[PATCH] D67356: [InstCombine] Simplify @llvm.usub.with.overflow+non-zero check (PR43251)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: spatel, nikic, vsk.
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.

In this particular case, given

  char* test(char& base, unsigned long offset) {
    return &base - offset;
  }

it will end up producing something like
https://godbolt.org/z/luGEju
which after optimizations reduces down to roughly

  declare void @use64(i64)
  define i1 @test(i8* dereferenceable(1) %base, i64 %offset) {
    %base_int = ptrtoint i8* %base to i64
    %adjusted = sub i64 %base_int, %offset
    call void @use64(i64 %adjusted)
    %not_null = icmp ne i64 %adjusted, 0
    %no_underflow = icmp ule i64 %adjusted, %base_int
    %no_underflow_and_not_null = and i1 %not_null, %no_underflow
    ret i1 %no_underflow_and_not_null
  }

Without D67122 <https://reviews.llvm.org/D67122> there was no `%not_null`,
and in this particular case we can "get rid of it", by merging two checks:
Here we are checking: `Base u>= Offset && (Base u- Offset) != 0`, but that is simply `Base u> Offset`

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

The `@llvm.usub.with.overflow` pattern itself is not handled here
because this is the main pattern, that we currently consider canonical.

https://bugs.llvm.org/show_bug.cgi?id=43251


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D67356

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

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