[LLVMdev] The use iterator not working...

[Daniel Berlin]

It has, AFAIK, always been this way.
It is also a common source of bugs, as I believe you have discovered.

If we had  llvm-specific "clang warnings", this would be one i added
(IE "Dereferencing use iterator of x gives x") :)

In the past 6 months, i did this twice by accident in passes and then
spent hours tracking it down.

It actually makes me wonder whether use_iterator should even have an
operator * for the use_iterator_impl<Use> case. ISTM to basically
always be a bug (though this is an idle thought, i haven't thought
through the implications at all).



On Tue, Jun 9, 2015 at 7:54 PM, Zack Waters <zswaters at gmail.com> wrote:
> Thanks Dan and Jon. I made an incorrect assumption that the "use" iterator
> was actually giving me the "user" when de-referencing it.
>
> Did it always have this behavior in previous LLVM versions? I've seen lots
> of examples of the "use" iterator being dereferenced and resulting
> Instruction pointer being treated as the "user"?
>
> Thanks,
> Zack
>
>
> On Tue, Jun 9, 2015 at 7:25 PM, Jonathan Roelofs <jonathan at codesourcery.com>
> wrote:
>>
>>
>>
>> On 6/9/15 8:02 PM, Zack Waters wrote:
>>>
>>> Hi,
>>>
>>> I'm having a problem with the use iterator. Each "use" that I see, when
>>> using the use_iterator, is the same as the "def". Meaning, in the code
>>> below the pDef is always equal to pUse pointer for every instruction in
>>> all basic blocks (except terminators).
>>>
>>>              for (auto i = inst_begin(f), ie = inst_end(f); i != ie; ++i)
>>>                  Instruction* pDef = &(*i);
>>>                  errs() << "Def: " << *pDef << "\n";
>>>
>>>                  for (auto ui = pDef->use_begin(), uie =
>>> pDef->use_end(); ui != uie; ++ui)
>>>                  {
>>
>>
>> 'user' != 'use'.
>>
>> Think of llvm::Use as the edge between the place where a value is
>> produced, and the place where that value is consumed. The consumer is the
>> 'User', and the Use points at it.
>>
>> http://llvm.org/docs/doxygen/html/classllvm_1_1Use.html
>>
>> The confusing thing that's happening below is that the llvm::Use is
>> implicitly converted via `llvm::Use::operator Value *() const` to a
>> `Value*`, and that `Value*` is `pDef`.
>>
>>
>> HTH,
>>
>> Jon
>>
>>>                      Instruction* pUse = dyn_cast<Instruction>(*ui);
>>>                      errs() << "  Use: \t" << *pUse << "\n";
>>>                  }
>>>              }
>>>
>>> However, everything works as expected when using the range-based use
>>> iterator with the following code.
>>>
>>>              for (auto i = inst_begin(f), ie = inst_end(f); i != ie; ++i)
>>>              {
>>>                  Instruction* pDef = &(*i);
>>>                  errs() << "Def: " << *pDef << "\n";
>>>
>>>                  for (User* pUser : pDef->users())
>>>                  {
>>>                      Instruction* pUse = dyn_cast<Instruction>(pUser);
>>>                      errs() << "  Use: \t" << *pUse << "\n";
>>>                  }
>>>              }
>>>
>>> Also, the code is executed inside a function pass. So was initially
>>> thinking I somehow screwed up the use information in a previous pass.
>>> However, I would assume the range-based iterator would not work as well
>>> but it does.
>>>
>>> Finally, I'm currently using LLVM 3.5.1 built for Windows. Google hasn't
>>> been much help. Anybody have any suggestions as to why the first example
>>> above doesn't work?
>>>
>>> Thanks,
>>> Zack
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>
>> --
>> Jon Roelofs
>> jonathan at codesourcery.com
>> CodeSourcery / Mentor Embedded
>
>
>
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