Re: New option to turn off stack reuse for temporaries

[Xinliang David Li <davidxl@google.com>]

(re-post in plain text)

Moving this to cfgexpand time is simple and it can also be extended to
handle scoped variables. However Jakub raised a good point about this
being too late as stack space overlay is not the only way to cause
trouble when the lifetime of a stack object is extended beyond the
clobber stmt.

thanks,

David

On Tue, Jun 26, 2012 at 1:28 AM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Mon, Jun 25, 2012 at 6:25 PM, Xinliang David Li <davidxl@google.com> wrote:
>> Are there any more concerns about this patch? If not, I'd like to check it in.
>
> No - the fact that the flag is C++ specific but in common.opt is odd enough
> and -ftemp-reuse-stack sounds very very generic - which in fact it is not,
> it's a no-op in C.  Is there a more formal phrase for the temporary kind that
> is affected?  For me "temp" is synonymous to "auto" so I'd have expected
> the switch to turn off stack slot sharing for
>
>  {
>   int a[5];
>  }
>  {
>   int a[5];
>  }
>
> but that is not what it does.  So - a little kludgy but probably more to what
> I'd like it to be would be to move the option to c-family/c.opt enabled only
> for C++ and Obj-C++ and export it to the middle-end via a new langhook
> (the gimplifier code should be in Frontend code that lowers to GENERIC
> really and the WITH_CLEANUP_EXPR code should be C++ frontend specific ...).
>
> Thanks,
> Richard.
>
>> thanks,
>>
>> David
>>
>> On Fri, Jun 22, 2012 at 8:51 AM, Xinliang David Li <davidxl@google.com> wrote:
>>> On Fri, Jun 22, 2012 at 2:39 AM, Richard Guenther
>>> <richard.guenther@gmail.com> wrote:
>>>> On Fri, Jun 22, 2012 at 11:29 AM, Jason Merrill <jason@redhat.com> wrote:
>>>>> On 06/22/2012 01:30 AM, Richard Guenther wrote:
>>>>>>>
>>>>>>> What other issues? It enables more potential code motion, but on the
>>>>>>> other hand, causes more conservative stack reuse. As far I can tell,
>>>>>>> the handling of temporaries is added independently after the clobber
>>>>>>> for scoped variables are introduced. This option can be used to
>>>>>>> restore the older behavior (in handling temps).
>>>>>>
>>>>>>
>>>>>> Well, it does not really restore the old behavior (if you mean before
>>>>>> adding
>>>>>> CLOBBERS, not before the single patch that might have used those for
>>>>>> gimplifying WITH_CLEANUP_EXPR).  You say it disables stack-slot sharing
>>>>>> for those decls but it also does other things via side-effects of no
>>>>>> longer
>>>>>> emitting the CLOBBER.  I say it's better to disable the stack-slot
>>>>>> sharing.
>>>>>
>>>>>
>>>>> The patch exactly restores the behavior of temporaries from before my change
>>>>> to add CLOBBERs for temporaries.  The primary effect of that change was to
>>>>> provide stack-slot sharing, but if there are other effects they are probably
>>>>> desirable as well, since the broken code depended on the old behavior.
>>>>
>>>> So you see it as workaround option, like -fno-strict-aliasing, rather than
>>>> debugging aid?
>>>
>>> It can be used for both purposes -- if the violations are as pervasive
>>> as strict-aliasing cases (which looks like so).
>>>
>>> thanks,
>>>
>>> David
>>>
>>>>
>>>> Richard.
>>>>
>>>>> Jason