PING^6 [PATCH v2] combine: Tweak the condition of last_set invalidation

[Kewen.Lin]

Hi,

Gentle ping this:

https://gcc.gnu.org/pipermail/gcc-patches/2021-June/572555.html

BR,
Kewen

>>>>> on 2021/6/11 下午9:16, Kewen.Lin via Gcc-patches wrote:
>>>>>> Hi Segher,
>>>>>>
>>>>>> Thanks for the review!
>>>>>>
>>>>>> on 2021/6/10 上午4:17, Segher Boessenkool wrote:
>>>>>>> Hi!
>>>>>>>
>>>>>>> On Wed, Dec 16, 2020 at 04:49:49PM +0800, Kewen.Lin wrote:
>>>>>>>> Currently we have the check:
>>>>>>>>
>>>>>>>>       if (!insn
>>>>>>>> 	  || (value && rsp->last_set_table_tick >= label_tick_ebb_start))
>>>>>>>> 	rsp->last_set_invalid = 1; 
>>>>>>>>
>>>>>>>> which means if we want to record some value for some reg and
>>>>>>>> this reg got refered before in a valid scope,
>>>>>>>
>>>>>>> If we already know it is *set* in this same extended basic block.
>>>>>>> Possibly by the same instruction btw.
>>>>>>>
>>>>>>>> we invalidate the
>>>>>>>> set of reg (last_set_invalid to 1).  It avoids to find the wrong
>>>>>>>> set for one reg reference, such as the case like:
>>>>>>>>
>>>>>>>>    ... op regX  // this regX could find wrong last_set below
>>>>>>>>    regX = ...   // if we think this set is valid
>>>>>>>>    ... op regX
>>>>>>>
>>>>>>> Yup, exactly.
>>>>>>>
>>>>>>>> But because of retry's existence, the last_set_table_tick could
>>>>>>>> be set by some later reference insns, but we see it's set due
>>>>>>>> to retry on the set (for that reg) insn again, such as:
>>>>>>>>
>>>>>>>>    insn 1
>>>>>>>>    insn 2
>>>>>>>>
>>>>>>>>    regX = ...     --> (a)
>>>>>>>>    ... op regX    --> (b)
>>>>>>>>    
>>>>>>>>    insn 3
>>>>>>>>
>>>>>>>>    // assume all in the same BB.
>>>>>>>>
>>>>>>>> Assuming we combine 1, 2 -> 3 sucessfully and replace them as two
>>>>>>>> (3 insns -> 2 insns),
>>>>>>>
>>>>>>> This will delete insn 1 and write the combined result to insns 2 and 3.
>>>>>>>
>>>>>>>> retrying from insn1 or insn2 again:
>>>>>>>
>>>>>>> Always 2, but your point remains valid.
>>>>>>>
>>>>>>>> it will scan insn (a) again, the below condition holds for regX:
>>>>>>>>
>>>>>>>>   (value && rsp->last_set_table_tick >= label_tick_ebb_start)
>>>>>>>>
>>>>>>>> it will mark this set as invalid set.  But actually the
>>>>>>>> last_set_table_tick here is set by insn (b) before retrying, so it
>>>>>>>> should be safe to be taken as valid set.
>>>>>>>
>>>>>>> Yup.
>>>>>>>
>>>>>>>> This proposal is to check whether the last_set_table safely happens
>>>>>>>> after the current set, make the set still valid if so.
>>>>>>>
>>>>>>>> Full SPEC2017 building shows this patch gets more sucessful combines
>>>>>>>> from 1902208 to 1902243 (trivial though).
>>>>>>>
>>>>>>> Do you have some example, or maybe even a testcase?  :-)
>>>>>>>
>>>>>>
>>>>>> Sorry for the late reply, it took some time to get one reduced case.
>>>>>>
>>>>>> typedef struct SA *pa_t;
>>>>>>
>>>>>> struct SC {
>>>>>>   int h;
>>>>>>   pa_t elem[];
>>>>>> };
>>>>>>
>>>>>> struct SD {
>>>>>>   struct SC *e;
>>>>>> };
>>>>>>
>>>>>> struct SA {
>>>>>>   struct {
>>>>>>     struct SD f[1];
>>>>>>   } g;
>>>>>> };
>>>>>>
>>>>>> void foo(pa_t *k, char **m) {
>>>>>>   int l, i;
>>>>>>   pa_t a;
>>>>>>   l = (int)a->g.f[5].e;
>>>>>>   i = 0;
>>>>>>   for (; i < l; i++) {
>>>>>>     k[i] = a->g.f[5].e->elem[i];
>>>>>>     m[i] = "";
>>>>>>   }
>>>>>> }
>>>>>>
>>>>>> Baseline is r12-0 and the option is "-O3 -mcpu=power9 -fno-strict-aliasing",
>>>>>> with this patch, the generated assembly can save two rlwinm s.
>>>>>>
>>>>>>>> +  /* Record the luid of the insn whose expression involving register n.  */
>>>>>>>> +
>>>>>>>> +  int				last_set_table_luid;
>>>>>>>
>>>>>>> "Record the luid of the insn for which last_set_table_tick was set",
>>>>>>> right?
>>>>>>>
>>>>>>
>>>>>> But it can be updated later to one smaller luid, how about the wording like:
>>>>>>
>>>>>>
