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
