[LLVMdev] Pointer aliasing

[Roel Jordans]

Hi Brent,

Looking at your code I can see at least one reason why some of the store 
operations remain in the output since you are (through x, y, and z) 
writing in memory which exists outside of your function (p).

Constant propagation also seems to work in the first few lines, *y = *x 
+1 (%3) is stored directly.

The strange thing to me is that the same doesn't happen for *z = *x + 2. 
Here *x is loaded again and the addition is still performed...

 From this point on, constant propagation seems to stop working 
completely. Looking at the IR I would have expected something like the 
following:

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
    store double 4.000000e+00, double* %5, align 8, !tbaa !3
    ret double 8.000000e+00
}

!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}

I am afraid I can't really help you in telling what went wrong here and 
caused the missing optimizations, but I agree with you that the result 
in not what I would have expected.

Cheers,
Roel
On 01/23/2012 03:31 AM, Brent Walker wrote:
> 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
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