[PATCH] [Review Request] Improve getVTList() in SelectionDAG

[Owen Anderson]

OK, I can see that it works.  It seems like you’re basically implementing the same idiom as the existing FoldingSet class (http://llvm.org/docs/ProgrammersManual.html#llvm-adt-foldingset-h).  Can you see if you could make use of that rather than rolling your own solution?

—Owen

On Oct 10, 2013, at 10:26 PM, Wan, Xiaofei <xiaofei.wan at intel.com> wrote:

> Owen:
> 
> Thanks for your comments. 
> Patch updated http://llvm-reviews.chandlerc.com/D1127
> 
> SDVTList SelectionDAG::getVTList(EVT VT1, EVT VT2) {
>  const EVT tmpVTs[2] = {VT1, VT2};
>  return getVTList(tmpVTs, 2);
> }
> How does the lifetime of this array work out?  It looks like SDVTListInfo captures the pointer to the EVT array, which was stack allocated.  After this method returns, you're going to have a dangling pointer in the SDVTListInfo.
> [Xiaofei] Yes, tmpVTs is an array on the stack, but it is just used temporally to determine whether this SDVTList  exist in the DenseMap, it won't be referenced anymore after that.
> 
> SDVTList SelectionDAG::getVTList(const EVT *VTs, unsigned NumVTs) {
>  SDVTListInfo Key(VTs, NumVTs);
>  if (VTListMap.count(Key))
>    return VTListMap[Key];
> 
>  EVT *Array = Allocator.Allocate<EVT>(NumVTs);
>  std::copy(VTs, VTs + NumVTs, Array); -------------------------------------------------VTs is copied out to Array which will be stored in the DenseMap
>  SDVTList Result = makeVTList(Array, NumVTs);
>  Key.pVTs = Array; --------------------------------------------------VTs is not used any more.
>  VTListMap.insert(std::make_pair(Key, Result));
> 
>  return Result;
> }
> 
> Thanks
> Wan Xiaofei
> 
> -----Original Message-----
> From: Owen Anderson [mailto:resistor at mac.com] 
> Sent: Friday, October 11, 2013 1:30 AM
> To: Wan, Xiaofei
> Cc: llvm-commits at cs.uiuc.edu LLVM
> Subject: Re: [PATCH] [Review Request] Improve getVTList() in SelectionDAG
> 
> Some feedback:
> 
>> +  /// getEmptyKey() - A private constructor that returns an unknown 
>> + that is  /// not equal to the tombstone key or SDVTListInfo().
>> +  static SDVTListInfo getEmptyKey() {
>> +    SDVTListInfo SDVTInfo;
>> +    SDVTInfo.NumVTs = 0;
>> +    return SDVTInfo;
>> +  }
>> +
>> +  /// getTombstoneKey() - A private constructor that returns an 
>> + unknown that  /// is not equal to the empty key or SDVTListInfo().
>> +  static SDVTListInfo getTombstoneKey() {
>> +    SDVTListInfo SDVTInfo;
>> +    SDVTInfo.NumVTs = 0;
>> +    return SDVTInfo;
>> +  }
> 
> I don't think this is a valid implementation of the DenseMapInfo callbacks.  getEmptyKey() and getTombstoneKey() have to return values that don't compare equal to each other.
> 
>> +  const EVT pVTs[2] = {VT1, VT2};
>> +  return getVTList(pVTs, 2);
> 
> How does the lifetime of this array work out?  It looks like SDVTListInfo captures the pointer to the EVT array, which was stack allocated.  After this method returns, you're going to have a dangling pointer in the SDVTListInfo.
> 
> -Owen
> 
> <D1127.5.patch>

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