[llvm-dev] Preventing function call from being optimized out in LTO

[David Blaikie via llvm-dev]

On Tue, Jun 9, 2020 at 1:29 PM Shishir V Jessu <shishir.jessu at utexas.edu> wrote:
>
> Hi David,
>
> Thanks for your detailed comments.
>
>> but without any interprocedural analysis, nor any optimization of the body of the function, a call to an optnone function can still be removed if the call is dead.
>
>
> I am not concerned with calls to an optnone function being removed, but rather call-sites within an optnone function being removed. But although I place "optnone" on a given function F, dead code in F still sometimes seems to be potentially removed. Is there any way to prevent this from happening at the linking stage (since the bitcode emitted by clang still includes all my calls)?

Oh, sorry - my misunderstanding. Could you provide an example of that
happening? (a specific optnone function IR and a specific 'opt'
command that optimizes that IR and modifies the optnone function in an
unexpected/undesirable way) - that could be a bug.

- Dave

>
> Thanks,
> Shishir
>
> On Tue, Jun 9, 2020 at 2:12 PM David Blaikie <dblaikie at gmail.com> wrote:
>>
>> On Tue, Jun 9, 2020 at 11:54 AM Shishir V Jessu <shishir.jessu at utexas.edu> wrote:
>>>
>>> Hi David,
>>>
>>> By "dead" do you mean unreachable?
>>
>>
>> Yep!
>>
>>>
>>> My understanding was that the removal of dead code is simply another optimization,
>>
>>
>> It is, yes.
>>
>>>
>>> which should be disabled after adding "optnone"
>>
>>
>> Nah - optnone just means "don't optimize this function" (nor do interprocedural analysis to optimize the call site based on details of the implementation of this function - treat it as though this function were in another translation unit/module that the compiler has no visibility into) - but without any interprocedural analysis, nor any optimization of the body of the function, a call to an optnone function can still be removed if the call is dead.
>>
>>>
>>> (and adding the function to llvm.used so the function doesn't later get deleted entirely).
>>
>>
>> llvm.used is there to preserve functions that don't otherwise need to exist - it sounds to me like this isn't a useful tool for your situation - if you manage to preserve the call then the function will be retained (unless it's defined as "available externally" - then the definition of the funcion could be removed, leaving only a declaration). It sounds like you need to preserve the call - so if you succeed at that, then the function body/definition should be retained without the need for llvm.used.
>>
>>>
>>> I am instrumenting certain basic blocks in an LLVM pass, and would like to compile a binary which structures things the same way the LLVM pass does, to analyze some behavior.
>>
>>
>> Hmm, can't quite picture that from what you've described, sorry.
>>
>>>
>>> I observe that my calls are not removed on -O0 and -O1 for several programs, so it should be the case that the calls are removed on higher optimization levels - but then adding "optnone" isn't protecting all of those calls from being removed.
>>
>>
>> Any number of optimizations might make more code provably dead & then removed, losing your calls.
>>
>> - Dave
>>
>>>
>>>
>>> If you have any more insight I'd appreciate it! Thanks for your help.
>>>
>>> On Tue, Jun 9, 2020 at 1:13 PM David Blaikie <dblaikie at gmail.com> wrote:
>>>>
>>>> optnone on such functions should suffice - well, unless the calls turn out to be dead & I don't think there's anything you can do to thwart dead code removal. So what are you trying to preserve the function calls for?
>>>>
>>>> On Tue, Jun 9, 2020 at 11:01 AM Shishir V Jessu via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>> Hello,
>>>>>
>>>>> I am adding function calls to an LLVM link-time optimization (LTO) pass, using the IRBuilder::CreateCall method. I want these calls to remain in the final x86 binary at any optimization level, but on levels -O2 and -O3, some of these calls are being optimized out.
>>>>>
>>>>> So far, I've tried adding each function in the program (excluding LLVM intrinsics) to the llvm.used set, and I've also set noinline and optnone attributes on each function in the program. This has allowed me to retain most, but not all, of the calls I've added with IRBuilder::CreateCall.
>>>>>
>>>>> Furthermore, I have confirmed that all the calls I've created are present in the LLVM IR that results immediately after my pass. Thus, I know some future LTO pass is optimizing out some of these calls.
>>>>>
>>>>> How can I ensure that none of the calls I add are optimized out? Thanks for your help!
>>>>>
>>>>> Best,
>>>>> Shishir Jessu
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev