[LLVMdev] AliasAnalysis Documentation Ambiguity

[Thomas B. Jablin]

Hi,

The description of the alias method in AliasAnalysis reads:

alias - The main low level interface to the alias analysis implementation. Returns a Result indicating whether the two pointers are aliased to each other. This is the interface that must be implemented by specific alias analysis implementations. 

This implies that alias can run on ANY two Values without restrictions. However, line 696 of BasicAliasAnalysis reads:

// Arguments can't alias with local allocations or noalias calls.
if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
    (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))
  return NoAlias;

This reasoning is only valid if the Argument and the AllocaInst or NoAliasCall in question are part of the same function. Here is an example:

void foo(int *bar) {
}

void baz(void) {
 int qux[10];
 foo(qux);
}

In the above example, qux in baz MayAlias with bar in foo. However, BasicAliasAnalysis will return NoAlias.

Should there be a check in BasicAliasAnalysis to ensure that O1 and O2 belong to the same function, or should the documentation be updated to indicate the limits of the alias method? Thanks.

Tom