PING^5 [PATCH v2] combine: Tweak the condition of last_set invalidation

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

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.
>>>>>