>>>>>> +  /* Record the luid of the insn which uses register n, the insn should
>>>>>> +     be the first one using register n in that block of the insn which
>>>>>> +     last_set_table_tick was set for.  */
>>>>>>
>>>>>>
>>>>>>>> -static void update_table_tick (rtx);
>>>>>>>> +static void update_table_tick (rtx, int);
>>>>>>>
>>>>>>> Please remove this declaration instead, the function is not used until
>>>>>>> after its actual definition :-)
>>>>>>>
>>>>>>
>>>>>> Done.
>>>>>>
>>>>>>>> @@ -13243,7 +13247,21 @@ update_table_tick (rtx x)
>>>>>>>>        for (r = regno; r < endregno; r++)
>>>>>>>>  	{
>>>>>>>>  	  reg_stat_type *rsp = &reg_stat[r];
>>>>>>>> -	  rsp->last_set_table_tick = label_tick;
>>>>>>>> +	  if (rsp->last_set_table_tick >= label_tick_ebb_start)
>>>>>>>> +	    {
>>>>>>>> +	      /* Later references should not have lower ticks.  */
>>>>>>>> +	      gcc_assert (label_tick >= rsp->last_set_table_tick);
>>>>>>>
>>>>>>> This should be obvious, but checking it won't hurt, okay.
>>>>>>>
>>>>>>>> +	      /* Should pick up the lowest luid if the references
>>>>>>>> +		 are in the same block.  */
>>>>>>>> +	      if (label_tick == rsp->last_set_table_tick
>>>>>>>> +		  && rsp->last_set_table_luid > insn_luid)
>>>>>>>> +		rsp->last_set_table_luid = insn_luid;
>>>>>>>
>>>>>>> Why?  Is it conservative for the check you will do later?  Please spell
>>>>>>> this out, it is crucial!
>>>>>>>
>>>>>>
>>>>>> Since later the combinations involving this insn probably make the
>>>>>> register be used in one insn sitting ahead (which has smaller luid than
>>>>>> the one which was recorded before).  Yes, it's very conservative, this
>>>>>> ensure that we always use the luid of the insn which is the first insn
>>>>>> using this register in the block.  The last_set invalidation is going
>>>>>> to catch the case like:
>>>>>>
>>>>>>    ... regX  // avoid the set used here ...
>>>>>>    regX = ...
>>>>>>    ...
>>>>>>
>>>>>> Once we have the smallest luid one of all insns which use register X,
>>>>>> any unsafe regX sets should be caught.
>>>>>>
>>>>>> I updated the comments to:
>>>>>>
>>>>>> +              /* Since combination may generate some instructions
>>>>>> +                 to replace some foregoing instructions with the
>>>>>> +                 references to register r (using register r), we
>>>>>> +                 need to make sure we record the first instruction
>>>>>> +                 which is using register r, so always update with
>>>>>> +                 the lowest luid here.  If the given set happens
>>>>>> +                 before this recorded earliest reference, the set
>>>>>> +                 value should be safe to be used.  */
>>>>>>
>>>>>>>> @@ -13359,7 +13378,10 @@ record_value_for_reg (rtx reg, rtx_insn *insn, rtx value)
>>>>>>>>  
>>>>>>>>    /* Mark registers that are being referenced in this value.  */
>>>>>>>>    if (value)
>>>>>>>> -    update_table_tick (value);
>>>>>>>> +    {
>>>>>>>> +      gcc_assert (insn);
>>>>>>>> +      update_table_tick (value, DF_INSN_LUID (insn));
>>>>>>>> +    }
>>>>>>>
>>>>>>> Don't add that assert please.  If you really want one it should come
>>>>>>> right at the start of the function, not 60 lines later :-)
>>>>>>>
>>>>>>
>>>>>> Exactly, fixed.
>>>>>>
>>>>>>> Looks good if I understood this correctly :-)
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> Thanks again, I also updated the comments in func record_value_for_reg,
>>>>>> the new version is attached.
>>>>>>
>>>>>> BR,
>>>>>> Kewen
>>>>>> -----
>>>>>> gcc/ChangeLog:
>>>>>>
>>>>>> 	* combine.c (struct reg_stat_type): New member
>>>>>> 	last_set_table_luid.
>>>>>> 	(update_table_tick): Add one argument for insn luid and
>>>>>> 	set last_set_table_luid with it, remove its declaration.
>>>>>> 	(record_value_for_reg): Adjust the condition to set
>>>>>> 	last_set_invalid nonzero.
>>>>>>