Flush-to-zero in gfortran - fortran

Question

Is there a way to force flush-to-zero of underflows in gfortran?
I can't believe this is the first time someone has asked this, but I couldn't find anything on it anywhere. Mea culpa if this is a duplicate.

Answer 1

You can accomplish this with recent versions of gfortran that support the Fortran 2003 IEEE modules. The standard defines two underflow modes -- gradual and abrupt. Abrupt is the one you want which sets underflow to 0 and signals the underflow floating point exception. You can test for support of controlling the underflow mode with the function ieee_support_underflow_control(X) which tests for underflow control for the kind of real X is and returns a logical true if it is supported. If supported, you can then call ieee_set_underflow_mode(.false.) to set abrupt underflow mode.
Below is a test program you can use to test underflow control support for the default real kind:
program test
use, intrinsic :: ieee_arithmetic
use, intrinsic :: iso_fortran_env, only: compiler_version, compiler_options
implicit none
logical :: underflow_support, gradual, underflow
real :: fptest
integer :: i
print '(4a)', 'This file was compiled by ', &
compiler_version(), ' using the options ', &
compiler_options()
fptest = 0.0
underflow_support = ieee_support_underflow_control(fptest)
if (underflow_support) then
print *,'Underflow control supported for the default real kind'
else
stop 'no underflow control support'
end if
call ieee_set_underflow_mode(.false.)
call ieee_get_underflow_mode(gradual)
if (.not.gradual) then
print *,'Able to set abrupt underflow mode'
else
stop 'error setting underflow mode'
end if
fptest = 2e-36
do i=1,50 ! 50 iterations max
fptest = fptest * 0.5
print '(e15.10)',fptest
call ieee_get_flag(ieee_underflow,underflow)
if (underflow) print *,'Underflow exception signaling'
if (fptest == 0.0) exit
end do
end program test
Using gfortran version 5.2.0, this program outputs:
This file was compiled by GCC version 5.2.0 using the options -mtune=generic -march=x86-64 -fno-unsafe-math-optimizations -frounding-math -fsignaling-nans
Underflow control supported for the default real kind
Able to set abrubpt underflow mode
.1000000036E-35
.5000000180E-36
.2500000090E-36
.1250000045E-36
.6250000225E-37
.3125000112E-37
.1562500056E-37
.0000000000E+00
Underflow exception signaling
The compiler option flags -fno-unsafe-math-optimizations -frounding-math -fsignaling-nans are suggested by the gfortran 5.2 documentation to be used anytime the IEEE modules are used to ensure adherence to the standard.

Answer 2

A lazy way to "flush to zero" is to use to use gfortran's -funsafe-math-optimizations to:
Allow math optimizations that may violate IEEE or ISO standards
or in other words:
This mode enables optimizations that allow arbitrary reassociations and transformations with no accuracy guarantees. It also does not try to preserve the sign of zeros.
For example, small.f:
program test
real r
r=1e-40
print *,'r on next line'
print *,r
end program
Without any flags, a non-zero denormal (small) number is shown, without error:
$ gfortran -g small.f
$ ./a.out
r on next line
9.99994610E-41
Trap the denormalized number, which crashes when attempting to print the value:
$ gfortran -g -ffpe-trap=denorm small.f
$ ./a.out
r on next line
Program received signal SIGFPE: Floating-point exception - erroneous arithmetic operation.
Backtrace for this error:
#0 0x2aaaab05c26f in ???
#1 0x2aaaaac61aed in get_float_string
at ../../../libgfortran/io/write_float.def:1064
#2 0x2aaaaac6423d in list_formatted_write_scalar
at ../../../libgfortran/io/write.c:1889
#3 0x4008f1 in test
at /path/to/small.f:5
#4 0x400941 in main
at /path/to/small.f:6
Floating point exception
And added flag that flushes it to zero:
$ gfortran -g -ffpe-trap=denorm -funsafe-math-optimizations small.f
$ ./a.out
r on next line
0.00000000