[PATCH] D67846: [InstCombine] (a+b) <= a && (a+b) != 0 -> (0-b) < a (PR43259)

[Roman Lebedev via Phabricator via llvm-commits]

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

This is again motivated by D67122 <https://reviews.llvm.org/D67122> sanitizer check enhancement.
That patch seemingly worsens `-fsanitize=pointer-overflow`
overhead from 25% to 50%, which strongly implies missing folds.

This pattern isn't exactly what we get there
(strict vs. non-strict predicate), but this pattern does not
require known-bits analysis, so it is best to handle it first.

  Name: 0
    %adjusted = add i8 %base, %offset
    %not_null = icmp ne i8 %adjusted, 0
    %no_underflow = icmp ule i8 %adjusted, %base
    %r = and i1 %not_null, %no_underflow
  =>
    %neg_offset = sub i8 0, %offset
    %r = icmp ugt i8 %base, %neg_offset

https://rise4fun.com/Alive/knp

There are 3 other variants of this pattern,
they all will go into InstSimplify:
https://rise4fun.com/Alive/bIDZ

https://bugs.llvm.org/show_bug.cgi?id=43259


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D67846

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
  llvm/test/Transforms/InstCombine/result-of-add-of-negative-or-zero-is-non-zero-and-no-underflow.ll

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