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

[Michael Kruse via llvm-dev]

Your code and the one you have seen in
https://bugs.llvm.org/show_bug.cgi?id=44679 are structurally
different. In that latter, setting n to a constant value causes a
branch taken in the next iteration. Your's is a more classic for-loop
from 0 to 2. It is ScalarEvolution's job to find this out. SimplyCFG
job is just to normalize the IR into a form understood by
ScalarEvolution. That is, ScalarEvolution is the primary engine
enabling this.

Michael

Am Mo., 3. Feb. 2020 um 14:51 Uhr schrieb Karl Rehm <klrehm123 at gmail.com>:
>
> 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