[LLVMdev] Pointer aliasing

[Brent Walker]

Hi LLVMers,
I would like to ask a question regarding aliasing.  Suppose I have the
following program:

double f(double** p )
{
   double a,b,c;
   double * x = &a;
   double * y = &b;
   double * z = &c;

   *x = 1;
   *y = *x + 2;
   *z = *x + 3;

   return *x+*y+*z;
}

LLVM can tell that the three pointers do not alias each other so can
perform the constant folding at compile time.

define double @f(double** nocapture %p) nounwind uwtable readnone {
  ret double 8.000000e+00
}

Now consider the function below.  I know (in my particluar case) that
the pointers in the p array do not alias each other.  I tried to
communicate this information to llvm/clang via the __restrict__
qualifier but it does not seem to have an effect.  Can you please
suggest what is wrong.  How can I achieve what I want?

double f(double** p )
{
   double *__restrict__ x = p[0];
   double *__restrict__ y = p[1];
   double *__restrict__ z = p[2];

   *x = 1;
   *y = *x + 2;
   *z = *x + 3;

   return *x+*y+*z;
}

define double @f(double** nocapture %p) nounwind uwtable {
  %1 = load double** %p, align 8, !tbaa !0
  %2 = getelementptr inbounds double** %p, i64 1
  %3 = load double** %2, align 8, !tbaa !0
  %4 = getelementptr inbounds double** %p, i64 2
  %5 = load double** %4, align 8, !tbaa !0
  store double 1.000000e+00, double* %1, align 8, !tbaa !3
  store double 3.000000e+00, double* %3, align 8, !tbaa !3
  %6 = load double* %1, align 8, !tbaa !3
  %7 = fadd double %6, 3.000000e+00
  store double %7, double* %5, align 8, !tbaa !3
  %8 = load double* %1, align 8, !tbaa !3
  %9 = load double* %3, align 8, !tbaa !3
  %10 = fadd double %8, %9
  %11 = fadd double %10, %7
  ret double %11
}

!0 = metadata !{metadata !"any pointer", metadata !1}
!1 = metadata !{metadata !"omnipotent char", metadata !2}
!2 = metadata !{metadata !"Simple C/C++ TBAA", null}
!3 = metadata !{metadata !"double", metadata !1}

Thank you for any help.

Brent