[llvm-dev] Why can't we devirtulize this?

[Hubert Tong via llvm-dev]

Try -fstrict-vtable-pointers: https://godbolt.org/z/hfTe6e

-- HT

On Tue, Nov 3, 2020 at 9:12 PM Sameer Abu Asal via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> Hi,
>
> I came across this case that shows a bad case for de-virtualizeation
>
> https://godbolt.org/z/e7r6a8
>
> If you have a call to a virtual function inside of a Loop, llvm is unable
> to de-virt the call simply with Instr Combine. However, for the case where
> we put the virtual call inside a loop.
>
> You can recreate the output for inst-combine with:
>
> clang++ -mllvm -print-after-all -mllvm -filter-print-funcs=_Z10devirt_badm -S -O3 -emit-llvm ./test_devirt.cpp -mllvm -debug-only=instcombine -o ./test_devi
> rt.ll |& tee /tmp/log.bad
>
> clang++ -mllvm -print-after-all -mllvm -filter-print-funcs=_Z11devirt_goodv -S -O3 -emit-llvm ./test_devirt.cpp -mllvm -debug-only=instcombine -o ./test
> _devirt.ll |& tee /tmp/log.good
>
> It seems to me a problem with Inst combine not being able to hoist the
> call to get the virtual func pointer outside of the loop, is that right?
> Can we possibly do better than this?
>
> Thank you,
>
> --Sameer
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20201103/9fe79de0/attachment.html>