#pragma

[uunet!ai.mit.edu!rms]

    >> Consider: a #pragma that changes the semantics of a program will be
    >> implemented only by certain compilers.  The program will work
    >> incorrectly on other ANSI C systems.

    In the Fortran universe, there are many counterexamples to this.
    Consider the very common...

That part of my message was about uses of #pragma that change the
semantics.

I can't understand from your example exactly what `c$dir ivdep' is
supposed to tell the compiler, but I get the impression that it only
tells the compiler it need not be cautious about some optimization.
If so, then this is analogous to a pragma that DOES NOT change the
meaning of the program.

In this sort of case, #pragma is more portable than an original
extension, but it still has the problem of failing to work with
macros.  If Fortran does not have macros, `c$dir ivdep' is a
reasonable solution in Fortran.  But the analogous thing in C does
have a problem.