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From: "burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/51260] New: PARAMETER array with constructor initializer: Compile-time simplify single element access
Date: Mon, 21 Nov 2011 22:01:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51260

             Bug #: 51260
           Summary: PARAMETER array with constructor initializer:
                    Compile-time simplify single element access
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: burnus@gcc.gnu.org


Missed optimization in several ways for:
        integer, parameter:: is(10000)=(/(i,i=1,10000)/)
        print *,is(1)
        end

The main issue is that "is(1)" is not simplified into the constant "1".

Expected:

* The front end should already optimize is(1) to "1" and should not generate
the variable to store the parameter.

* One should somehow tell the middle end that only a single element is needed
for I/O. One currently has:
  _gfortran_transfer_integer_write (&dt_parm.0, &is[0], 4);
which requires the whole object though only the first 4 bytes are needed (=
&"1").

* The constructor should not be fully expanded, if one only needs a single
element, i.e. using
   integer, parameter:: is(*)=(/(i,i=1,10000000)/)
should be quick if one only needs the first element. In any case, no parameter
array should be written if one can simplify it at front-end compilation time.