[LLVMdev] Program order in inst_iterator?

[John Criswell]

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).

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