[LLVMdev] RFC: implicit null checks in llvm

[Andrew Trick]

> On Apr 24, 2015, at 4:14 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
> 
> I don't think we can expose the memory operations directly from a
> semantic, theoretical point of view.  Whether practically we can do
> this or not is a different question.
> 
> Does LLVM do optimizations like these at the machine instruction
> level?
> 
> 
>   if (condition)
>     T = *X  // normal load, condition guards against null
> 
>   EH_LABEL // clobbers all
>   U = *X  // implicit null check, branches out on fault
>   EH_LABEL // clobbers all
>   ...
> 
> =>
> 
>  since the second "load" from X always happens, X must be
>  dereferenceable
> 
> 
>   T = *X  // miscompile here
> 
>   EH_LABEL // clobbers all
>   U = *X  // implicit null check, branches out on fault
>   EH_LABEL // clobbers all
>   ...
> 
> The fundamental problem, of course, is that we're hiding the real
> control flow which is
> 
> if (!is_dereferenceable(X))  branch_out;
> U = *X

That’s a good description of the problem.

Lowering to real loads will *probably* just work because your are being saved by EH_LABEL instructions which are conservatively modeled as having unknown side effects. The feature that saves you will also defeat optimization of those loads. I don't see any advantage of this in terms of optimizing codegen. It is just a workaround to avoid defining pseudo instructions.

The optimal implementation would be to leave the explicit null check in place. Late in the pipeline, just before post-ra scheduling, a pass would combine and+cmp+br+load when it is profitable using target hooks like getLdStBaseRegImmOfsWidth(). Note that we still have alias information in the form of machine mem operands.

You could take a step in that direction without doing much backend work by lowering to pseudo-loads during ISEL instead of using EH_LABEL. Then the various load/store optimizations could be taught to explicitly optimize normal loads and stores over the pseudo loads but not among them.

Andy