[PATCH] Remove quite obvious dead assignments.

[Richard Sandiford]

Martin Liška <mliska@suse.cz> writes:
> On 6/26/19 1:39 PM, Jakub Jelinek wrote:
>> On Wed, Jun 26, 2019 at 12:57:15PM +0200, Martin Liška wrote:
>>> --- a/gcc/tree-vect-stmts.c
>>> +++ b/gcc/tree-vect-stmts.c
>>> @@ -3483,8 +3483,7 @@ vectorizable_call (stmt_vec_info stmt_info, gimple_stmt_iterator *gsi,
>>>  			= gimple_build_call_internal_vec (ifn, vargs);
>>>  		      gimple_call_set_lhs (call, half_res);
>>>  		      gimple_call_set_nothrow (call, true);
>>> -		      new_stmt_info
>>> -			= vect_finish_stmt_generation (stmt_info, call, gsi);
>>> +		      vect_finish_stmt_generation (stmt_info, call, gsi);
>>>  		      if ((i & 1) == 0)
>>>  			{
>>>  			  prev_res = half_res;
>>> @@ -3583,8 +3582,7 @@ vectorizable_call (stmt_vec_info stmt_info, gimple_stmt_iterator *gsi,
>>>  	      gcall *call = gimple_build_call_internal_vec (ifn, vargs);
>>>  	      gimple_call_set_lhs (call, half_res);
>>>  	      gimple_call_set_nothrow (call, true);
>>> -	      new_stmt_info
>>> -		= vect_finish_stmt_generation (stmt_info, call, gsi);
>>> +	      vect_finish_stmt_generation (stmt_info, call, gsi);
>>>  	      if ((j & 1) == 0)
>>>  		{
>>>  		  prev_res = half_res;
>> 
>> This looks wrong.
>> There should have been
>> SLP_TREE_VEC_STMTS (slp_node).quick_push (new_stmt_info);
>> in between for slp_node, or the usual code like:
>>           if (cond_for_first)
>>             STMT_VINFO_VEC_STMT (stmt_info) = *vec_stmt = new_stmt_info;
>>           else
>>             STMT_VINFO_RELATED_STMT (prev_stmt_info) = new_stmt_info;
>> 	  prev_stmt_info = new_stmt_info;
>> otherwise.  In any case, I think this should be dealt with separately.
>
> Likewise here:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91017

I think this part of the patch is OK.  SLP_TREE_VEC_STMTS and
STMT_VINFO_VEC_STMT should only receive the final stmt for each vector
result, whereas these stmts are producing intermediate results.

The assignments were introduced as part of a mechanical patch to avoid
using vinfo_for_stmt to get stmt_infos that we'd just created.  I'd
missed that in these cases we didn't actually use vinfo_for_stmt.

Off-hand, I'm not sure whether we actually need to create the
stmt_info here as things stand, or whether we're really just using
vect_finish_stmt_generation to insert the stmt in the right way.
I did have some WIP patches that would need the stmt_info to be
created though.

Thanks,
Richard