[Bug fortran/24406] New: EQUIVALENCE broken in 32-bit code with optimization -O2

["mick at nag dot co dot uk" <gcc-bugzilla@gcc.gnu.org>]

This program demonstrates a problem with Fortran EQUIVALENCE
in 32-bit compilations. The equivalenced value in variable rteps
gets set correctly (as evidenced by the write statement) but too
late - an incorrect value has already been used in the earlier
comparison statement.

Moving the statement
      write (*,*) 'rteps = ', rteps
to just before the comparison avoids the problem.

Compiling with a level of optimization lower than -O2 also
avoids the problem.

The problem does not show up in 64-bit compiles.

      program main
      double precision rteps
      integer irt(2)
      equivalence (rteps,irt)
c     This bit pattern sets rteps = 1.0d0
      irt(1) = Z'00000000'
      irt(2) = Z'3FF00000'
      if (rteps.gt.0.99d0 .and. rteps.lt.1.1d0) then
         write (*,*) '0.99 < rteps < 1.1 : rteps OK'
      else
         write (*,*) 'rteps <= 0.99  or   rteps >= 1.1 : rteps BAD'
      end if
      write (*,*) 'rteps = ', rteps
      end

% gfortran -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc/configure --prefix=/var/tmp/gfortran-20051007/irun
--enable-languages=c,f95
Thread model: posix
gcc version 4.1.0 20051007 (experimental)

% gfortran -Wall -m32 -O0 equiv.f
% ./a.out
 0.99 < rteps < 1.1 : rteps OK
 rteps =    1.00000000000000     

% gfortran -Wall -m32 -O2 equiv.f
% ./a.out
 rteps <= 0.99  or   rteps >= 1.1 : rteps BAD
 rteps =    1.00000000000000


-- 
           Summary: EQUIVALENCE broken in 32-bit code with optimization -O2
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mick at nag dot co dot uk


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24406