted_by_p (e, PRED_LOOP_GUARD_WITH_RECURSION, taken))
> - return;
> - /* Consider PRED_LOOP_GUARD_WITH_RECURSION superrior to LOOP_GUARD. */
> - if (pred == PRED_LOOP_GUARD_WITH_RECURSION)
> +
> + /* Allow double prediction in certain circumstances. See PR98782. */
> + if (!param_ipa_cp_allow_multi_edge_costing)
> {
> - edge_prediction **preds = bb_predictions->get (e->src);
> - if (preds)
> - filter_predictions (preds, not_loop_guard_equal_edge_p, e);
> + if (pred == PRED_LOOP_GUARD
> + && edge_predicted_by_p (e, PRED_LOOP_GUARD_WITH_RECURSION, taken))
> + return;
> + /* Consider PRED_LOOP_GUARD_WITH_RECURSION superrior to LOOP_GUARD. */
> + if (pred == PRED_LOOP_GUARD_WITH_RECURSION)
> + {
> + edge_prediction **preds = bb_predictions->get (e->src);
> + if (preds)
> + filter_predictions (preds, not_loop_guard_equal_edge_p, e);
> + }
> }
> +
> predict_edge_def (e, pred, taken);
> }
> /* Predict edges to successors of CUR whose sources are not postdominated by
>
>
> --
