PING^4 [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.
>>>>