[PATCH] D67849: [InstCombine] (a+b) < a && (a+b) != 0 -> (0-b) < a iff a/b != 0 (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.

For

  #include <cassert>
  char* test(char& base, signed long offset) {
    __builtin_assume(offset < 0);
    return &base + offset;
  }

We produce

https://godbolt.org/z/r40U47

and again those two icmp's can be merged:

  Name: 0
  Pre: C != 0
    %adjusted = add i8 %base, C
    %not_null = icmp ne i8 %adjusted, 0
    %no_underflow = icmp ult i8 %adjusted, %base
    %r = and i1 %not_null, %no_underflow
  =>
    %neg_offset = sub i8 0, C
    %r = icmp ugt i8 %base, %neg_offset

https://rise4fun.com/Alive/ALap

There are 3 other variants of this pattern,
i believe they all will go into InstSimplify.

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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D67849

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

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