> -----Original Message-----
> From: Kumar, Venkataramanan <[email protected]>
> Sent: Tuesday, June 23, 2026 10:50 PM
> To: Hongtao Liu <[email protected]>; Cui, Lili <[email protected]>
> Cc: [email protected]; Liu, Hongtao <[email protected]>;
> [email protected]; Jan Hubicka <[email protected]>
> Subject: RE: [PATCH] x86: Increase generic tune branch misprediction cost
>
> AMD General
>
> Hi,
>
> > -----Original Message-----
> > From: Kumar, Venkataramanan
> > Sent: Tuesday, May 19, 2026 11:22 PM
> > To: 'Hongtao Liu' <[email protected]>; 'Lili Cui'
> > <[email protected]>
> > Cc: '[email protected]' <[email protected]>;
> > '[email protected]' <[email protected]>; '[email protected]'
> > <[email protected]>; 'Jan Hubicka' <[email protected]>
> > Subject: RE: [PATCH] x86: Increase generic tune branch misprediction
> > cost
> >
> > Hi,
> >
> > > -----Original Message-----
> > > From: Kumar, Venkataramanan
> > > Sent: Tuesday, May 19, 2026 6:26 PM
> > > To: 'Hongtao Liu' <[email protected]>; Lili Cui
> > > <[email protected]>
> > > Cc: [email protected]; [email protected];
> > > [email protected]; Jan Hubicka <[email protected]>
> > > Subject: RE: [PATCH] x86: Increase generic tune branch misprediction
> > > cost
> > >
> > > Hi
> > >
> > > > -----Original Message-----
> > > > From: Hongtao Liu <[email protected]>
> > > > Sent: Tuesday, May 19, 2026 2:04 PM
> > > > To: Lili Cui <[email protected]>
> > > > Cc: [email protected]; [email protected];
> > > > [email protected]; Jan Hubicka <[email protected]>
> > > > Subject: Re: [PATCH] x86: Increase generic tune branch
> > > > misprediction cost
> > > >
> > > > Caution: This message originated from an External Source. Use
> > > > proper caution when opening attachments, clicking links, or responding.
> > > >
> > > >
> > > > On Fri, May 15, 2026 at 3:02 PM Lili Cui <[email protected]> wrote:
> > > > >
> > > > > Hi,
> > > > >
> > > > > This patch increases the branch misprediction scale for generic
> > > > > tuning to
> > > > better reflect the cost on modern CPUs with deeper pipelines.
> > > > >
> > > > > Bootstrapped and regression tested on x86_64-pc-linux-gnu. OK
> > > > > for
> > trunk?
> > > >
> > > > Any comments from AMD folks?
> > >
> > > Yes, planning to run few benchmarks, will get back to you.
>
> We ran SPEC2017, SPEC2026, and a few selected benchmarks from Phoronix
> on Zen5.
> * 544.nab_r shows an improvement of ~10–12%.
> * Other SPEC2017 and SPEC2026 benchmarks are largely neutral.
> * From the Phoronix suite, all benchmarks except Hint are also neutral.
>
> We are okay with the cost change.
> The outlier is Hint (https://openbenchmarking.org/test/pts/hint), which
> regresses by ~30% with -O2 -march=generic -mtune=znver5/graniterapids.
> Based on offline discussions, we will address this separately.
Ok for the patch.
>
> Regards,
> Venkat
> > >
> > > Regards,
> > > Venkat.
> > >
> > > >
> > > > >
> > > > > Thanks,
> > > > > Lili.
> > > > >
> > > > >
> > > > > Increase the branch misprediction scale for generic tuning from
> > > > > COSTS_N_INSNS (2) to COSTS_N_INSNS (2) + 3.
> > > > >
> > > > > Modern CPUs have deeper pipelines, making branch mispredictions
> > > > > more expensive. Increasing this cost encourages if-conversion,
> > > > > avoiding pipeline stalls from mispredicted branches.
> > > > >
> > > > > This improves 544.nab_r (-O2) by 12.7% on GNR and 12.1% on
> > > > > Znver5 with single-copy.
> > > > >
> > > > > gcc/ChangeLog:
> > > > >
> > > > > * config/i386/x86-tune-costs.h (generic_cost): Increase branch
> > > > > mispredict scale from COSTS_N_INSNS (2) to COSTS_N_INSNS (2) +
> 3.
> > > > > ---
> > > > > gcc/config/i386/x86-tune-costs.h | 2 +-
> > > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/gcc/config/i386/x86-tune-costs.h
> > > > > b/gcc/config/i386/x86-tune-costs.h
> > > > > index 7819fdf7c02..0c687a11a74 100644
> > > > > --- a/gcc/config/i386/x86-tune-costs.h
> > > > > +++ b/gcc/config/i386/x86-tune-costs.h
> > > > > @@ -4274,7 +4274,7 @@ struct processor_costs generic_cost = {
> > > > > "16", /* Func alignment. */
> > > > > 4, /* Small unroll limit. */
> > > > > 2, /* Small unroll factor. */
> > > > > - COSTS_N_INSNS (2), /* Branch mispredict scale.
> > > > > */
> > > > > + COSTS_N_INSNS (2) + 3, /* Branch mispredict scale.
> > > > > */
> > > > > };
> >
> > My reading of the generic tune change: br_mispredict_scale goes from
> > COSTS_N_INSNS(2) (8) to COSTS_N_INSNS(2) + 3 (11), and with default
> > branch_cost == 3 for generic, ix86_max_noce_ifcvt_seq_cost() for
> > unpredictable edges rises from 24 to 33, so RTL if-conversion should
> > accept costlier cmov/predicated sequences instead of compare-and-branch.
> >
> > Could you clarify how +3 was chosen? Was it tuned empirically (e.g.
> > SPEC / 544.nab_r on GNR and Znver5), or tied to a model of mis predict
> > penalty on recent cores (e.g. GNR)?
> >
> > Regards,
> > Venkat.
> >
> > > > >
> > > > > /* core_cost should produce code tuned for Core familly of CPUs.
> > > > > */
> > > > > --
> > > > > 2.34.1
> > > > >
> > > >
> > > >
> > > > --
> > > > BR,
> > > > Hongtao
