llvm dev - Nov 2021 - Missed optimization of bitwise expressions

Hi,

I tried searching for small bitwise expressions using AND OR XOR and
NOT that "opt -O3" fails to optimize to a simpler form. For example:

(A^B)|~A --> ~(A&B)
https://alive2.llvm.org/ce/z/HpWfzp

A|B|(A^B) --> A|B
https://alive2.llvm.org/ce/z/UCC6uM

((A|B)^C)&A --> A&~C (actually I don't understand why this one is
OK,
even if B might be poison, but alive2 says it is OK)
https://alive2.llvm.org/ce/z/mkvW6p

I can file bugs for specific examples but I wondered if there was any
interest in a more systematic approach to finding these missed
optimizations? My approach was to write some very hacky Python
(https://gist.github.com/jayfoad/94f4c68fa3a9aa908db79dbd7e9df80d,
please don't read it) to exhaustively generate programs; then take all
the programs with a given truth table, run them all through "opt -O3",
and check that they all got optimized to the same (or at least equally
short) output.

Thanks,
Jay.

Nice test program! I don't know python well enough to understand that yet,
but no reason to be ashamed of hacks. :)

If we're missing 2 variable logic reductions, those are common/easy enough
that we should have those in instcombine. So yes, it would be great if you
file bugs for those, and they could be marked with the 'beginner'
keyword
too as a potential easy patch for newcomers to LLVM.

There's also a set of recent patch proposals for 3 variable logic
reductions -- for example, https://reviews.llvm.org/D112276 -- these were
inspired by a logic lookup table function as discussed in the comments.
The extra-use and commuted variations make these harder. IMO, this is where
there should be a dedicated pass/solver for logic folds if we want those
optimizations to be complete. Otherwise, there's an explosion of possible
pattern match combinations.


On Thu, Nov 11, 2021 at 6:34 AM Jay Foad <jay.foad at gmail.com> wrote:
> Hi,
>
> I tried searching for small bitwise expressions using AND OR XOR and
> NOT that "opt -O3" fails to optimize to a simpler form. For
example:
>
> (A^B)|~A --> ~(A&B)
> https://alive2.llvm.org/ce/z/HpWfzp
>
> A|B|(A^B) --> A|B
> https://alive2.llvm.org/ce/z/UCC6uM
>
> ((A|B)^C)&A --> A&~C (actually I don't understand why this
one is OK,
> even if B might be poison, but alive2 says it is OK)
> https://alive2.llvm.org/ce/z/mkvW6p
>
> I can file bugs for specific examples but I wondered if there was any
> interest in a more systematic approach to finding these missed
> optimizations? My approach was to write some very hacky Python
> (https://gist.github.com/jayfoad/94f4c68fa3a9aa908db79dbd7e9df80d,
> please don't read it) to exhaustively generate programs; then take all
> the programs with a given truth table, run them all through "opt
-O3",
> and check that they all got optimized to the same (or at least equally
> short) output.
>
> Thanks,
> Jay.
>
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