Re: New option to turn off stack reuse for temporaries

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

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On Wed, Jul 4, 2012 at 8:01 AM, Xinliang David Li <davidxl@google.com> wrote:
> Comment?
>
> David
>
> On Mon, Jul 2, 2012 at 4:30 PM, Xinliang David Li <davidxl@google.com> wrote:
>> I extended the patch a little so that the option can be used to set
>> multiple stack reuse levels: -fstack-reuse=[all|name_vars|none]
>>
>> all: enable stack reuse for all local vars (named vars and compiler
>> generated temporaries) which live in memory;
>> name_vars: enable stack reuse only for user declared local vars with names;
>> none: disable stack reuse completely.
>>
>> Note the patch still chooses to suppress clobber statement generation
>> instead of just ignoring them in stack layout. This has the additional
>> advantage of allowing more aggressive code motion when stack use is
>> disabled.
>>
>> The documentation will be updated when the patch is agreed upon.
>>
>> thanks,
>>
>> David
>>
>>
>> On Thu, Jun 28, 2012 at 10:43 PM, Xinliang David Li <davidxl@google.com> wrote:
>>> (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