The ANSI FORTRAN 77 language defines generic and specific intrinsics. In short, the distinctions are:
Typically, a generic intrinsic has a return type that is determined by the type of one or more of its arguments.
The GNU Fortran language generalizes these concepts somewhat,
especially by providing intrinsic subroutines and generic
intrinsics that are treated as either a specific intrinsic subroutine
or a specific intrinsic function (e.g.
However, GNU Fortran avoids generalizing this concept to the point where existing code would be accepted as meaning something possibly different than what was intended.
ABS is a generic intrinsic, so all working
code written using
ABS of an
INTEGER return value.
Similarly, all such code expects that
ABS of an
argument returns an
INTEGER*2 return value.
IABS is a specific intrinsic that accepts only
Code that passes something other than an
IABS is not valid GNU Fortran code, because
it is not clear what the author intended.
For example, if `J' is
is not defined by the GNU Fortran language, because the programmer
might have used that construct to mean any of the following, subtly
INTEGER(KIND=1)first (as if `IABS(INT(J))' had been written).
INTEGER(KIND=1)(as if `INT(ABS(J))' had been written).
The distinctions matter especially when types and values wider than
INTEGER(KIND=1) (such as
INTEGER(KIND=2)), or when
operations performing more “arithmetic” than absolute-value, are involved.
The following sample program is not a valid GNU Fortran program, but might be accepted by other compilers. If so, the output is likely to be revealing in terms of how a given compiler treats intrinsics (that normally are specific) when they are given arguments that do not conform to their stated requirements:
PROGRAM JCB002 C Version 1: C Modified 1999-02-15 (Burley) to delete my email address. C Modified 1997-05-21 (Burley) to accommodate compilers that implement C INT(I1-I2) as INT(I1)-INT(I2) given INTEGER*2 I1,I2. C C Version 0: C Written by James Craig Burley 1997-02-20. C C Purpose: C Determine how compilers handle non-standard IDIM C on INTEGER*2 operands, which presumably can be C extrapolated into understanding how the compiler C generally treats specific intrinsics that are passed C arguments not of the correct types. C C If your compiler implements INTEGER*2 and INTEGER C as the same type, change all INTEGER*2 below to C INTEGER*1. C INTEGER*2 I0, I4 INTEGER I1, I2, I3 INTEGER*2 ISMALL, ILARGE INTEGER*2 ITOOLG, ITWO INTEGER*2 ITMP LOGICAL L2, L3, L4 C C Find smallest INTEGER*2 number. C ISMALL=0 10 I0 = ISMALL-1 IF ((I0 .GE. ISMALL) .OR. (I0+1 .NE. ISMALL)) GOTO 20 ISMALL = I0 GOTO 10 20 CONTINUE C C Find largest INTEGER*2 number. C ILARGE=0 30 I0 = ILARGE+1 IF ((I0 .LE. ILARGE) .OR. (I0-1 .NE. ILARGE)) GOTO 40 ILARGE = I0 GOTO 30 40 CONTINUE C C Multiplying by two adds stress to the situation. C ITWO = 2 C C Need a number that, added to -2, is too wide to fit in I*2. C ITOOLG = ISMALL C C Use IDIM the straightforward way. C I1 = IDIM (ILARGE, ISMALL) * ITWO + ITOOLG C C Calculate result for first interpretation. C I2 = (INT (ILARGE) - INT (ISMALL)) * ITWO + ITOOLG C C Calculate result for second interpretation. C ITMP = ILARGE - ISMALL I3 = (INT (ITMP)) * ITWO + ITOOLG C C Calculate result for third interpretation. C I4 = (ILARGE - ISMALL) * ITWO + ITOOLG C C Print results. C PRINT *, 'ILARGE=', ILARGE PRINT *, 'ITWO=', ITWO PRINT *, 'ITOOLG=', ITOOLG PRINT *, 'ISMALL=', ISMALL PRINT *, 'I1=', I1 PRINT *, 'I2=', I2 PRINT *, 'I3=', I3 PRINT *, 'I4=', I4 PRINT * L2 = (I1 .EQ. I2) L3 = (I1 .EQ. I3) L4 = (I1 .EQ. I4) IF (L2 .AND. .NOT.L3 .AND. .NOT.L4) THEN PRINT *, 'Interp 1: IDIM(I*2,I*2) => IDIM(INT(I*2),INT(I*2))' STOP END IF IF (L3 .AND. .NOT.L2 .AND. .NOT.L4) THEN PRINT *, 'Interp 2: IDIM(I*2,I*2) => INT(DIM(I*2,I*2))' STOP END IF IF (L4 .AND. .NOT.L2 .AND. .NOT.L3) THEN PRINT *, 'Interp 3: IDIM(I*2,I*2) => DIM(I*2,I*2)' STOP END IF PRINT *, 'Results need careful analysis.' END
No future version of the GNU Fortran language
will likely permit specific intrinsic invocations with wrong-typed
arguments (such as
IDIM in the above example), since
it has been determined that disagreements exist among
many production compilers on the interpretation of
These disagreements strongly suggest that Fortran programmers,
and certainly existing Fortran programs, disagree about the
meaning of such invocations.
The first version of
JCB002 didn't accommodate some compilers'
treatment of `INT(I1-I2)' where `I1' and `I2' are
In such a case, these compilers apparently convert both
INTEGER*4 and then do an
instead of doing an
INTEGER*2 subtraction on the
original values in `I1' and `I2'.
However, the results of the careful analyses done on the outputs of programs compiled by these various compilers show that they all implement either `Interp 1' or `Interp 2' above.
Specifically, it is believed that the new version of
above will confirm that:
If you get different results than the above for the stated compilers, or have results for other compilers that might be worth adding to the above list, please let us know the details (compiler product, version, machine, results, and so on).