[Bug middle-end/39298] New: Optimize away only set but not used variable

[Bug middle-end/39298] New: Optimize away only set but not used variable

[burnus at gcc dot gnu dot org]

In the following Fortran program, the variable "foo" is never used, it is only
set.

Result: Using "gfortran -O3", "foo" is not optimized away.
Expected: "foo" is optimized away as it happens with "ifort -O2".

      program a
      implicit none
      integer*8 it,two
      parameter(it=1073741824,two=2)
      complex foo(it*two-1)

      foo(10)=1.
      write(*,*) ''
      end program a


-- 
           Summary: Optimize away only set but not used variable
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: burnus at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39298

[Bug middle-end/39298] Optimize away only set but not used variable

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2009-02-25 09:43 -------
Is there a reason the Fortran frontend gives function local variables static
storage duration?

a ()
{
  struct __st_parameter_dt dt_parm.1;
  static integer(kind=4) options.0[8] = {68, 255, 0, 0, 0, 1, 0, 1};
  static complex(kind=4) foo[2147483647];

<bb 2>:
  _gfortran_set_options (8, &options.0);
  foo[9] = __complex__ (1.0e+0, 0.0);


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2009-02-25 09:43:40
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39298

[Bug middle-end/39298] Optimize away only set but not used variable

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2009-02-25 09:51 -------
It works with it == 1024 in which case foo is not static.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39298

[Bug middle-end/39298] Optimize away only set but not used variable

[burnus at gcc dot gnu dot org]

------- Comment #3 from burnus at gcc dot gnu dot org  2009-02-25 10:21 -------
> Is there a reason the Fortran frontend gives function local variables static
> storage duration?

For huge arrays, there is a problem if the memory is allocated on the stack, as
one quickly hits stack-size limits. Thus gfortran puts large local arrays in
static memory, except when a procedure can be called recursively/simultaneously
(RECURSIVE attribute, -frecursive, -fopenmp). The size for which this happens
is controlled by -fmax-stack-var-size=<n>.

Actually, I don't quite see why the "static" matters: As local variable it
cannot be accessed from elsewhere and if it is not accessed in the procedure
...

Additionally, C's main() and Fortran's PRORGRAM (= "MAIN__") are special
because they are only called once.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39298

[Bug middle-end/39298] Optimize away only set but not used variable

[burnus at gcc dot gnu dot org]

------- Comment #4 from burnus at gcc dot gnu dot org  2009-02-25 10:35 -------
I did some testing with sunf95, icc and ifort.

sunf95 also puts the variable in .bss as gfortran does, while ifort puts it on
the stack (unless explicitly declared as static ["SAVE"]). If the variable is
static, neither of the compilers optimizes it away.

a) Why are static variables not optimized away? (Not even in main()/MAIN_?)
b) Is there something what one could do to get the advantage of not having huge
variables on the stack but still allowing to optimize the variable away?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39298

[Bug middle-end/39298] Optimize away only set but not used variable

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2009-02-25 11:21 -------
Optimizing dead local static variables requires special handling which probably
is not thought to be worth the trouble.  If the variable is unused the
programmer can as well remove it.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39298

Re: [Bug middle-end/39298] Optimize away only set but not used variable

[Andrew Thomas Pinski]

Sent from my iPhone

On Feb 25, 2009, at 1:43 AM, "rguenth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org 
 > wrote:

>
>
> ------- Comment #1 from rguenth at gcc dot gnu dot org  2009-02-25  
> 09:43 -------
> Is there a reason the Fortran frontend gives function local  
> variables static
> storage duration?
>

Yes, it is larger than the threshhold. Remember fortran has no  
recursive functions except for the ones which marked as such.

> a ()
> {
>  struct __st_parameter_dt dt_parm.1;
>  static integer(kind=4) options.0[8] = {68, 255, 0, 0, 0, 1, 0, 1};
>  static complex(kind=4) foo[2147483647];
>
> <bb 2>:
>  _gfortran_set_options (8, &options.0);
>  foo[9] = __complex__ (1.0e+0, 0.0);
>
>
> -- 
>
> rguenth at gcc dot gnu dot org changed:
>
>           What    |Removed                     |Added
> --- 
> --- 
> ----------------------------------------------------------------------
>             Status|UNCONFIRMED                 |NEW
>     Ever Confirmed|0                           |1
>   Last reconfirmed|0000-00-00 00:00:00         |2009-02-25 09:43:40
>               date|                            |
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39298
>

[Bug middle-end/39298] Optimize away only set but not used variable

[pinskia at gmail dot com]

------- Comment #6 from pinskia at gmail dot com  2009-02-25 13:37 -------
Subject: Re:  Optimize away only set but not used variable



Sent from my iPhone

On Feb 25, 2009, at 1:43 AM, "rguenth at gcc dot gnu dot org"
<gcc-bugzilla@gcc.gnu.org 
 > wrote:

>
>
> ------- Comment #1 from rguenth at gcc dot gnu dot org  2009-02-25  
> 09:43 -------
> Is there a reason the Fortran frontend gives function local  
> variables static
> storage duration?
>

Yes, it is larger than the threshhold. Remember fortran has no  
recursive functions except for the ones which marked as such.

> a ()
> {
>  struct __st_parameter_dt dt_parm.1;
>  static integer(kind=4) options.0[8] = {68, 255, 0, 0, 0, 1, 0, 1};
>  static complex(kind=4) foo[2147483647];
>
> <bb 2>:
>  _gfortran_set_options (8, &options.0);
>  foo[9] = __complex__ (1.0e+0, 0.0);
>
>
> -- 
>
> rguenth at gcc dot gnu dot org changed:
>
>           What    |Removed                     |Added
> --- 
> --- 
> ----------------------------------------------------------------------
>             Status|UNCONFIRMED                 |NEW
>     Ever Confirmed|0                           |1
>   Last reconfirmed|0000-00-00 00:00:00         |2009-02-25 09:43:40
>               date|                            |
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39298
>


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39298

[Bug middle-end/39298] Optimize away only set but not used variable

[pinskia at gcc dot gnu dot org]

------- Comment #7 from pinskia at gcc dot gnu dot org  2009-02-27 00:00 -------
The fortran front-end needs to be able to tell the middle-end that the function
cannot be recursive and then the middle-end needs to use that info.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39298