[polly] r225326 - Do not use a const Twine here

[Mehdi Amini]

> On Jan 6, 2015, at 4:39 PM, Tobias Grosser <tobias at grosser.es> wrote:
> 
> On 07.01.2015 01:33, David Blaikie wrote:
>> 
>> 
>> On Tue, Jan 6, 2015 at 4:30 PM, Tobias Grosser <tobias at grosser.es
>> <mailto:tobias at grosser.es>> wrote:
>> 
>>    Author: grosser
>>    Date: Tue Jan  6 18:30:01 2015
>>    New Revision: 225326
>> 
>>    URL: http://llvm.org/viewvc/llvm-project?rev=225326&view=rev
>>    Log:
>>    Do not use a const Twine here
>> 
>>    This has caused segfaults when using Polly in the context of Julia,
>>    that are
>>    not reproducible on my machine in 'make check-polly'.
>> 
>>    Modified:
>>         polly/trunk/lib/CodeGen/IslExprBuilder.cpp
>> 
>>    Modified: polly/trunk/lib/CodeGen/IslExprBuilder.cpp
>>    URL:
>>    http://llvm.org/viewvc/llvm-project/polly/trunk/lib/CodeGen/IslExprBuilder.cpp?rev=225326&r1=225325&r2=225326&view=diff
>>    ==============================================================================
>>    --- polly/trunk/lib/CodeGen/IslExprBuilder.cpp (original)
>>    +++ polly/trunk/lib/CodeGen/IslExprBuilder.cpp Tue Jan  6 18:30:01 2015
>>    @@ -112,7 +112,7 @@ Value *IslExprBuilder::createAccessAddre
>>        const ScopArrayInfo *SAI = ScopArrayInfo::getFromId(BaseId);
>>        Base = SAI->getBasePtr();
>>        assert(Base->getType()->isPointerTy() && "Access base should be
>>    a pointer");
>>    -  const Twine &BaseName = Base->getName();
>>    +  auto BaseName = Base->getName();
>> 
>> 
>> So what type does getName return? StringRef?
> 
> Base is an llvm::Value and llvm::Value's getName() function returns a StringRef, yes.
> 
> Are you saying I should not use 'auto' here? Or do you have an explanation for the memory corruption I am seeing?

Isn’t it possible that the issue was taking a Twine by reference?
I mean the return object is a temporary StringRef, it will be destroyed unless bound to a temporary. However there is a Twine constructor which is called for the conversion here, so the temp is bound to the argument of the Twine ctor and its lifetime ends with the ctor.
You Twine reference a destroyed StringRef after that.

Mehdi



> 
> Cheers,
> Tobias
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