[LLVMdev] Pointer aliasing issue

[Duncan Sands]

Hi Artiom,

> Unfortunately since in this example function arguments are array of pointers,
> __restrict doesn't help.

it does help, though perhaps not in the way you had in mind.  The IR changes
from (llvm-gcc at -O4)

define void @factorial(float** nocapture %a, float** nocapture %b) nounwind {
entry:
   %0 = getelementptr inbounds float** %b, i64 2
   %1 = load float** %0, align 8
   %2 = load float** %a, align 8
   %3 = getelementptr inbounds float* %2, i64 2
   %4 = load float* %3, align 4
   %5 = getelementptr inbounds float* %1, i64 2
   store float %4, float* %5, align 4
   %6 = load float** %0, align 8
   %7 = load float** %a, align 8
   %8 = getelementptr inbounds float* %7, i64 1
   %9 = load float* %8, align 4
   %10 = getelementptr inbounds float* %6, i64 2
   store float %9, float* %10, align 4
   ret void
}

to

define void @factorial(float** noalias nocapture %a, float** noalias nocapture 
%b) nounwind {
entry:
   %0 = getelementptr inbounds float** %b, i64 2
   %1 = load float** %0, align 8
   %2 = load float** %a, align 8
   %3 = getelementptr inbounds float* %2, i64 2
   %4 = load float* %3, align 4
   %5 = getelementptr inbounds float* %1, i64 2
   store float %4, float* %5, align 4
   %6 = getelementptr inbounds float* %2, i64 1
   %7 = load float* %6, align 4
   store float %7, float* %5, align 4
   ret void
}

Ciao,

Duncan.

> Yes, using 2 stores into same location is intentional.
>
> On Mon, Oct 11, 2010 at 12:20 PM, Duncan Sands <baldrick at free.fr
> <mailto:baldrick at free.fr>> wrote:
>
>     Hi Artiom,
>
>      > I have following C code:
>      >
>      >     void factorial(float **a, float **b)
>      >     {
>      >        b[2][2] = a[0][2];
>      >        b[2][2] = a[0][1];
>      >     }
>     ...
>      > Assuming that a and b arrays contains pointers that are not aliased,
>      > is it possible to remove the first store by using some C or LLVM IR specific
>      > declarations?
>
>     try this:
>
>     void factorial(float ** __restrict a, float ** __restrict b)
>     {
>        b[2][2] = a[0][2];
>        b[2][2] = a[0][1];
>     }
>
>     Ciao,
>
>     Duncan.
>
>     PS: Did you mean to use b[2][2] in both lines?
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>
>
>
> --
> Artiom