[llvm-dev] Missed optimization of bitwise expressions

[Jay Foad via llvm-dev]

Now that that one is fixed (thanks!) my script can't find any more
cases that we fail to optimize where the input has two variables and
three instructions.

Moving on to the two-variable four-instruction cases, there are
various missing simplifications like:

A | B | ~(A ^ B) --> -1
(A ^ B) & (~A | B) --> ~A & B
(((A | B) ^ B) & A) ^ (A | B) --> B
(((A & B) ^ A) & B) | (A & B) --> A & B

Taking the last of these as an example: https://alive2.llvm.org/ce/z/wZtbjM
I'm surprised that we don't simplify it in stages:
1. (A & B) ^ A --> A & ~B // perhaps this is not considered profitable?
2. (A & ~B) & B --> 0 // perhaps we don't manage to reassociate this
to see that it contains B & ~B ?
3. 0 | (A & B) --> A & B

Jay.

On Tue, 16 Nov 2021 at 09:09, Jay Foad <jay.foad at gmail.com> wrote:
>
> Thanks for the fixes so far! Here's another simple two variable case:
>
> https://bugs.llvm.org/show_bug.cgi?id=52518 "[InstCombine] Failure to
> simplify (~A|B)^A --> ~(A&B)"
>
>
> Jay.
>
> On Thu, 11 Nov 2021 at 19:59, Jay Foad <jay.foad at gmail.com> wrote:
> >
> > OK, I've filed the first couple of beginner bugs:
> > https://bugs.llvm.org/show_bug.cgi?id=52478
> > https://bugs.llvm.org/show_bug.cgi?id=52479
> >
> > Thanks,
> > Jay.
> >
> > On Thu, 11 Nov 2021 at 16:35, Sanjay Patel <spatel at rotateright.com> wrote:
> > >
> > > 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.