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

[Shishir V Jessu via llvm-dev]

Hi David,

By "dead" do you mean unreachable? My understanding was that the removal of
dead code is simply another optimization, which should be disabled after
adding "optnone" (and adding the function to llvm.used so the function
doesn't later get deleted entirely).

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. 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.

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
>>
>
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