This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: avoiding recursive calls of calloc due to optimization


On Mon, Sep 21, 2015 at 3:33 PM, Daniel Gutson
<daniel.gutson@tallertechnologies.com> wrote:
> On Mon, Sep 21, 2015 at 2:26 PM, Andrew Pinski <pinskia@gmail.com> wrote:
>> On Mon, Sep 21, 2015 at 10:20 AM, Daniel Gutson
>> <daniel.gutson@tallertechnologies.com> wrote:
>>> This is derived from https://gcc.gnu.org/ml/gcc-help/2015-03/msg00091.html
>>>
>>> Currently, gcc provides an optimization that transforms a call to
>>> malloc and a call to memset into a call to calloc.
>>> This is fine except when it takes place within the calloc() function
>>> implementation itself, causing a recursive call.
>>> Two alternatives have been proposed: -fno-malloc-builtin and disable
>>> optimizations in calloc().
>>> I think the former is suboptimal since it affects all the code just
>>> because of the implementation of one function (calloc()),
>>> whereas the latter is suboptimal too since it disables the
>>> optimizations in the whole function (calloc too).
>>> I think of two alternatives: either make -fno-calloc-builtin to

BTW, this patch implements this alternative.


>>> disable the optimization, or make the optimization aware of the
>>> function context where it is operating and prevent it to do the
>>> transformation if the function is calloc().
>>>
>>> Please help me to find the best alternative so we can implent it.
>>
>> Did you try the optimize attribute?
>
> That is the 2nd alternative I mentioned as proposed. And I'm also aware that
> the optimize attribute is somehow broken. Anyway, as I mentioned, I'd like to
> avoid disabling (all the other) optimizations for calloc's implementation.
> (FWIW, setting optimize as function attribute to O1 "fixes" the issue).
>
>>
>> Also you can try the following:
>>
>> size_t ns = size*elements;
>> If (ns / elements != size)
>>   return NULL;
>> void *ptr = malloc (ns);
>> asm ("":"+r"(ptr));
>> memset (ptr, 0, ns);
>>
>> Notice I put in a check for overflow in there.
>
> Thanks, it's a nice workaround.
>
>>
>> Thanks,
>> Andrew Pinski
>>
>>
>>>
>>> Thanks,
>>>
>>>    Daniel.
>
>
>
> --
>
> Daniel F. Gutson
> Chief Engineering Officer, SPD
>
> San Lorenzo 47, 3rd Floor, Office 5
> CÃrdoba, Argentina
>
> Phone:   +54 351 4217888 / +54 351 4218211
> Skype:    dgutson
> LinkedIn: http://ar.linkedin.com/in/danielgutson



-- 

Daniel F. Gutson
Chief Engineering Officer, SPD

San Lorenzo 47, 3rd Floor, Office 5
CÃrdoba, Argentina

Phone:   +54 351 4217888 / +54 351 4218211
Skype:    dgutson
LinkedIn: http://ar.linkedin.com/in/danielgutson

Attachment: no_builtin_calloc.diff
Description: Text document


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]