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Is there a FINDLOC() equivalent that can be used for IEEE NAN values? It is my understanding I cannot use FINDLOC with IEEE_VALUE( 1,0, IEEE_QUIET_NAN ), because the IEEE NAN values (signaling or quiet) don't equal anything, "including itself".

I can obviously just put in a DO loop across the array and check for IEEE_IS_NAN, but I'm assuming (possibly erroneously?) that FINDLOC() has a more efficient implementation under the hood.

Link Copied

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It is probably not what you are looking for, but here is a concise method:

`write(*,*) findloc( ieee_is_nan(array) )`

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While that's concise in terms of written code, it's very expensive, as IEEE_IS_NAN() will need to operate on the entirety of *array* before returning to FINDLOC(). So if *array* is very large and you have a NAN value fairly early, it will still perform the check on all the remaining members of *array*, whereas (theoretically) FINDLOC() will short-circuit.

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Presumably a NAN will be infrequent in your code.

IF(IEEE_IS_NAN(SUM(array))) THEN

! look for it

...

ENDIF

.OR.

the SUM(array) can be replaced by something in the prior code that contains the accumulation of what was in the array. e.g. total momentum, total energy, ...

Jim Dempsey

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Unfortunately, they are a little more frequent than usual, as this particular array we're initializing to NaN at startup (so as to not confuse with a "perfectly valid" 0.0). The code block in question is to find the first "unused" member of the array.

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A computed NAN is not a specific value, whereas a compiler generated NAN is a specific value.

The computed NAN will have

s111 1111 1qxx xxxx xxxx xxxx xxxx xxxx (single precision)

s111 1111 1111 qxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx (double)

s=sign (don't care)

1's exponent of all 1's

q=1=QNAN, q=0=SNAN

x's remaining bits of any value except all 0's

It would be recommended that your code should differentiate between the two.

I would suggest you generate a unique signalling NAN that is not the compiler generated signaling NAN and is not (likely) a computer signaling NAN. Such as (single precision). Such as

0xFF89ABCD (single precision)

0xFFF789ABCDEF0123 (double precision)

These are signaling NANs, if they are inadvertently used in computations, the execution will stop.

These signaling NANs are unlikely to be generated (due to trap) unless something mistakenly overwrites this with trash data (with same values).

You would then treat (cast) the array as integer(4) or integer(8) as the case may be, and then search for magic number that indicates a free location.

Jim Dempsey

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@jimdempseyatthecove in this case, I am not looking for a computed NAN; I simply want if a NAN is used in an operation, for all subsequent (floating-point) operations to carry-forward that NAN. I have yet to decide if I want to implement it as a signaling NAN to catch immediately, as that may have other implications I may not want to get into.

This brings up an interesting quirk though: If I have a structure with default initializers as such:

```
TYPE, BIND(C) :: FOO_PACKET
INTEGER(KIND=C_INT32_T) :: ID = -1
REAL(KIND=C_FLOAT) :: X = Z'FFFFFFFF'
REAL(KIND=C_FLOAT) :: Y = Z'FFFFFFFF'
END TYPE
```

...Intel Fortran doesn't complain, and happily compiles the routine that includes this block. However, if I compile with gfortran, it complains about the BOZ literal constants as being invalid in this context. A quick check on the GNU site indicates that the use of BOZ here is not standards-conforming.

So, which is right: GNU gfortran or ifort? Is this only valid syntax after a certain Fortran standard version?

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warning #6915: Fortran 2018 does not allow data values on a type statement. [Z'FFFFFFFF'] . You need standards checking, as otherwise it is assuming an intel language extension.

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Hmm... "-stand" isn't in the ifort Linux manual page, but it's in the HTML documentation; that's why I missed it. Thanks

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Look at FAST_IS_NAN in the following.

