Re: [PATCH, i386]: Do not limit the cost of moves to/from XMM register to minimum 8.

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Sep 3, 2019 at 1:24 PM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Tue, Sep 3, 2019 at 9:57 AM Hongtao Liu <crazylht@gmail.com> wrote:
> >
> > On Mon, Sep 2, 2019 at 4:41 PM Uros Bizjak <ubizjak@gmail.com> wrote:
> > >
> > > On Mon, Sep 2, 2019 at 10:13 AM Hongtao Liu <crazylht@gmail.com> wrote:
> > > >
> > > > > which is not the case with core_cost (and similar with skylake_cost):
> > > > >
> > > > >   2, 2, 4,                /* cost of moving XMM,YMM,ZMM register */
> > > > >   {6, 6, 6, 6, 12},            /* cost of loading SSE registers
> > > > >                        in 32,64,128,256 and 512-bit */
> > > > >   {6, 6, 6, 6, 12},            /* cost of storing SSE registers
> > > > >                        in 32,64,128,256 and 512-bit */
> > > > >   2, 2,                    /* SSE->integer and integer->SSE moves */
> > > > >
> > > > > We have the same cost of moving between integer registers (by default
> > > > > set to 2), between SSE registers and between integer and SSE register
> > > > > sets. I think that at least the cost of moves between regsets should
> > > > > be substantially higher, rs6000 uses 3x cost of intra-regset moves;
> > > > > that would translate to the value of 6. The value should be low enough
> > > > > to keep the cost below the value that forces move through the memory.
> > > > > Changing core register allocation cost of SSE <-> integer to:
> > > > >
> > > > > --cut here--
> > > > > Index: config/i386/x86-tune-costs.h
> > > > > ===================================================================
> > > > > --- config/i386/x86-tune-costs.h        (revision 275281)
> > > > > +++ config/i386/x86-tune-costs.h        (working copy)
> > > > > @@ -2555,7 +2555,7 @@ struct processor_costs core_cost = {
> > > > >                                            in 32,64,128,256 and 512-bit */
> > > > >    {6, 6, 6, 6, 12},                    /* cost of storing SSE registers
> > > > >                                            in 32,64,128,256 and 512-bit */
> > > > > -  2, 2,                                        /* SSE->integer and
> > > > > integer->SSE moves */
> > > > > +  6, 6,                                        /* SSE->integer and
> > > > > integer->SSE moves */
> > > > >    /* End of register allocator costs.  */
> > > > >    },
> > > > >
> > > > > --cut here--
> > > > >
> > > > > still produces direct move in gcc.target/i386/minmax-6.c
> > > > >
> > > > > I think that in addition to attached patch, values between 2 and 6
> > > > > should be considered in benchmarking. Unfortunately, without access to
> > > > > regressed SPEC tests, I can't analyse these changes by myself.
> > > > >
> > > > > Uros.
> > > >
> > > > Apply similar change to skylake_cost, on skylake workstation we got
> > > > performance like:
> > > > ---------------------------
> > > > version                                                            |
> > > > 548_exchange_r score
> > > > gcc10_20180822:                                           |   10
> > > > apply remove_max8                                       |   8.9
> > > > also apply increase integer_tofrom_sse cost |   9.69
> > > > -----------------------------
> > > > Still 3% regression which is related to _gfortran_mminloc0_4_i4 in
> > > > libgfortran.so.5.0.0.
> > > >
> > > > I found suspicious code as bellow, does it affect?
> > >
> > > Hard to say without access to the test, but I'm glad that changing the
> > > knob has noticeable effect. I think that (as said by Alan) a fine-tune
> > > of register pressure calculation will be needed to push this forward.
> > >
> > > Uros.
> > >
> > > > ------------------
> > > > modified   gcc/config/i386/i386-features.c
> > > > @@ -590,7 +590,7 @@ general_scalar_chain::compute_convert_gain ()
> > > >    if (dump_file)
> > > >      fprintf (dump_file, "  Instruction conversion gain: %d\n", gain);
> > > >
> > > > -  /* ???  What about integer to SSE?  */
> > > > +  /* ???  What about integer to SSE?  */???
> > > >    EXECUTE_IF_SET_IN_BITMAP (defs_conv, 0, insn_uid, bi)
> > > >      cost += DF_REG_DEF_COUNT (insn_uid) * ix86_cost->sse_to_integer;
> > > > ------------------
> > > > --
> > > > BR,
> > > > Hongtao
> >
> > Note:
> > Removing limit of cost would introduce lots of regressions in SPEC2017 as follow
> > --------------------------------
> > 531.deepsjeng_r  -7.18%
> > 548.exchange_r  -6.70%
> > 557.xz_r -6.74%
> > 508.namd_r -2.81%
> > 527.cam4_r -6.48%
> > 544.nab_r -3.99%
> >
> > Tested on skylake server.
> > -------------------------------------
> > How about  changing cost from 2 to 8 until we figure out a better number.
>
> Certainly works for me.  Note the STV code uses the "other" sse_to_integer
> number and the numbers in question here are those for the RA.  There's
> a multitude of values used in the tables here, including some a lot larger.
> So the overall bumping to 8 certainly was the wrong thing to do and instead
> individual numbers should have been adjusted (didn't look at the history
> of that bumping).

For reference:

r125951 | uros | 2007-06-22 19:51:06 +0200 (Fri, 22 Jun 2007) | 6 lines

    PR target/32413
    * config/i386/i386.c (ix86_register_move_cost): Rise the cost of
    moves between MMX/SSE registers to at least 8 units to prevent
    ICE caused by non-tieable SI/HI/QImodes in SSE registers.

should probably have been "twice the cost of X" or something like that
instead where X be some reg-reg move cost.

>  For example Pentium4 has quite high bases for move
> costs, like xmm <-> xmm move costing 12 and SSE->integer costing 20
> while the opposite 12.
>
> So yes, we want to revert the patch by applying its effect to the
> individual cost tables so we can revisit this for the still interesting
> micro-architectures.
>
> Richard.
>
> > --
> > BR,
> > Hongtao