[llvm-commits] [llvm] r89421 - /llvm/trunk/lib/Analysis/CaptureTracking.cpp

[Duncan Sands]

Hi Dan,

> +      // Don't count comparisons of the original value against null as captures.
> +      // This allows us to ignore comparisons of malloc results with null,
> +      // for example.
> +      if (isIdentifiedObject(V))
> +        if (ConstantPointerNull *CPN =
> +              dyn_cast<ConstantPointerNull>(I->getOperand(1)))
> +          if (CPN->getType()->getAddressSpace() == 0)
> +            break;

I think this is wrong, consider the following pseudocode example:

i1 equals_null (i8* %P noalias) {
   return %P == 0
}

Thanks to your change, this function will be marked nocapture, because
a noalias parameter counts as an identified object.

Now consider

i8* @capture (i8* %P noalias) {
   n = 0
   loop:
     %Q = GEP %P, -n
     if (equals_null(%Q))
       break;
     n = n + 1;
     goto loop;
   return GEP null, n
}

I think it is correct to pass %Q to equals_null given that %P is noalias.
So this example shows that %P can be captured even though CaptureTracking
says it is not with your change.  A variant of this is to have %P be some
noalias value like the return value of malloc.

Ciao,

Duncan.