[PATCH] D153270: [BasicAA] Fix nsw handling for negated scales (PR63266)

[Nikita Popov via Phabricator via llvm-commits]

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We currently preserve the nsw flag when negating scales, which is incorrect for INT_MIN.

However, just dropping the NSW flag in this case makes BasicAA behavior unreliable and asymmetric, because we may or may not drop the NSW flag depending on which side gets subtracted.

Instead, leave the Scale alone and add an additional IsNegated flag, which indicates that the who VarIndex should be interpreted as a subtraction. This allows us to retain the NSW flag.

When accumulating the offset range, we need to use subtraction instead of adding for IsNegated indices. Everything else works on the absolute value of the scale, so the negation does not matter there.

Fixes https://github.com/llvm/llvm-project/issues/63266.


https://reviews.llvm.org/D153270

Files:
  llvm/lib/Analysis/BasicAliasAnalysis.cpp
  llvm/test/Analysis/BasicAA/range.ll

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