This example is not vectorized, but I have vectorized it in a different project.

```
! TestNaN.f90
module MOD_NEGATIVE_IS_NAN
integer, parameter :: IEEE754_SP_SIGN = Z'80000000'
integer, parameter :: IEEE754_SP_EXPONENT = Z'7F800000'
! *** Due to compiler (Intel 2020u1/u2) optimization bug we cannot use a parameter
! integer, parameter :: IEEE754_SP_MANTISSA = Z'007FFFFF'
! instead using memory stored variable works
! at some point this can be restored to parameter
integer :: IEEE754_SP_MANTISSA = Z'007FFFFF'
integer(8), parameter :: IEEE754_DP_SIGN = Z'8000000000000000'
integer(8), parameter :: IEEE754_DP_EXPONENT = Z'7FF0000000000000'
! *** Due to compiler (Intel 2020u1/u2) optimization bug we cannot use a parameter
integer(8), parameter :: IEEE754_DP_MANTISSA = Z'000FFFFFFFFFFFFF'
! instead using memory stored variable works
! at some point this can be restored to parameter
! integer(8) :: IEEE754_DP_MANTISSA = Z'000FFFFFFFFFFFFF'
! Function that returns a negative integer (of size of REAL/DOUBLE PRECISION)
! when input argument is a NaN (QNaN as SNaN would have caused an abort)
! these functions are elemental and capable of operating on vectors
interface NEGATIVE_IS_NAN
MODULE PROCEDURE NEGATIVE_IS_NAN_SP
MODULE PROCEDURE NEGATIVE_IS_NAN_DP
end interface NEGATIVE_IS_NAN
interface FAST_IS_NAN
MODULE PROCEDURE FAST_IS_NAN_SP
MODULE PROCEDURE FAST_IS_NAN_DP
end interface FAST_IS_NAN
CONTAINS
INTEGER(4) ELEMENTAL FUNCTION NEGATIVE_IS_NAN_SP(X)
REAL, INTENT(IN) :: X
NEGATIVE_IS_NAN_SP = TRANSFER(ABS(X),0)+IEEE754_SP_MANTISSA
END FUNCTION NEGATIVE_IS_NAN_SP
INTEGER(8) ELEMENTAL FUNCTION NEGATIVE_IS_NAN_DP(X)
DOUBLE PRECISION, INTENT(IN) :: X
NEGATIVE_IS_NAN_DP = TRANSFER(ABS(X),0_8)+IEEE754_DP_MANTISSA
END FUNCTION NEGATIVE_IS_NAN_DP
LOGICAL FUNCTION FAST_IS_NAN_SP(X)
REAL, INTENT(IN) :: X
FAST_IS_NAN_SP = (NEGATIVE_IS_NAN_SP(X) < 0)
END FUNCTION FAST_IS_NAN_SP
LOGICAL FUNCTION FAST_IS_NAN_DP(X)
DOUBLE PRECISION, INTENT(IN) :: X
FAST_IS_NAN_DP = (NEGATIVE_IS_NAN_DP(X) < 0_8)
END FUNCTION FAST_IS_NAN_DP
end module MOD_NEGATIVE_IS_NAN
program TestNaN
use, intrinsic :: ieee_arithmetic, only : ieee_value, ieee_quiet_nan, ieee_is_nan
use, intrinsic :: ieee_exceptions, only : ieee_set_halting_mode, ieee_divide_by_zero
use MOD_NEGATIVE_IS_NAN
use omp_lib
implicit none
integer, parameter :: N = 100000000 ! some number larger than Last Level Cache
integer, parameter :: reps = 50 ! number of times to scann for NaN
! integer, parameter :: N = 1000 ! some number larger than Last Level Cache
! integer, parameter :: reps = 5000000 ! number of times to scann for NaN
real, allocatable :: SP(:)
real :: SP_NAN, SP_R, SP_DR
double precision, allocatable :: DP(:)
real(8) :: DP_NAN, DP_R, DP_DR
double precision :: T0, T1
integer :: I, J
LOGICAL :: FOUND_NAN
INTEGER :: COUNT_NANS
ALLOCATE(SP(N), DP(N))
CALL RANDOM_NUMBER(SP)
CALL RANDOM_NUMBER(SP_R)
SP_DR = SP_R / 10.0
where ( (SP > (SP_R-SP_DR)).and.(SP < (SP_R+SP_DR)) )
SP = ieee_value( SP_R, ieee_quiet_nan )
end where
CALL RANDOM_NUMBER(DP)
CALL RANDOM_NUMBER(DP_R)
DP_DR = DP_R / 10.0
where ( (DP > (DP_R-DP_DR)).and.(DP < (DP_R+DP_DR)) )
DP = ieee_value( DP_R, ieee_quiet_nan )
end where
! ASSURE CONSTANTS ARE CORRECT
SP_NAN = TRANSFER(IEEE754_SP_EXPONENT+1234, 0.0) ! Arbitrary QNaN
FOUND_NAN = IEEE_IS_NAN(SP_NAN)
PRINT *,'IEEE_IS_NAN(SP_NAN)', FOUND_NAN
FOUND_NAN = (NEGATIVE_IS_NAN(SP_NAN) < 0)
PRINT *,'NEGATIVE_IS_NAN(SP_NAN)', FOUND_NAN
FOUND_NAN = FAST_IS_NAN(SP_NAN)
PRINT *,'FAST_IS_NAN(SP_NAN)', FOUND_NAN
! ASSURE CONSTANTS ARE CORRECT
DP_NAN = TRANSFER(IEEE754_DP_EXPONENT+1234_8, 0.0_8) ! Arbitrary QNaN
FOUND_NAN = IEEE_IS_NAN(DP_NAN)
PRINT *,'IEEE_IS_NAN(DP_NAN)', FOUND_NAN
FOUND_NAN = (NEGATIVE_IS_NAN(DP_NAN) < 0_8)
PRINT *,'NEGATIVE_IS_NAN(DP_NAN)', FOUND_NAN
FOUND_NAN = FAST_IS_NAN(DP_NAN)
PRINT *,'FAST_IS_NAN(DP_NAN)', FOUND_NAN
! Make pass without report to precondition cache to some stable state
COUNT_NANS = 0
DO I=1,N
SP(I) = SP(I) * 1.00001
IF(IEEE_IS_NAN(SP(I))) COUNT_NANS = COUNT_NANS + 1
END DO
! Now timed passes
T0 = OMP_GET_WTIME()
DO J=1, reps
COUNT_NANS = 0
DO I=1,N
SP(I) = SP(I) * 1.00001
IF(IEEE_IS_NAN(SP(I))) COUNT_NANS = COUNT_NANS + 1
END DO
END DO
T1 = OMP_GET_WTIME()
PRINT *, "IEEE_IS_NAN(SP(I)) ", T1-T0, COUNT_NANS
! Make pass without report to precondition cache to some stable state
COUNT_NANS = 0
DO I=1,N
SP(I) = SP(I) * 1.00001
IF(NEGATIVE_IS_NAN(SP(I)) < 0) COUNT_NANS = COUNT_NANS + 1
END DO
! Now timed passes
T0 = OMP_GET_WTIME()
DO J=1, reps
COUNT_NANS = 0
DO I=1,N
SP(I) = SP(I) * 1.00001
IF(NEGATIVE_IS_NAN(SP(I)) < 0) COUNT_NANS = COUNT_NANS + 1
END DO
END DO
T1 = OMP_GET_WTIME()
PRINT *, "NEGATIVE_IS_NAN(SP(I)) < 0 ", T1-T0, COUNT_NANS
! Make pass without report to precondition cache to some stable state
COUNT_NANS = 0
DO I=1,N
SP(I) = SP(I) * 1.00001
IF(IEEE_IS_NAN(SP(I))) COUNT_NANS = COUNT_NANS + 1
END DO
! Now timed passes
T0 = OMP_GET_WTIME()
DO J=1, reps
COUNT_NANS = 0
BLOCK
INTEGER :: COUNT_NANS_4(0:15)
INTEGER :: J
COUNT_NANS_4 = 0
DO I=1,N - MOD(N,16), 16
DO J=0,15
SP(I+J) = SP(I+J) * 1.00001
IF(NEGATIVE_IS_NAN(SP(I+J)) < 0) COUNT_NANS_4(J) = COUNT_NANS_4(J) + 1
END DO
END DO
DO J=0,15
COUNT_NANS = COUNT_NANS + COUNT_NANS_4(J)
END DO
DO I=N-MOD(N,16)+1,N
SP(I) = SP(I) * 1.00001
