[PATCH] D72007: [InstCombine] try to pull 'not' of select into compare operands

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: lebedev.ri, nikic, xbolva00.
Herald added subscribers: hiraditya, mcrosier.
Herald added a project: LLVM.

not (select Cond, (cmp TPred, ?, ?), FVal --> select Cond, (cmp TPred', ?, ?), (not FVal)

(If both sides of the select are cmps, we can remove an instruction.)

We have a more general 'isFreeToInvert' analysis, but I'm not seeing a way to use that more widely without inducing infinite looping (opposing transforms).
Here, we flip the compare predicate directly, so we should not have any danger by creating extra intermediate 'not' ops.

Alive proofs:
https://rise4fun.com/Alive/jKa

  Name: both select values are compares - invert predicates
    %tcmp = icmp sle i32 %x, %y  
    %fcmp = icmp ugt i32 %z, %w
    %sel = select i1 %cond, i1 %tcmp, i1 %fcmp
    %not = xor i1 %sel, true
  =>
    %tcmp_not = icmp sgt i32 %x, %y  
    %fcmp_not = icmp ule i32 %z, %w
    %not = select i1 %cond, i1 %tcmp_not, i1 %fcmp_not
    
  Name: false val is compare - invert/not 
    %fcmp = icmp ugt i32 %z, %w
    %sel = select i1 %cond, i1 %tcmp, i1 %fcmp
    %not = xor i1 %sel, true
  =>
    %tcmp_not = xor i1 %tcmp, -1  
    %fcmp_not = icmp ule i32 %z, %w
    %not = select i1 %cond, i1 %tcmp_not, i1 %fcmp_not


https://reviews.llvm.org/D72007

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
  llvm/test/Transforms/InstCombine/not.ll

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