[PATCH] D122686: Fold calls to strnlen (PR46455)

[Martin Sebor via Phabricator via llvm-commits]

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This patch adds support for folding simple `strnlen` calls to `SimplifyLibCalls.cpp`.  It extends the `optimizeStringLength` member function to accept the `bound` AKA `maxlen` argument to `strnlen` and attempt to fold calls to it analogously to `strlen`.  To keep `optimizeStringLength` from growing unwieldy the patch factors out the string length logic into a new helper function, `minStringLength`, and adds most of the `strnlen` folding there.

The new `minStringLength` function can be called recursively to simplify some less than completely trivial calls with conditional expressions such as `strnlen(x ? "123" : "12345" + i, n)`.  In case one of the recursive calls fails, rather than creating and inserting the simplified instructions into the IR as it goes, the function returns a pointer to a function/lambda that does that.  On success the caller then evaluates the lambda to create and insert the instructions (or constants) into the IR.

I have tested the patch by bootstrapping LLVM + Clang on x86_64-linux and running `check-all`.

If there are better solutions than the lambda approach above I'd be happy to learn about them and adjust the patch (and follow up on the question I posted about this on Discourse: https://discourse.llvm.org/t/61309).

The new `minStringLength` function can also be called to implement `wcsnlen` folding.  I have done that in a separate patch that I'll submit if/when this is accepted.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D122686

Files:
  llvm/include/llvm/Transforms/Utils/SimplifyLibCalls.h
  llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
  llvm/test/Transforms/InstCombine/strlen-4.ll
  llvm/test/Transforms/InstCombine/strnlen-1.ll
  llvm/test/Transforms/InstCombine/strnlen-2.ll
  llvm/test/Transforms/InstCombine/strnlen-3.ll
  llvm/test/Transforms/InstCombine/strnlen-4.ll
  llvm/test/Transforms/InstCombine/strnlen-5.ll
  llvm/test/Transforms/InstCombine/strnlen-6.ll
  llvm/test/Transforms/InstCombine/wcslen-5.ll

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