IF(NEGATIVE_IS_NAN(SP(I)) < 0) COUNT_NANS = COUNT_NANS + 1
END DO
END BLOCK
END DO
T1 = OMP_GET_WTIME()
PRINT *, "NEGATIVE_IS_NAN(SP(I)) COUNT_NANS_4", T1-T0, COUNT_NANS
! Make pass without report to precondition cache to some stable state
COUNT_NANS = 0
DO I=1,N
SP(I) = SP(I) * 1.00001
IF(FAST_IS_NAN(SP(I))) COUNT_NANS = COUNT_NANS + 1
END DO
! Now timed passes
T0 = OMP_GET_WTIME()
DO J=1, reps
COUNT_NANS = 0
DO I=1,N
SP(I) = SP(I) * 1.00001
IF(FAST_IS_NAN(SP(I))) COUNT_NANS = COUNT_NANS + 1
END DO
END DO
T1 = OMP_GET_WTIME()
PRINT *, "FAST_IS_NAN(SP(I)) ", T1-T0, COUNT_NANS
! Make pass without report to precondition cache to some stable state
COUNT_NANS = 0
DO I=1,N
DP(I) = DP(I) * 1.00001_8
IF(IEEE_IS_NAN(DP(I))) COUNT_NANS = COUNT_NANS + 1
END DO
! Now timed passes
T0 = OMP_GET_WTIME()
DO J=1, reps
COUNT_NANS = 0
DO I=1,N
DP(I) = DP(I) * 1.00001_8
IF(IEEE_IS_NAN(DP(I))) COUNT_NANS = COUNT_NANS + 1
END DO
END DO
T1 = OMP_GET_WTIME()
PRINT *, "IEEE_IS_NAN(DP(I)) ", T1-T0, COUNT_NANS
! Make pass without report to precondition cache to some stable state
COUNT_NANS = 0
DO I=1,N
DP(I) = DP(I) * 1.00001_8
IF(NEGATIVE_IS_NAN(DP(I)) < 0_8) COUNT_NANS = COUNT_NANS + 1
END DO
! Now timed passes
T0 = OMP_GET_WTIME()
DO J=1, reps
COUNT_NANS = 0
DO I=1,N
DP(I) = DP(I) * 1.00001_8
IF(NEGATIVE_IS_NAN(DP(I)) < 0_8) COUNT_NANS = COUNT_NANS + 1
END DO
END DO
T1 = OMP_GET_WTIME()
PRINT *, "NEGATIVE_IS_NAN(DP(I)) < 0_8", T1-T0, COUNT_NANS
! Make pass without report to precondition cache to some stable state
COUNT_NANS = 0
DO I=1,N
DP(I) = DP(I) * 1.00001_8
IF(NEGATIVE_IS_NAN(DP(I)) < 0_8) COUNT_NANS = COUNT_NANS + 1
END DO
! Now timed passes
T0 = OMP_GET_WTIME()
DO J=1, reps
COUNT_NANS = 0
BLOCK
INTEGER(8) :: COUNT_NANS_8(0:7)
INTEGER :: J
COUNT_NANS_8 = 0_8
DO I=1,N - MOD(N,8), 8
DO J=0,7
DP(I+J) = DP(I+J) * 1.00001_8
IF(NEGATIVE_IS_NAN(DP(I+J)) < 0_8) COUNT_NANS_8(J) = COUNT_NANS_8(J) + 1_8
END DO
END DO
DO J=0,7
COUNT_NANS = COUNT_NANS + COUNT_NANS_8(J)
END DO
DO I=N-MOD(N,8)+1,N
DP(I) = DP(I) * 1.00001_8
IF(NEGATIVE_IS_NAN(DP(I)) < 0_8) COUNT_NANS = COUNT_NANS + 1
END DO
END BLOCK
END DO
T1 = OMP_GET_WTIME()
PRINT *, "NEGATIVE_IS_NAN(DP(I)) COUNT_NANS_8", T1-T0, COUNT_NANS
! Make pass without report to precondition cache to some stable state
FOUND_NAN = .FALSE.
DO I=1,N
DP(I) = DP(I) * 1.00001_8
IF(FAST_IS_NAN(DP(I))) FOUND_NAN = .TRUE.
END DO
! Now timed passes
T0 = OMP_GET_WTIME()
DO J=1, reps
COUNT_NANS = 0
DO I=1,N
DP(I) = DP(I) * 1.00001_8
IF(FAST_IS_NAN(DP(I))) COUNT_NANS = COUNT_NANS + 1
END DO
END DO
T1 = OMP_GET_WTIME()
PRINT *, "FAST_IS_NAN(DP(I))", T1-T0, COUNT_NANS
end program TestNaN
```

