Re: patch implementing a new pass for register-pressure relief through live range shrinkage

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

On Tue, Nov 5, 2013 at 4:35 PM, Vladimir Makarov <vmakarov@redhat.com> wrote:
>   I'd like to add a new experimental optimization to the trunk.  This
> optimization was discussed on RA BOF of this summer GNU Cauldron.
>
>   It is a register pressure relief through live-range shrinkage.  It
> is implemented on the scheduler base and uses register-pressure insn
> scheduling infrastructure.  By rearranging insns we shorten pseudo
> live-ranges and increase a chance to them be assigned to a hard
> register.
>
>   The code looks pretty simple but there are a lot of works behind
> this patch.  I've tried about ten different versions of this code
> (different heuristics for two currently existing register-pressure
> algorithms).
>
>   I think it is *upto target maintainers* to decide to use or not to
> use this optimization for their targets.  I'd recommend to use this at
> least for x86/x86-64.  I think any OOO processor with small or
> moderate register file which does not use the 1st insn scheduling
> might benefit from this too.
>
>   On SPEC2000 for x86/x86-64 (I use Haswell processor, -O3 with
> general tuning), the optimization usage results in smaller code size
> in average (for floating point and integer benchmarks in 32- and
> 64-bit mode).  The improvement better visible for SPECFP2000 (although
> I have the same improvement on x86-64 SPECInt2000 but it might be
> attributed mostly mcf benchmark unstability).  It is about 0.5% for
> 32-bit and 64-bit mode.  It is understandable, as the optimization has
> more opportunities to improve the code on longer BBs.  Different from
> other heuristic optimizations, I don't see any significant worse
> performance.  It gives practically the same or better performance (a
> few benchmarks imporoved by 1% or more upto 3%).
>
>   The single but significant drawback is additional compilation time
> (4%-6%) as the 1st insn scheduling pass is quite expensive.  So I'd
> recommend target maintainers to switch it on only for -Ofast.

Generally I'd not recomment viewing -Ofast as -O4 but as -O3
plus generally "unsafe" optimizations.  So I'd not enable it for -Ofast
but for -O3 - possibly also with -Os if indeed the main motivation is
also code-size improvements (-Os is a similar beast as -O3, spend
as much time as you can on optimizing size).

Btw, thanks for working on this.  How does it relate to
-fsched-pressure?  Does it treat all register classes the same?
On x86 mostly the few fixed registers for some of the integer pipeline
instructions hurt, x86_64 has enough general and FP registers?

Richard.

>  If
> somebody finds that the optimization works on processors which uses
> 1st insn scheduling by default (in which I slightly doubt), we could
> improve the compilation time by reusing data for this optimization and
> the 1st insn scheduling.
>
>   Any comments, questions, thoughts are appreciated.
>
> 2013-11-05  Vladimir Makarov  <vmakarov@redhat.com>
>
>         * tree-pass.h (make_pass_live_range_shrinkage): New external.
>         * timevar.def (TV_LIVE_RANGE_SHRINKAGE): New.
>         * sched-rgn.c (gate_handle_live_range_shrinkage): New.
>         (rest_of_handle_live_range_shrinkage): Ditto
>         (class pass_live_range_shrinkage): Ditto.
>         (pass_data_live_range_shrinkage): Ditto.
>         (make_pass_live_range_shrinkage): Ditto.
>         * sched-int.h (sched_relief_p): New external.
>         * sched-deps.c (create_insn_reg_set): Make void return value.
>         * passes.def: Add pass_live_range_shrinkage.
>         * ira.c (update_equiv_regs): Don't move if
>         flag_live_range_shrinkage.
>         * haifa-sched.c (sched_relief_p): New.
>         (rank_for_schedule): Add code for pressure relief through live
>         range shrinkage.
>         (schedule_insn): Print more debug info.
>         (sched_init): Setup SCHED_PRESSURE_WEIGHTED for pressure relief
>         through live range shrinkage.
>         * doc/invoke.texi (-flive-range-shrinkage): New.
>         * common.opt (flive-range-shrinkage): New.
>