[PATCH] Remove also 2nd argument for unused delete operator (PR tree-optimization/91270).

[Martin Liška]

On 7/30/19 3:09 PM, Marc Glisse wrote:
> On Tue, 30 Jul 2019, Martin Liška wrote:
> 
>> On 7/30/19 1:35 PM, Marc Glisse wrote:
>>> +          /* Some delete operators have size as 2nd argument.  */
>>>
>>> Some delete operators have 3 arguments. From cp/decl.c:
>>>
>>> Â Â Â Â Â Â Â Â Â Â Â  /* operator delete (void *, size_t, align_val_t); */
>>>
>>
>> Yep, I know. The patch I installed expects at least 2 arguments:
>>
>> +                 /* Some delete operators have size as 2nd argument.  */
>> +Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  if (is_delete_operator && gimple_call_num_args (stmt) >= 2)
> 
> True, I guess I am a bit confused why the second argument (which could be either size or alignment) needs special handling (mark_operand_necessary) while the third one does not (it is usually a constant).

Ah, that's bad, both of them need a care:

diff --git a/gcc/tree-ssa-dce.c b/gcc/tree-ssa-dce.c
index bec13cd5930..80d5f5c30f7 100644
--- a/gcc/tree-ssa-dce.c
+++ b/gcc/tree-ssa-dce.c
@@ -824,13 +824,16 @@ propagate_necessity (bool aggressive)
 			   || DECL_FUNCTION_CODE (def_callee) == BUILT_IN_CALLOC))
 		      || DECL_IS_REPLACEABLE_OPERATOR_NEW_P (def_callee)))
 		{
-		  /* Some delete operators have size as 2nd argument.  */
+		  /* Delete operators can have alignment and (or) size as next
+		     arguments.  When being a SSA_NAME, they must be marked
+		     as necessary.  */
 		  if (is_delete_operator && gimple_call_num_args (stmt) >= 2)
-		    {
-		      tree size_argument = gimple_call_arg (stmt, 1);
-		      if (TREE_CODE (size_argument) == SSA_NAME)
-			mark_operand_necessary (size_argument);
-		    }
+		    for (unsigned i = 1; i < gimple_call_num_args (stmt); i++)
+		      {
+			tree arg = gimple_call_arg (stmt, i);
+			if (TREE_CODE (arg) == SSA_NAME)
+			  mark_operand_necessary (arg);
+		      }
 
 		  continue;
 		}

> 
> I tried to experiment to understand, but it is complicated because including <new> disables the optimization:
> 
> #include <new>
> void fn1() {
> Â Â Â  char*p=new char;
> Â Â Â  delete p;
> }
> 
> This ICEs with -O -std=c++17:
> 
> int a = 64;
> std::align_val_t b{64};
> void fn1() {
> Â  void *s = operator new(a,b);
> Â  operator delete(s,8+*(unsigned long*)s,b);
> }
> 
> 

I can't see it on current master. Can you?

Martin