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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/56937] New: Unnecessarily temporary with array-vector assignments
Date: Fri, 12 Apr 2013 18:12:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56937

             Bug #: 56937
           Summary: Unnecessarily temporary with array-vector assignments
    Classification: Unclassified
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: burnus@gcc.gnu.org


For assignments of the form

   array(vec_idx) = array(vec_idx) + nonaliasing_expr

no temporary should be generated.

In other words, the following program should print "1.0" and not "0.5". It
prints "1.0" with PGI and Intel (which do this optimization) and prints "0.5"
with pathf95, g95, gfortran and NAG, which don't:

real :: r(4), p(4)
integer :: idx(4)
p = [0.25, 0.25, 0.25, 0.25]
idx = [2,2,3,2]
r = 0
r(idx) = r(idx) + p
print *, sum(r)
end

The reason for the value is that gfortran (et al.) create a temporary and use:
  tmp(2) = r(1) + 0.25  ! r(1:4) == 0
  tmp(2) = r(2) + 0.25
  tmp(3) = r(3) + 0.25
  tmp(2) = r(4) + 0.25
  r(2) = tmp(2)         ! = 0.25
  r(2) = tmp(2)         ! = 0.25
  r(3) = tmp(3)         ! = 0.25
  r(2) = tmp(2)         ! = 0.25
-> sum(r) = 0.5

Without temporary one gets:
  r(2) = 0 + 0.25
  r(2) = (0.25) + 0.25
  r(3) = 0 + 0.25
  r(4) = (0.75) + 0.25
-> sum(r) = 1.0


The reason that the optimization is valid is given by the following restriction
in the Fortran standard:

"If a vector subscript has two or more elements with the same value, an array
section with that vector subscript shall not appear in a variable definition
context (16.6.7)."   (Fortran 2008, 6.5.3.3.2 Vector subscript, paragraph 3)