🚀 go-pugleaf

In article <14334@lambda.UUCP> jlg@lambda.UUCP (Jim Giles) writes:

| From article <8960014@hpfcso.HP.COM>, by mjs@hpfcso.HP.COM (Marc Sabatella):
| > [...]
| >>II) Feature differences.
| > [...]
| > What about
| > c) math intrinsics and built-in I/O (including implied do-loops)
| 
| This is a discussion of implementation difficulty.  Math intrinsics CAN
| be implemented as procedure calls -just as C does.  Built-in I/O _IS_
| implemented with procedure calls on every system I've ever worked with
| - just as C does.  Implied do-loops are a red herring in this discussion:
| they are simply do loops that are completely simulated at compile time.
| What's so hard about that?
| 
| > [...]
| > d) equivalence
| 
| Already discussed in other postings.  In C, can you say 'union'?  I thought
| so.

NO.  Fortran equivalences are much harder to implement than unions.
Unions always start off at offset 0 from the start of the union.  In
FORTRAN, equivalences require much more support within the compiler,
in order to support things like equivalencing A[1] to B[100] and
equivalencing C to A[20].  Check out section 7.9 in the Red Dragon
book for more details.

| > [...]
| > e) run-time dimensionable arrays
| 
| Again, in the context of simplicity of implementation, these are no harder
| than statically declared arrays.  The dimension values are just variables
| instead of constants.  Now, in an _optimizing_ compiler, these dynamic
| array bounds limit constant folding to an extent (the bounds aren't constant).

Again no.  To implement arrays with static bounds, you only need to
keep the bounds (as integer constants) within the symbol table.  For
full run-time dimensionable arrays, you must keep around an indication
that the array is dynamic, and where the bound is stored.  You must
expand this bound expression within the tree as you are evaluating
arrays and such.  PL/I is much harder in this respect, since it allows
variable sized arrays within structures, and all members following the
array must be appropriately offset.

--
Michael Meissner	email: meissner@osf.org		phone: 617-621-8861
Open Software Foundation, 11 Cambridge Center, Cambridge, MA

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