The Argument Alias Problem

- In Fortran, it is a mistake to invoke copy with overlapping arguments. The compiler will perform optimizations assuming A and B are not aliases over the computational range.
- In C, argument aliases are allowed. Therefore optimizations (SWP) changing the original order of loads and stores are not possible. There are several ways to remove this restriction:
  - the ivdep pragma
  - the compiler optimization flag: -OPT:alias=memory-access-model
  - the restrict keyword

SUBROUTINE COPY(A,B,N)

REAL*8 A(N),B(N)

DO I=1,N

B(I) = A(I)

ENDDO

END

void copy(double *a, double *b, int n)

{ int i;

for(i=1; i<n; i++) b[i] = a[i];

}

In Fortran, compiler assumes

A and B do not overlap

In C, compiler assumes pointers a

and b can point to the same address