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