Using the -fpe or /fpe Compiler Option

The -fpen (Linux* and Mac OS*) or /fpe:n (Windows*) option allows some control over the results of floating-point exceptions.

-fpe0 or /fpe:0 restricts floating-point exceptions by enabling the overflow, the divide-by-zero, and the invalid floating-point exceptions. The program will print an error message and abort if any of these exceptions occurs. If a floating underflow occurs, the result is set to zero and execution continues. This is called flush-to-zero. This option sets -IPF_fp_speculationstrict (Linux and Mac OS) or /QIPF_fp_speculationstrict (Windows) if no specific -IPF_fp_speculation or /QIPF_fp_speculation option is specified. The -fpe0 or /fpe:0 option sets -ftz (Linux and Mac OS) /Qftz (Windows). To get more detailed location information about where the exception occurred, use -traceback (Linux and Mac OS) or /traceback (Windows).

Note

On systems based on the IA-32 and Intel® 64 architectures , explicitly setting -fpe0 or /fpe:0 can degrade performance since the generated code stream must be synchronized after each floating-point instruction to allow for abrupt underflow fix-up.

-fpe1 or /fpe:1 restricts only floating-point underflow. Floating-point overflow, floating-point divide-by-zero, and floating-point invalid produce exceptional values (NaN and signed Infinities) and execution continues. If a floating-point underflow occurs, the result is set to zero and execution continues. The /fpe:1 option sets -ftz or /Qftz.

Note

On systems based on the IA-32 and Intel® 64 architectures , explicitly setting -fpe1 or /fpe:1 can degrade performance since the generated code stream must be synchronized after each floating-point instruction to allow for abrupt underflow fix-up.

-fpe3 or /fpe:3 is the default on all processors, which allows full floating-point exception behavior. Floating-point overflow, floating-point divide-by-zero, and floating-point invalid produce exceptional values (NaN and signed Infinities) and execution continues. Floating underflow is gradual:  denormalized values are produced until the result becomes 0.

The -fpe or /fpe option enables exceptions in the Fortran main program only. The floating-point exception behavior set by the Fortran main program remains in effect throughout the execution of the entire program unless changed by the programmer. If the main program is not Fortran, the user can use the Fortran intrinsic FOR_SET_FPE to set the floating-point exception behavior.

When compiling different routines in a program separately, you should use the same value of n in -fpen or /fpe:n.

An example follows:

       IMPLICIT NONE

       real*4 res_uflow, res_oflow

       real*4 res_dbyz, res_inv

       real*4 small, big, zero, scale

       small = 1.0e-30

       big   = 1.0e30

       zero  = 0.0

       scale = 1.0e-10

!      IEEE underflow condition (Underflow Raised)

       res_uflow = small * scale

       write(6,100)"Underflow: ",small, " *", scale, " = ", res_uflow

!      IEEE overflow condition (Overflow Raised)

       res_oflow = big * big

       write(6,100)"Overflow: ", big, " *", big, " = ", res_oflow

!      IEEE divide-by-zero condition (Divide by Zero Raised)

       res_dbyz = -big / zero

       write(6,100)"Div-by-zero: ", -big, " /", zero, " = ", res_dbyz

!      IEEE invalid condition (Invalid Raised)

       res_inv = zero / zero

       write(6,100)"Invalid: ", zero, " /", zero, " = ", res_inv

 100   format(A14,E8.1,A2,E8.1,A2,E10.1)

       end

Consider the following command line:

ifort fpe.f -fpe0 -g (Linux and Mac OS)

ifort fpe.for /fpe:0 /traceback (Windows)

The following output is produced:

Windows:

   Underflow:  0.1E-29 * 0.1E-09 =   0.1E-39

forrtl: error (73): floating overflow

Image              PC          Routine     Line        Source

fpe.exe            0040122B    _MAIN__      47        test.f90

fpe.exe            0044223D    Unknown    Unknown     Unknown

fpe.exe            0043583C    Unknown    Unknown     Unknown

kernel32.dll       77E8141A   Unknown     Unknown     Unknown

Linux and Mac OS:

./a.out

   Underflow:  0.1E-29 * 0.1E-09 =   0.0E+00

forrtl: error (72): floating overflow

Image       PC        Routine     Line        Source

a.out       0804A063  Unknown     Unknown  Unknown

a.out       08049E78  Unknown     Unknown  Unknown

Unknown     B746B748  Unknown     Unknown  Unknown

a.out       08049D31  Unknown     Unknown  Unknown

Aborted

The following command line uses /fpe1:

ifort fpe.f -fpe1 -g (Linux and Mac OS)
ifort fpe.for /fpe:1 /traceback (Windows)

The following output is produced:

   Underflow:  0.1E-29 * 0.1E-09 =   0.0E+00

    Overflow:  0.1E+31 * 0.1E+31 = Infinity

 Div-by-zero: -0.1E+31 / 0.0E+00 = -Infinity

     Invalid:  0.0E+00 / 0.0E+00 = NaN      

The following command line uses /fpe3:

ifort fpe.f -fpe3 -g (Linux and Mac OS)

ifort fpe.for /fpe:3 /traceback (Windows)

The following output is produced:

Underflow:  0.1E-29 * 0.1E-09 =   0.1E-39

    Overflow:  0.1E+31 * 0.1E+31 = Infinity

 Div-by-zero: -0.1E+31 / 0.0E+00 = -Infinity

     Invalid:  0.0E+00 / 0.0E+00 = NaN