[llvm] r183532 - R600: Fix a potential iterator invalidation issue.

[Vincent Lejeune]

Ok, I though vector::erase(Iterator) left vector untouched if Iterator was equal to vector::end()


----- Mail original -----
> De : Benjamin Kramer <benny.kra at gmail.com>
> À : Vincent Lejeune <vljn at ovi.com>
> Cc : llvm-commits at cs.uiuc.edu
> Envoyé le : Vendredi 7 juin 2013 18h37
> Objet : Re: [llvm] r183532 - R600: Fix a potential iterator invalidation issue.
> 
> 
> On 07.06.2013, at 18:27, Vincent Lejeune <vljn at ovi.com> wrote:
> 
>>  Hi,
>> 
>>  I initially wrote the code almost like this way, using std::find instead of
>>  std::remove (because there is at most one element that is equal to chan in 
> UpdatedUndef) but
>>  I got memory corruption error in some llvm IR.
> 
> If there's at most one equal element in the vector something like
> 
>     std::vector<unsigned>::iterator ChanPos =
>         std::find(UpdatedUndef.begin(), UpdatedUndef.end(), Chan);
>     if (ChanPos != UpdatedUndef.end())
>       UpdatedUndef.erase(ChanPos);
> 
> should work. It would be a little bit faster because std::find exit early. The 
> std::remove sequence is equivalent to the previous code but avoids the iterator 
> invalidation issue.
> 
> - Ben
> 
>> 
>>  Vincent
>> 
>>  ----- Mail original -----
>>>  De : Benjamin Kramer <benny.kra at googlemail.com>
>>>  À : llvm-commits at cs.uiuc.edu
>>>  Cc : 
>>>  Envoyé le : Vendredi 7 juin 2013 18h13
>>>  Objet : [llvm] r183532 - R600: Fix a potential iterator invalidation 
> issue.
>>> 
>>>  Author: d0k
>>>  Date: Fri Jun  7 11:13:49 2013
>>>  New Revision: 183532
>>> 
>>>  URL: http://llvm.org/viewvc/llvm-project?rev=183532&view=rev
>>>  Log:
>>>  R600: Fix a potential iterator invalidation issue.
>>> 
>>>  As a bonus this reduces the loop from O(n^2) to O(n).
>>> 
>>>  Modified:
>>>      llvm/trunk/lib/Target/R600/R600OptimizeVectorRegisters.cpp
>>> 
>>>  Modified: llvm/trunk/lib/Target/R600/R600OptimizeVectorRegisters.cpp
>>>  URL: 
>>> 
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/R600/R600OptimizeVectorRegisters.cpp?rev=183532&r1=183531&r2=183532&view=diff
>>> 
> ==============================================================================
>>>  --- llvm/trunk/lib/Target/R600/R600OptimizeVectorRegisters.cpp 
> (original)
>>>  +++ llvm/trunk/lib/Target/R600/R600OptimizeVectorRegisters.cpp Fri Jun  
> 7 
>>>  11:13:49 2013
>>>  @@ -198,11 +198,9 @@ MachineInstr *R600VectorRegMerger::Rebui
>>>           .addReg(SubReg)
>>>           .addImm(Chan);
>>>       UpdatedRegToChan[SubReg] = Chan;
>>>  -    for (std::vector<unsigned>::iterator RemoveIt = 
> UpdatedUndef.begin(),
>>>  -        RemoveE = UpdatedUndef.end(); RemoveIt != RemoveE; ++ 
> RemoveIt) {
>>>  -      if (*RemoveIt == Chan)
>>>  -        UpdatedUndef.erase(RemoveIt);
>>>  -    }
>>>  +    UpdatedUndef.erase(
>>>  +        std::remove(UpdatedUndef.begin(), UpdatedUndef.end(), Chan),
>>>  +        UpdatedUndef.end());
>>>       DEBUG(dbgs() << "    ->"; Tmp->dump(););
>>>       (void)Tmp;
>>>       SrcVec = DstReg;
>>> 
>>> 
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>