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: Functions that are CSEable but not pure


On Thu, Oct 4, 2012 at 5:22 PM, Jason Merrill <jason@redhat.com> wrote:
> On 10/04/2012 09:07 AM, Richard Guenther wrote:
>>
>> Ugh.  Especially with the above (you can DCE those calls) makes this
>> severly mis-specified ... and any implementation error-prone (look what
>> mess our losely defined 'malloc' attribute opened ...).
>>
>> I thought of a testcase like
>>
>>   int *p = get_me ();
>>   .. = *p;
>>   int *q = get_me ();
>>   .. = *q;
>>
>> and get_me allocating/initalizing and returning a singleton.
>
>
> Right.
>
>
>> But you tell me it's more complicated and get_me () needs to
>> be a barrier for any load/store (as it may modify arbitrary memory,
>> but only on the "first" call).
>
>
> Yes, because the initialization is user-written code.
>
>
>> I think that "may modify arbitrary memory" isn't going to fly and
>> my answer would be, better don't try to optimize anything here,
>> at least not in generic terms.  How would you handle this in
>> the alias oracle?  How would you then make CSE recognize
>> two functions return the same value and are CSEable?
>
>
> For aliasing purposes, the call is like a call to a normal function. For CSE
> purposes, we want to recognize identical calls and combine them.  I don't
> know the GCC bits well enough to be any more specific.
>
>
>> Can you come up with a short but complete testcase illustrating the issue
>> better (preferably C, so I don't need to read-in magic points where
>> construction
>> happens)?
>
>
> int init_count;
> int data;
>
> void init()
> {
>   static int initialized;
>   if (!initialized)
>     {
>       data = ++init_count;
>       initialized = 1;
>     }
> }
>
> inline int *get_me() __attribute ((pure));
> inline int *get_me()
> {
>   init ();
>   return &data;
> }
>
> int sink;
>
> int main()
> {
>   sink = init_count;
>   int *p = get_me();
>   if (init_count != 1)
>     __builtin_abort();
>   int *q = get_me();
>   if (init_count != 1)
>     __builtin_abort();
>   return *p + *q;
> }
>
> On this testcase, gcc -O2 doesn't reload init_count after the call to get_me
> because it thinks that the call can't have modified init_count. I want the
> compiler to know that it is safe to discard the redundant assignment, but
> not make assumptions about memory.

But isn't it a fact that it _cannot_ modify init_count?  If the second call
is CSEable then it cannot have side-effects that are observable at
the call site.  Is the following an example you would consider to fall
under your CSEing?

int init_count;
int data;
int initialized;

void init()
{
  if (!initialized)
    {
      data = ++init_count;
      initialized = 1;
    }
}

inline int *get_me() __attribute ((pure));
inline int *get_me()
{
  init ();
  return &data;
}

int sink;

int main()
{
  sink = init_count;
  int *p = get_me();
  if (init_count != 1)
    __builtin_abort();
  initialized = 0;
  int *q = get_me();
  if (init_count != 2)
    __builtin_abort();
  return *p + *q;
}

?  If so, then why can we assume get_me returns the same pointer even here?

Richard.

>
> On 10/04/2012 08:59 AM, Jakub Jelinek wrote:> On Thu, Oct 04, 2012 at
> 08:56:03AM -0400, Jason Merrill wrote:
>> Sure, but I thought you want to inline the wrapper function as soon as
>> possible.  Or do you want to keep it as some kind of builtin that gets
>> expanded during expansion to the test and call?
>
> Ah, I see your point; if get_me is inlined we end up with two calls to init,
> so it would be good to mark init with the same hypothetical attribute.
>
> Jason
>


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