[PATCH] D124692: [InstCombin] fix miscompile when casting int->FP->int

[Sanjay Patel via Phabricator via llvm-commits]

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As shown in https://github.com/llvm/llvm-project/issues/55150 - the existing fold may be wrong when converting to a signed value.
This is a quick fix to avoid the miscompile.

I added tests/comments for all of the signed/unsigned combinations at the border width, and tried to confirm with Alive2:
https://alive2.llvm.org/ce/z/3p9DSu

There are already some TODO items in the test file that suggest possible refinements, so the regression with ui->FP->si is probably ok? It seems unlikely that we'd see these kind of edge cases with non-byte width integer types in real code. The potential miscompile went undetected for several years.


https://reviews.llvm.org/D124692

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
  llvm/test/Transforms/InstCombine/sitofp.ll

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