I am trying to convert some old FORTRAN code to C++.

I am about three sub-routines deep into the code and one of the sub-routines includes the following line (twice):

```
CDIR$ IVDEP
```

This is the first time I have come across this command, or directive, or whatever it is.

What does it do? It looks like an "IF" statement because the next instruction is a "RETURN."

But I don't know which option can be omitted.

Any suggestions?

The entire routine follows below, if relevant. It is part of a bigger program that computes a Discrete Fourier (Forward) Transform:

```
SUBROUTINE RADF5(IDO,L1,CC,CH,WA1,WA2,WA3,WA4)
C***BEGIN PROLOGUE RADF5
C***REFER TO RFFTF
C***ROUTINES CALLED (NONE)
C***END PROLOGUE RADF5
DIMENSION CC(IDO,L1,5) ,CH(IDO,5,L1) ,
1 WA1(*) ,WA2(*) ,WA3(*) ,WA4(*)
C***FIRST EXECUTABLE STATEMENT RADF5
PI = 4.*ATAN(1.)
TR11 = SIN(.1*PI)
TI11 = SIN(.4*PI)
TR12 = -SIN(.3*PI)
TI12 = SIN(.2*PI)
DO 101 K=1,L1
CR2 = CC(1,K,5)+CC(1,K,2)
CI5 = CC(1,K,5)-CC(1,K,2)
CR3 = CC(1,K,4)+CC(1,K,3)
CI4 = CC(1,K,4)-CC(1,K,3)
CH(1,1,K) = CC(1,K,1)+CR2+CR3
CH(IDO,2,K) = CC(1,K,1)+TR11*CR2+TR12*CR3
CH(1,3,K) = TI11*CI5+TI12*CI4
CH(IDO,4,K) = CC(1,K,1)+TR12*CR2+TR11*CR3
CH(1,5,K) = TI12*CI5-TI11*CI4
101 CONTINUE
IF (IDO .EQ. 1) RETURN
IDP2 = IDO+2
IF((IDO-1)/2.LT.L1) GO TO 104
DO 103 K=1,L1
CDIR$ IVDEP
DO 102 I=3,IDO,2
IC = IDP2-I
DR2 = WA1(I-2)*CC(I-1,K,2)+WA1(I-1)*CC(I,K,2)
DI2 = WA1(I-2)*CC(I,K,2)-WA1(I-1)*CC(I-1,K,2)
DR3 = WA2(I-2)*CC(I-1,K,3)+WA2(I-1)*CC(I,K,3)
DI3 = WA2(I-2)*CC(I,K,3)-WA2(I-1)*CC(I-1,K,3)
DR4 = WA3(I-2)*CC(I-1,K,4)+WA3(I-1)*CC(I,K,4)
DI4 = WA3(I-2)*CC(I,K,4)-WA3(I-1)*CC(I-1,K,4)
DR5 = WA4(I-2)*CC(I-1,K,5)+WA4(I-1)*CC(I,K,5)
DI5 = WA4(I-2)*CC(I,K,5)-WA4(I-1)*CC(I-1,K,5)
CR2 = DR2+DR5
CI5 = DR5-DR2
CR5 = DI2-DI5
CI2 = DI2+DI5
CR3 = DR3+DR4
CI4 = DR4-DR3
CR4 = DI3-DI4
CI3 = DI3+DI4
CH(I-1,1,K) = CC(I-1,K,1)+CR2+CR3
CH(I,1,K) = CC(I,K,1)+CI2+CI3
TR2 = CC(I-1,K,1)+TR11*CR2+TR12*CR3
TI2 = CC(I,K,1)+TR11*CI2+TR12*CI3
TR3 = CC(I-1,K,1)+TR12*CR2+TR11*CR3
TI3 = CC(I,K,1)+TR12*CI2+TR11*CI3
TR5 = TI11*CR5+TI12*CR4
TI5 = TI11*CI5+TI12*CI4
TR4 = TI12*CR5-TI11*CR4
TI4 = TI12*CI5-TI11*CI4
CH(I-1,3,K) = TR2+TR5
CH(IC-1,2,K) = TR2-TR5
CH(I,3,K) = TI2+TI5
CH(IC,2,K) = TI5-TI2
CH(I-1,5,K) = TR3+TR4
CH(IC-1,4,K) = TR3-TR4
CH(I,5,K) = TI3+TI4
CH(IC,4,K) = TI4-TI3
102 CONTINUE
103 CONTINUE
RETURN
104 DO 106 I=3,IDO,2
IC = IDP2-I
CDIR$ IVDEP
DO 105 K=1,L1
DR2 = WA1(I-2)*CC(I-1,K,2)+WA1(I-1)*CC(I,K,2)
DI2 = WA1(I-2)*CC(I,K,2)-WA1(I-1)*CC(I-1,K,2)
DR3 = WA2(I-2)*CC(I-1,K,3)+WA2(I-1)*CC(I,K,3)
DI3 = WA2(I-2)*CC(I,K,3)-WA2(I-1)*CC(I-1,K,3)
DR4 = WA3(I-2)*CC(I-1,K,4)+WA3(I-1)*CC(I,K,4)
DI4 = WA3(I-2)*CC(I,K,4)-WA3(I-1)*CC(I-1,K,4)
DR5 = WA4(I-2)*CC(I-1,K,5)+WA4(I-1)*CC(I,K,5)
DI5 = WA4(I-2)*CC(I,K,5)-WA4(I-1)*CC(I-1,K,5)
CR2 = DR2+DR5
CI5 = DR5-DR2
CR5 = DI2-DI5
CI2 = DI2+DI5
CR3 = DR3+DR4
CI4 = DR4-DR3
CR4 = DI3-DI4
CI3 = DI3+DI4
CH(I-1,1,K) = CC(I-1,K,1)+CR2+CR3
CH(I,1,K) = CC(I,K,1)+CI2+CI3
TR2 = CC(I-1,K,1)+TR11*CR2+TR12*CR3
TI2 = CC(I,K,1)+TR11*CI2+TR12*CI3
TR3 = CC(I-1,K,1)+TR12*CR2+TR11*CR3
TI3 = CC(I,K,1)+TR12*CI2+TR11*CI3
TR5 = TI11*CR5+TI12*CR4
TI5 = TI11*CI5+TI12*CI4
TR4 = TI12*CR5-TI11*CR4
TI4 = TI12*CI5-TI11*CI4
CH(I-1,3,K) = TR2+TR5
CH(IC-1,2,K) = TR2-TR5
CH(I,3,K) = TI2+TI5
CH(IC,2,K) = TI5-TI2
CH(I-1,5,K) = TR3+TR4
CH(IC-1,4,K) = TR3-TR4
CH(I,5,K) = TI3+TI4
CH(IC,4,K) = TI4-TI3
105 CONTINUE
106 CONTINUE
RETURN
END
```