🚀 go-pugleaf

Another thread I probably should avoid....

  Suppose that I have a FORTRAN subroutine which takes two parameters
which it may modify, X and Y.  Assume that the array A is declared
somehow; exactly how isn't important.  Suppose you have the code:

	SUBROUTINE BLAH(X,Y)
	INTEGER X,Y
	X = 3
	Y = 2
	IF (A(X) .EQ. 0) ...

  Translating this to C, you get:

	blah(int *x, int *y)
	{
	  *x = 3;
	  *y = 2;
	  if (a(*x) == 0) ...

  A compiler for FORTRAN can optimize the test to be A(3) since it
knows that the assignment to Y can never change X.  A compiler for C,
in the absence of interprocedural aliasing information, cannot do the
same.

  The interprocedural aliasing problem is NP-hard in the presence of
multi-level pointers or reference parameters (sorry, my refs are at
home right now, but if any cares e-mail me), and so various
approximation algorithms are used.  For any approximation algorithm, I
can write (pathological) FORTRAN code which will result in worse code
being generated by the C compiler.

	Anton
   
+---------------------------+------------------+-------------+
| Anton Rang (grad student) | rang@cs.wisc.edu | UW--Madison |
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