Re: [RFC/PATCH v3] ira: Support more matching constraint forms with param [PR100328]

["Kewen.Lin" <linkw@linux.ibm.com>]

Hi Richard,

on 2021/6/30 下午11:42, Richard Sandiford wrote:
> "Kewen.Lin" <linkw@linux.ibm.com> writes:
>> on 2021/6/28 下午3:20, Hongtao Liu wrote:
>>> On Mon, Jun 28, 2021 at 3:12 PM Hongtao Liu <crazylht@gmail.com> wrote:
>>>>
>>>> On Mon, Jun 28, 2021 at 2:50 PM Kewen.Lin <linkw@linux.ibm.com> wrote:
>>>>>
>>>>> Hi!
>>>>>
>>>>> on 2021/6/9 下午1:18, Kewen.Lin via Gcc-patches wrote:
>>>>>> Hi,
>>>>>>
>>>>>> PR100328 has some details about this issue, I am trying to
>>>>>> brief it here.  In the hottest function LBM_performStreamCollideTRT
>>>>>> of SPEC2017 bmk 519.lbm_r, there are many FMA style expressions
>>>>>> (27 FMA, 19 FMS, 11 FNMA).  On rs6000, this kind of FMA style
>>>>>> insn has two flavors: FLOAT_REG and VSX_REG, the VSX_REG reg
>>>>>> class have 64 registers whose foregoing 32 ones make up the
>>>>>> whole FLOAT_REG.  There are some differences for these two
>>>>>> flavors, taking "*fma<mode>4_fpr" as example:
>>>>>>
>>>>>> (define_insn "*fma<mode>4_fpr"
>>>>>>   [(set (match_operand:SFDF 0 "gpc_reg_operand" "=<Ff>,wa,wa")
>>>>>>       (fma:SFDF
>>>>>>         (match_operand:SFDF 1 "gpc_reg_operand" "%<Ff>,wa,wa")
>>>>>>         (match_operand:SFDF 2 "gpc_reg_operand" "<Ff>,wa,0")
>>>>>>         (match_operand:SFDF 3 "gpc_reg_operand" "<Ff>,0,wa")))]
>>>>>>
>>>>>> // wa => A VSX register (VSR), vs0…vs63, aka. VSX_REG.
>>>>>> // <Ff> (f/d) => A floating point register, aka. FLOAT_REG.
>>>>>>
>>>>>> So for VSX_REG, we only have the destructive form, when VSX_REG
>>>>>> alternative being used, the operand 2 or operand 3 is required
>>>>>> to be the same as operand 0.  reload has to take care of this
>>>>>> constraint and create some non-free register copies if required.
>>>>>>
>>>>>> Assuming one fma insn looks like:
>>>>>>   op0 = FMA (op1, op2, op3)
>>>>>>
>>>>>> The best regclass of them are VSX_REG, when op1,op2,op3 are all dead,
>>>>>> IRA simply creates three shuffle copies for them (here the operand
>>>>>> order matters, since with the same freq, the one with smaller number
>>>>>> takes preference), but IMO both op2 and op3 should take higher priority
>>>>>> in copy queue due to the matching constraint.
>>>>>>
>>>>>> I noticed that there is one function ira_get_dup_out_num, which meant
>>>>>> to create this kind of constraint copy, but the below code looks to
>>>>>> refuse to create if there is an alternative which has valid regclass
>>>>>> without spilled need.
>>>>>>
>>>>>>       default:
>>>>>>       {
>>>>>>         enum constraint_num cn = lookup_constraint (str);
>>>>>>         enum reg_class cl = reg_class_for_constraint (cn);
>>>>>>         if (cl != NO_REGS
>>>>>>             && !targetm.class_likely_spilled_p (cl))
>>>>>>           goto fail
>>>>>>
>>>>>>        ...
>>>>>>
>>>>>> I cooked one patch attached to make ira respect this kind of matching
>>>>>> constraint guarded with one parameter.  As I stated in the PR, I was
>>>>>> not sure this is on the right track.  The RFC patch is to check the
>>>>>> matching constraint in all alternatives, if there is one alternative
>>>>>> with matching constraint and matches the current preferred regclass
>>>>>> (or best of allocno?), it will record the output operand number and
>>>>>> further create one constraint copy for it.  Normally it can get the
>>>>>> priority against shuffle copies and the matching constraint will get
>>>>>> satisfied with higher possibility, reload doesn't create extra copies
>>>>>> to meet the matching constraint or the desirable register class when
>>>>>> it has to.
>>>>>>
>>>>>> For FMA A,B,C,D, I think ideally copies A/B, A/C, A/D can firstly stay
>>>>>> as shuffle copies, and later any of A,B,C,D gets assigned by one
>>>>>> hardware register which is a VSX register (VSX_REG) but not a FP
>>>>>> register (FLOAT_REG), which means it has to pay costs once we can NOT
>>>>>> go with VSX alternatives, so at that time it's important to respect