The above should get you started.

Jim Dempsey

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Test NaN on Core i7 2600K

```
IEEE_IS_NAN(SP_NAN) T
NEGATIVE_IS_NAN(SP_NAN) T
FAST_IS_NAN(SP_NAN) T
IEEE_IS_NAN(DP_NAN) T
NEGATIVE_IS_NAN(DP_NAN) T
FAST_IS_NAN(DP_NAN) T
IEEE_IS_NAN(SP(I)) 42.8022058000788 16763089
NEGATIVE_IS_NAN(SP(I)) < 0 4.22263870015740 16763089
NEGATIVE_IS_NAN(SP(I)) COUNT_NANS_4 2.59221769985743 16763089
FAST_IS_NAN(SP(I)) 12.2822704999708 16763089
IEEE_IS_NAN(DP(I)) 45.0686576000880 16628001
NEGATIVE_IS_NAN(DP(I)) < 0_8 9.07150449999608 16628001
NEGATIVE_IS_NAN(DP(I)) COUNT_NANS_8 5.25665029999800 16628001
FAST_IS_NAN(DP(I)) 12.9640327000525 16628001
```

Jim Dempsey

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I should mention that the compiler vectorized the COUNT_NANS_4 and COUNT_NANS_8.

Orignally written on system with AVX512F, but as run above on system with AVX (AVX1).

Jim Dempsey

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I haven't experimented with this...

integer and logical are interchangeable

You might try:

```
LOGICAL FUNCTION FAST_IS_NAN_SP(X)
REAL, INTENT(IN) :: X
FAST_IS_NAN_SP = ISHL(NEGATIVE_IS_NAN_SP(X), -31)
END FUNCTION FAST_IS_NAN_SP
LOGICAL FUNCTION FAST_IS_NAN_DP(X)
DOUBLE PRECISION, INTENT(IN) :: X
FAST_IS_NAN_DP = ISHL(NEGATIVE_IS_NAN_DP(X), -63)
END FUNCTION FAST_IS_NAN_DP
```

Try that, and report back.

In the codes that I used this in, I used the return of the NEGATIVE_IS_NAN as opposed to the LOGICAL.

Jim Dempsey

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If zero is a reasonable number, but you want to know it is calculated use a weird random number that is unlikely to set your array -- extremely unlikely

1.23456789

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@JohnNichols no, floating point equality tests are almost universally discouraged (unless comparing to 0.0), as the stored value goes through a discretization step when it is fetched to the floating point registers, and isn't guaranteed to exactly match your compile-time constant. You could define an explicit bit value, but then you might as well just use NANs.

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@ereisch noted Unfortunately, they are a little more frequent than usual, as this particular array we're initializing to NaN at startup (so as to not confuse with a "perfectly valid" 0.0). The code block in question is to find the first "unused" member of the array.

----------------------------------------------------------------------------------------------------------------

In the types of coding I work on as an engineer there are usually a valid range of numbers for the results.

It is then reasonably easy to pick a number to initialize that is miles away from the usual range that if it pops up, it is some form of error.

The problem with the NaN is occasionally you get them in the calculations - not often but statistically it is an annoying potential problem.

Either way there are challenges, but it is fun to follow this thread.

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So this brings up a related question -- is there a "standards compliant" method of initializing derived type members to a NAN value?

As pointed out earlier, the following is not compliant (but works with ifort):

```
TYPE, BIND(C) :: FOO_PACKET
REAL(KIND=C_FLOAT) :: X = Z'FFFFFFFF'
REAL(KIND=C_FLOAT) :: Y = Z'FFFFFFFF'
END TYPE
```

Similarly, the following end-around also seems illegal (though accepted in ifort), as BOZ literals are not allowed as arguments to 'transfer':

```
TYPE, BIND(C) :: FOO_PACKET
REAL(KIND=C_FLOAT) :: X = TRANSFER(Z'FFFFFFFF', 1.0)
REAL(KIND=C_FLOAT) :: Y = TRANSFER(Z'FFFFFFFF', 1.0)
END TYPE
```

I read that there *might* be a compliant way to override the default initializer, and then BOZ literals might be able to used inside that (since it would just be a regular function), but I can't find a good documentation source on how to do this. Would the derived type then need to be wrapped up in a module to do this, or is there a less intrusive way of going about it?

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Taking you code and placing it in a standard program

```
program Console11
use Base
implicit none
real X1
logical yes
TYPE, BIND(C) :: FOO_PACKET
REAL(KIND=dp) :: X = Z'FFFFFFFF'
REAL(KIND=dp) :: Y = Z'FFFFFFFF'
END TYPE
Type(FOO_PACKET) :: P1
yes = .false.
X1 = 1.0
! Variables
! Body of Console11
print *, 'Hello World'
yes = ISNAN(X1)
print *,yes
yes = ISNAN(P1%X)
print *,yes
end program Console11
```

The X and Y are assigned as

Name | Value | Type | |
---|---|---|---|

P1%X | 2.121995790471207D-314 | REAL(8) |

Did I make a mistake in understanding what you are saying?

if I set the z'FFF to an integer i it is equal to -1.

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If I do not assign values the type has two zero values. You can never be sure what Intel Fort will set variables to if you just create them -- it is a pain when you bring in old programs that assumed some value.

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