[LLVMdev] Module::getOrInsertFunction determinism

[Philip Reames]

On 06/03/2014 02:50 AM, Kuchta, Tomasz wrote:
> Hi Philip,
>
> I would like to ask a follow-up question about code generation.
> Do you know if is expected that if we take the same bit code modules and
> link them together in the same order (programatically), but on different
> machines (assuming the same version of LLVM and
> roughly the same OS), the output may differ with respect to order of
> function definitions inside module?
This falls in to the category of "not really surprising". There's enough
variation between machines that it's not uncommon for an otherwise
mostly-deterministic process to behave slightly differently. I wouldn't
call the change in order "expected", but it also doesn't sound like an
obvious bug.

Also, from what you said previously, you're working with a very old
version of LLVM. Most of my own experience is with newer versions so it
may be my expectations are badly off. I'd really recommend you upgrade.
You'll get much more applicable advice. :)
>
> Thank you very much in advance for help,
>
> Best regards
> Tomasz Kuchta
>
> On 29/05/2014 22:22, "Kuchta, Tomasz" <t.kuchta12 at imperial.ac.uk> wrote:
>
>> Hi Philip,
>>
>> Thank you very much for the reply. I need to add one important detail -
>> currently I¹m working on an outdated version of
>> LLVM, so I will need to first make sure that it also appears on the
>> current one. 
>>
>> The use case that I have in mind is to be able to uniquely identify
>> instruction in the bit code.
>> For example let¹s say we have an ³add² instruction in some function and I
>> want to know that this is the same
>> instruction in two separate runs. If the function is put in some different
>> place as a result of inserting function and linking in the second run,
>> how would I identify that instruction? It seems to me that cannot rely on
>> the offset of the instruction within the binary and I cannot rely on the
>> instruction count.
Have you considered trying either metadata or using an intrinsic
function as a marker for the interesting instruction? Depending on your
use case, either might work. (f.y.i. I believe Metadata has changed
radically since earlier versions.)
>> The problem in my case was that for the same source code base, I was
>> getting different resulting LLVM IR when doing the same
>> sequence of getOrInsertFunction and linking with other bit code modules on
>> each of runs.
>>
>> Thank you,
>> Tomek
>>
>> On 29/05/2014 19:34, "Philip Reames" <listmail at philipreames.com> wrote:
>>
>>> On 05/29/2014 11:06 AM, Tim Northover wrote:
>>>> Hi Tomek,
>>>>
>>>>> I¹ve got a question about Module::getOrInsertFunction().
>>>>> I got an impression that it is not deterministic where exactly in the
>>>>> bit
>>>>> code module the new function will be inserted.
>>>> Looking at the code (not exhaustively), it seems a new function will
>>>> always be added to the end of a module.
>>>>
>>>> Documenting that probably wouldn't be a terrible idea, but it
>>>> shouldn't affect anything except the human-readability of LLVM IR. Are
>>>> you trying to do something where it is actually causing problems?
>>>>
>>>> Cheers.
>>>>
>>>> Tim.
>>> I would argue in favor of leaving this explicitly undocumented.  I don't
>>> see the use case in knowing where in a module it got added, and it
>>> restricts future implementations in ways we can't predict. Documenting
>>> that it must be deterministic is fine.  Documenting where it decides to
>>> place it is not.
>>>
>>> Tomek, could you spell out why you need the position?  Maybe there's
>>> another option here.
>>>
>>> Philip
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