[Bug fortran/44856] New: Usage of array PARAMETERs: Literal copy vs. global variable

[Bug fortran/44856] New: Usage of array PARAMETERs: Literal copy vs. global variable

[burnus at gcc dot gnu dot org]

Currently, gfortran saves scalar constants (PARAMETER) only in the .mod file
while array parameters are saved as global, static variable in the .o file and
additionally as expression in the .mod file.

This has all kind of funny consequences. For instance:

   print *, array_par, (array_par), shape(array_par)

Here, the first item is accessed via the global variable, for the second one a
static variable is produced in place, which is then accessed, and for the third
one, the front end simplifies the expression and generates an variable
containing the shape.

Similarly for:
  a = array_par(1)
  a = array_par(1) + 0
In the first case, one accesses the global variable while in the second case,
one generates a temporary local variable, initialized by the literal.


There are a couple of issues:

a) For intrinsic modules, it does not work (cf. PR 40571) as there is no .o
file.

b) Accessing the global variable in case of taking a scalar does not make
sense:
  a = parameter_array(1)
should always use (a local variable with) the literal.

c) Also in other cases, it can make sense to generate a local copy of the
literal - as done in case of "(parameter_array)" - though, one probably wants
to limit the inline size.


Comments? Suggestions?


-- 
           Summary: Usage of array PARAMETERs:  Literal copy vs. global
                    variable
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: burnus at gcc dot gnu dot org
OtherBugsDependingO 40571
             nThis:


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44856

[Bug fortran/44856] Usage of array PARAMETERs: Literal copy vs. global variable

[burnus at gcc dot gnu dot org]

------- Comment #1 from burnus at gcc dot gnu dot org  2010-07-07 14:47 -------
My idea is:

a) For scalar expressions, generate simply always the scalar variable.

b) For array accesses, generate a const variable at the beginning of the
current block (subroutine, function, BLOCK) - if the variable is smaller than
"-farray-parameter-inline-limit" or if from an intrinsic module.

The trick for (b) is to collect this information in the symbol attribute (i.e.
not needed if never access or when only scalars are accessed). Additionally,
one needs to make sure that then in the current block the copied parameter is
accessed. I think a reasonable value for the limit could be something like 10,
20.

(I am actually not sure whether the FE does mark the module global parameters
variable as "restricted" or as constant. If one and if possible, one should do
so.)


An example. The trick is to make sure that  (ModulePara(1))  does not simply
generate, e.g., "D.1558 = modulepara[0];" which is pointless.


module m
  integer, parameter :: ModulePara(2) = [ 1, 2 ]
end module m

program test
  use m
  implicit none

  integer :: aaaa(2),i
  i=1

  aaaa = ModulePara         ! access modulepara (1)
  print *, ModulePara       ! access modulepara (2)
  print *, ModulePara(i)    ! access modulepara (3)
  print *, ModulePara(i)+i  ! access modulepara (4)
  print *, ModulePara(i)+1  ! access modulepara (5)

  aaaa(1) = (ModulePara(1)) ! use literal
  aaaa(1) = ModulePara(1)   ! use literal
  print *, ModulePara(1)    ! use literal
end program test

Dump contains the (correct!) six accesses to modulepara (5 + "extern").


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44856

[Bug fortran/44856] Usage of array PARAMETERs: Literal copy vs. global variable

[burnus at gcc dot gnu dot org]

------- Comment #2 from burnus at gcc dot gnu dot org  2010-07-07 15:23 -------
Note: The tricky part for a simple fix is:
   moduleArray(i)

If one simply wraps in resolve_code the code->expr(1,2) with parentheses (e =
gfc_get_parentheses (e)") this leads to "D.123 = modulearray[i]", which does
not help for internal modules (cf. PR 40571) ...


-- 

burnus at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44856

[Bug fortran/44856] Usage of array PARAMETERs: Literal copy vs. global variable

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44856

--- Comment #3 from Tobias Burnus <burnus at gcc dot gnu.org> 2010-12-08 16:28:07 UTC ---
Other example - from James,
cf.
http://groups.google.com/group/comp.lang.fortran/browse_thread/thread/1a695db0fcfaa3e9

program kindtest
   use ISO_FORTRAN_ENV
   implicit none
   integer i
   REAL_KINDS(1) = 42
   i = sum([(kind(real(0,REAL_KINDS(i))),i=1,size(REAL_KINDS))])
   write(*,*) i
end program kindtest

which is rejected with
  Error: 'kind' argument of 'real' intrinsic at (1) must be a constant

A quick test shows that for locally defined parameter arrays, only NAG f95 is
able to use it as KIND= argument to REAL. While g95, Intel, pathf95 and openf95
have the same problem as gfortran.