[llvm-dev] Questions about jump threading optimization and what we can do

[Karl Rehm via llvm-dev]

I think SimplifyCFG could be abusing the undef value for this. If num is
defined before the loop, it no longer becomes optimized:
https://godbolt.org/z/woHRQf

On Mon, Feb 3, 2020 at 3:50 PM Karl Rehm <klrehm123 at gmail.com> wrote:

> Hm. I assumed that JumpThreading would be the primary factor in optimizing
> code like this. Guess not. I'll need to look into SimplifyCFG to see what
> prevents it from doing the same thing to the other loop:
> https://godbolt.org/z/F6NjdG
>
> On Mon, Feb 3, 2020 at 3:09 PM Michael Kruse <llvmdev at meinersbur.de>
> wrote:
>
>> I assumed the LLVM-IR behind the godbolt link represented the C code
>> you used before.
>>
>> SimplifyCFG actually helps the loop being removed:
>> https://godbolt.org/z/5v7SXh
>>
>> But this doesn't even involve JumpThreading.
>>
>> Michael
>>
>
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