[patch for gcc12 stage1][version 2] add -ftrivial-auto-var-init and variable attribute "uninitialized" to gcc

[Qing Zhao]

> 
>> 
>>> @@ -11950,6 +12088,72 @@ lower_bound_in_type (tree outer, tree inner)
>>>    }
>>> }
>>> 
>>> +/* Returns true when the given TYPE has padding inside it.
>>> +   return false otherwise.  */
>>> +bool
>>> +type_has_padding (tree type)
>> 
>> Would it be possible to reuse __builtin_clear_padding here?
> 
> Not sure, where can I get more details on __builtin_clear_padding? I can study a little bit more on this to make sure this.

After some study, my understanding is, the call to __builtin_clear_padding is expanded during gimplification phase.  
And there is no __bultin_clear_padding expanding during rtx expanding phase.

Is the above understanding correct? 

If so,  for -ftrivial-auto-var-init, padding initialization should be done both in gimplification phase and rtx expanding phase. 
And since the __builtin_clear_padding might not be good for rtx expanding, reusing __builtin_clear_padding might not work.

Let me know if I misunderstand something.

Qing

>>