[LLVMdev] TBAA: Propagating aliasing information to derived pointers

[Johan Tibell]

On Wed, Jan 18, 2012 at 6:22 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Wed, Jan 18, 2012 at 6:13 PM, Johan Tibell <johan.tibell at gmail.com> wrote:
>> While adding support for TBAA to GHC's LLVM backend, we ran into the
>> following issue: given a pointer that we know has certain aliasing
>> properties (e.g. it points to the stack, not the heap) we'd like to
>>
>>  * annotate all loads and stores to that pointer with a TBAA type "stack", and
>>  * also annotate all loads and stores to pointers derived from that
>> pointer as "stack".
>>
>> For example, given Sp, which is the GHC stack pointer, and this pseudo-code
>>
>>    tmp = Sp + 4
>>    tmp2 = load tmp  // should be marked as "stack"
>>
>> we'd like to not only annotate loads/stores to Sp as "stack" but also
>> all loads/stores to tmp.
>>
>> Is this something that can be achieved within the current framework or
>> should we propagate the pointer aliasing information ourselves?
>
> In the current TBAA implementation, the marking only applies to the
> given load; it doesn't give any special properties to the loaded
> pointer.

Is there a particular reason the current implementation doesn't do it
(i.e. is it unsafe in general?) or is it just that no one has gotten
around to implementing it?

Cheers,
Johan