[Bug tree-optimization/126946] [14/15/16/17 Regression] Phiopt lengthening dependency chain in loop for MIN/MAX operations

[ktkachov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126946

--- Comment #6 from ktkachov at gcc dot gnu.org ---
(In reply to rguenther@suse.de from comment #5)
> > Am 19.08.2026 um 16:35 schrieb ktkachov at gcc dot gnu.org <gcc-bugzilla@gcc.gnu.org>:
> > 
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126946
> > 
> > --- Comment #4 from ktkachov at gcc dot gnu.org ---
> > (In reply to rguenther@suse.de from comment #3)
> >>> On Wed, 19 Aug 2026, ktkachov at gcc dot gnu.org wrote:
> >>> 
> >>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126946
> >>> 
> >>> --- Comment #2 from ktkachov at gcc dot gnu.org ---
> >>> (In reply to Richard Biener from comment #1)
> >>>> So why does RTL if-conversion not produce the fcsel?  (and why do we have
> >>>> such strange BB order)
> >>>> 
> >>>> The COND_EXPR allows the x86 cmov expander to pattern-match its FP MIN/MAX
> >>>> operations which match IEEE semantics of m < a ? a : m
> >>>> 
> >>>> On trunk I do see fcsel being used on aarch64 just fine for your testcase,
> >>>> so what "fixed" it there?
> >>>> 
> >>>> .L3:
> >>>>        ldr     s31, [x1, x2, lsl 2]
> >>>>        add     x2, x2, 1
> >>>>        fabs    s31, s31
> >>>>        fcmpe   s31, s0
> >>>>        fcsel   s0, s31, s0, gt
> >>>>        cmp     x0, x2
> >>>>        bne     .L3
> >>>>        ret
> >>>> 
> >>>> We expand from
> >>>> 
> >>>>  _4 = MEM[(const float *)x_9(D) + _20 * 4];
> >>>>  a_10 = ABS_EXPR <_4>;
> >>>>  _12 = a_10 > m_16;
> >>>>  _11 = _12 ? a_10 : m_16;
> >>> 
> >>> In this case we do not want fcsel. GCC 13 kept the well-predicted branch
> >>> whereas GCC 14 starting using fcsel unconditionally because COND_EXPR expansion
> >>> goes through the movcc optabs
> >> 
> >> How do you know the branch is well-predicted?  In general I'd
> >> expect its probability to change during the iteration given
> >> m grows assuming even distributed x[i].
> > 
> > In the full application we measured the misprediction rates with HW counters
> > and did an A/B comparison with just that fcsel/branch decision changed to
> > measure the speedup.
> > In this reduced example I think the argument is that the mispredict happens
> > whenever the max is updated. Therefore each time it's updated it becomes less
> > likely to update again i.e. mispredict since the max is raised so there's fewer
> > values left in the domain that are greater than the new max. So for large
> > enough n the mispredict rate should be dropping, whereas the fact that the
> > running max is a loop recurrence means that a wide core suffers from the
> > increased dependency chain of the fcsel.
> 
> That would then suggest any such MAX (like with fast-math) are problematic
> (but required for vectorization).  So why does the movcc expander not turn
> this back to a branch?  Likewise when expanding from max()?

Yes, with my measurements (on Grace) the scalar fmax is slower than a branch
when it's a loop-carried recurrence (about 2x slower vs the 4x slower version
of the fcsel)

I think there is logic during expansion for choosing between branch and cmov in
expand_expr_real_2 e.g. there's expand_cond_expr_using_cmove, but it doesn't
take loop recurrence into account