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

[David Blaikie via llvm-dev]

ooh, neat!

On Tue, Nov 3, 2020 at 7:36 PM Hubert Tong via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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
>>
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