[LLVMdev] Program order in inst_iterator?

[Anirudh Sivaraman]

On Tue, Jun 16, 2015 at 6:30 AM, John Criswell <jtcriswel at gmail.com> wrote:
> On 6/16/15 1:09 AM, Nick Lewycky wrote:
>>
>> Anirudh Sivaraman wrote:
>>>
>>> On Mon, Jun 15, 2015 at 10:50 AM, mats petersson<mats at planetcatfish.com>
>>> wrote:
>>>>
>>>> It will iterate over the instructions in the order that they are stored
>>>> in
>>>> the module/function/basicblock that they belong to. And that SHOULD,
>>>> assuming llvm-dis does what it is expected to do, be the same order.
>>>>
>>>
>>> Thanks for the reply. What about instruction ordering across basic
>>> blocks? Let's say instructions IA and IB belong to basic blocks BBa
>>> and BBb, where BBa is before BBb in program order. Then, will IA be
>>> printed before IB?
>>
>>
>> Yes, llvm-dis does print in the same order that inst_iterator visits. The
>> llvm::Function has a linked list of llvm::BasicBlock objects. Both llvm-dis
>> and the inst_iterator will iterate over that list, and then for each basic
>> block iterate over the linked list of llvm::Instruction objects. Both of
>> these lists are ordered, and their order is preserved through .ll and .bc
>> files.
>>
>> However, I wouldn't call that program order, though I couldn't find a
>> definition of the term easily. The only guarantee on the order is that the
>> first block visited/printed is the entry block. It's entirely possible to
>> have a three block program where A branches to C branches to B, but will be
>> visited/printed in order A B C.
>
>
> More importantly, there's no guarantee that inst_iterator and llvm-dis will
> iterate over blocks in the same order.  It's possible (though unlikely) that
> llvm-dis could change to print instructions in a different order.
>
> If you need to examine instructions in a certain order in LLVM IR, you
> probably want to visit definitions before uses (for data flow) or visit
> basic blocks in an order consistent with the control-flow graph (for control
> flow).
>

Thanks for these responses. So, in summary, if I need a particular
order when visiting instructions or basic blocks, I need to do it
myself, without relying on the LLVM API to guarantee it.

Anirudh

> Regards,
>
> John Criswell
>
>
>>
>>>
>>> Anirudh
>>>
>>>> --
>>>> Mats
>>>>
>>>> On 15 June 2015 at 18:10, Anirudh Sivaraman<sk.anirudh at gmail.com>
>>>> wrote:
>>>>>
>>>>>
>>>>> Does inst_iterator
>>>>>
>>>>>
>>>>> (http://llvm.org/docs/ProgrammersManual.html#iterating-over-the-instruction-in-a-function)
>>>>> guarantee that the iterated instructions are in program order: the
>>>>> order of instructions printed by llvm-dis?
>>>>>
>>>>> Thanks in advance,
>>>>> Anirudh
>>>>> _______________________________________________
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>>>>
>>>>
>>>>
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>
>
> --
> John Criswell
> Assistant Professor
> Department of Computer Science, University of Rochester
> http://www.cs.rochester.edu/u/criswell
>