[LLVMdev] AliasSetTracker and UnknownInst's (callsites mostly) problem

[Philip Reames]

This was discussed a while back in the thread titled "alias set collapse 
and LICM".

Philip

On 07/28/2015 06:13 AM, Andrew Zhogin wrote:
> Hi all,
>
> There is a problem about how AliasSetTracker merging AliasSet's when 
> meet UnknownInst.
>
> When adding new pointer it looks for existing AliasSet's aliased with 
> new pointer. And merging them together. It is ok for pointers: if %A 
> mayalias %B and %B mayalias %C then %A mayalias %C.
>
> But the same logic when adding callsite is wrong 
> (findAliasSetForUnknownInst).
>
> Callsite may be known to readonly access one pointer and change the 
> content of other pointer.
> Why to merge together two noalias pointers if they have some mod/ref 
> relationships with common callsite?
>
> If modrefinfo(%A, %call1) == REF and modrefinfo(%B, %call1) == MOD
> it doesn't mean %A may/must alias %B.
> AliasSet's for %A and %B must stay independent with corrected access 
> lattice: aliasset(%A).Access |= RefAccess, aliasset(%B).Access |= 
> ModAccess.
> And I think new UnknownInst must be added to both AliasSet's.
>
> Or maybe UnknownInst's must be collected in central part - 
> AliasSetTracker, not AliasSet's.
>
> Am I right?
>
> -- 
> Best regards,
> Andrew Zhogin
>
>
>
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