Re: Warn when returning the address of a temporary (middle-end) v2

[Jeff Law <law@redhat.com>]

On 06/30/14 15:37, Marc Glisse wrote:
> On Mon, 30 Jun 2014, Jeff Law wrote:
>
>> On 06/29/14 03:22, Marc Glisse wrote:
>>>
>>> After looking at PR 61597, I updated the 2 conditions to:
>>>
>>> +          if ((TREE_CODE (valbase) == VAR_DECL
>>> +               && !is_global_var (valbase))
>>> +              || TREE_CODE (valbase) == PARM_DECL)
>>>
>>> a PARM_DECL is a local variable and returning its address is wrong,
>>> isn't it?
>> Right.  It can live in either a caller or callee allocated slot.
>
> The "caller" case scares me a bit. Is it really wrong to return the
> address in that case? The slot still exists after returning if the
> caller is responsible for it.
The slot exists, but its contents are undefined.  The caller never even 
knows if the callee ever flushed an object back to those slots or if it 
did flush back, which objects were flushed and in what state.

There was a time when I was pondering using those slots for saving 
callee saved registers on the PA so that leafs that needed a few stack 
slots wouldn't need to allococate  a new frame, instead it'd just use 
those convenient 4 words.  But this turned out to just not be important 
and it would totally hose the unwinding mechanisms that were in use on 
the PA at the time.

Jeff