[PATCH 03/11] Record a pointer to the AliasSet

[Tobias Grosser]

On 10/06/2014 13:41, Andreas Simbuerger wrote:
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> On 06/10/2014 09:21 AM, Tobias Grosser wrote:
>> On 09/06/2014 02:42, Andreas Simbuerger wrote:
>>> Instead of taking the pointer of the AliasSet reference we
>>> directly retreive a pointer from the AliasSetTracker. This gives
>>> us a pointer that should be valid at least until the
>>> ScopDetection is destroyed. --- lib/Analysis/ScopDetection.cpp |
>>> 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/lib/Analysis/ScopDetection.cpp
>>> b/lib/Analysis/ScopDetection.cpp index cab7af7..fbaf07c 100644
>>> --- a/lib/Analysis/ScopDetection.cpp +++
>>> b/lib/Analysis/ScopDetection.cpp @@ -471,8 +471,15 @@ bool
>>> ScopDetection::isValidMemoryAccess(Instruction &Inst, // alias,
>>> if -basicaa is not available. They actually do not, but as we
>>> can // not proof this without -basicaa we would fail. We disable
>>> this check to // not cause irrelevant verification failures. -
>>> if (!AS.isMustAlias()) -    return invalid<ReportAlias>(Context,
>>> /*Assert=*/true, &Inst, &AS); +  if (!AS.isMustAlias()) { +    //
>>> If we want to be able to use this for diagnostics outside of +
>>> // ScopDetection, we need to fetch the AliasSet as a pointer as
>>> soon as it +    // is created above. +    AliasSet *ASPtr =
>>> Context.AST.getAliasSetForPointerIfExists( +        BaseValue,
>>> AliasAnalysis::UnknownSize, +
>>> Inst.getMetadata(LLVMContext::MD_tbaa)); +    return
>>> invalid<ReportAlias>(Context, /*Assert=*/false, &Inst, ASPtr); +
>>> }
>>
>> I am slightly confused. Are you saying this will yield another
>> pointer than deriving the pointer via '&AS'? Why is this the case?
>> Looking at the implementations of getAliasSetForPointer() and
>> getAliasSetForPointerIfExists() both call findAliasSetForPointer(),
>> the first returning the plain pointer, the second dereferencing it
>> and returning it as reference.
>
>
> Ah you're right. For some odd reason it fixed a problem in my jit
> compiler. However, this is not the case anymore, weird.
>
> The problem here is that the reference is only valid as long as the
> DetectionContext is valid. So this means a segfault for any user of
> the RejectLog, when he tries to get the debug message.

The pointer we pass to the DiagnosticReport points to an AliasSet
and should be independent of the DetectionContext. Thoigh, tt may still 
become invalid for other reasons. A test case might be helpful here.

Tobias