>>>>>> the matching constraint then we can increase the freq for the remaining
>>>>>> copies related to this (A/B, A/C, A/D).  This idea requires some side
>>>>>> tables to record some information and seems a bit complicated in the
>>>>>> current framework, so the proposed patch aggressively emphasizes the
>>>>>> matching constraint at the time of creating copies.
>>>>>>
>>>>>
>>>>> Comparing with the original patch (v1), this patch v3 has
>>>>> considered: (this should be v2 for this mail list, but bump
>>>>> it to be consistent as PR's).
>>>>>
>>>>>   - Excluding the case where for one preferred register class
>>>>>     there can be two or more alternatives, one of them has the
>>>>>     matching constraint, while another doesn't have.  So for
>>>>>     the given operand, even if it's assigned by a hardware reg
>>>>>     which doesn't meet the matching constraint, it can simply
>>>>>     use the alternative which doesn't have matching constraint
>>>>>     so no register move is needed.  One typical case is
>>>>>     define_insn *mov<mode>_internal2 on rs6000.  So we
>>>>>     shouldn't create constraint copy for it.
>>>>>
>>>>>   - The possible free register move in the same register class,
>>>>>     disable this if so since the register move to meet the
>>>>>     constraint is considered as free.
>>>>>
>>>>>   - Making it on by default, suggested by Segher & Vladimir, we
>>>>>     hope to get rid of the parameter if the benchmarking result
>>>>>     looks good on major targets.
>>>>>
>>>>>   - Tweaking cost when either of matching constraint two sides
>>>>>     is hardware register.  Before this patch, the constraint
>>>>>     copy is simply taken as a real move insn for pref and
>>>>>     conflict cost with one hardware register, after this patch,
>>>>>     it's allowed that there are several input operands
>>>>>     respecting the same matching constraint (but in different
>>>>>     alternatives), so we should take it to be like shuffle copy
>>>>>     for some cases to avoid over preferring/disparaging.
>>>>>
>>>>> Please check the PR comments for more details.
>>>>>
>>>>> This patch can be bootstrapped & regtested on
>>>>> powerpc64le-linux-gnu P9 and x86_64-redhat-linux, but have some
>>>>> "XFAIL->XPASS" failures on aarch64-linux-gnu.  The failure list
>>>>> was attached in the PR and thought the new assembly looks
>>>>> improved (expected).
>>>>>
>>>>> With option Ofast unroll, this patch can help to improve SPEC2017
>>>>> bmk 508.namd_r +2.42% and 519.lbm_r +2.43% on Power8 while
>>>>> 508.namd_r +3.02% and 519.lbm_r +3.85% on Power9 without any
>>>>> remarkable degradations.
>>>>>
>>>>> Since this patch likely benefits x86_64 and aarch64, but I don't
>>>>> have performance machines with these arches at hand, could
>>>>> someone kindly help to benchmark it if possible?
>>>> I can help test it on Intel cascade lake and AMD milan.
>>
>>
>> Thanks for your help, Hongtao!
>>
>>
>>> And could you rebase your patch on the lastest trunk, i got several
>>> failures when applying the patch
>>> ~ git apply ira-v3.diff
>>> error: patch failed: gcc/doc/invoke.texi:13845
>>> error: gcc/doc/invoke.texi: patch does not apply
>>> error: patch failed: gcc/ira-conflicts.c:233
>>> error: gcc/ira-conflicts.c: patch does not apply
>>> error: patch failed: gcc/ira-int.h:971
>>> error: gcc/ira-int.h: patch does not apply
>>> error: patch failed: gcc/ira.c:1922
>>> error: gcc/ira.c: patch does not apply
>>> error: patch failed: gcc/params.opt:330
>>> error: gcc/params.opt: patch does not apply
>>>
>>
>> I think it's due to unexpected git stat lines in previously attached diff.
>>
>> I have attached the format-patch file.  Please have a check.  Thanks!
> 
> FWIW, this seems to be neutral for SPEC 2017 on AArch64.  The SVE
> XFAIL->XPASS transitions mean it's definitely a good thing for
> AArch64 in that respect though.

Thanks for the information!  It gives us the confidence to turn it on by
default now, I've removed those xfails in the latest version
https://gcc.gnu.org/pipermail/gcc-patches/2021-July/574315.html.

BR,
Kewen