[PATCH] D69217: [ConstantRange] makeGuaranteedNoWrapRegion(): `shl` support

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: nikic, sanjoy.
lebedev.ri added a project: LLVM.
Herald added a subscriber: hiraditya.

Shift by more than bitwidth-1 produces poison.
Therefore if maximal shift amount that we are asked about is more than bitwidth-1,
the overflow will happen regardless of the value being shifted,
in other words there is no such value for which the overflow
[when shifting by more than bitwidth-1] does not happen.

For unsigned range not not overflow, we must not shift out any set bits,
and the actual limit for `x` can be computed by backtransforming
the maximal value we could ever get out of the `shl` - `-1` through
`lshr`. If the `x` is any larger than that then it will overflow.

Likewise for signed range, but just in signed domain..

This is based on the general idea outlined by @nikic in https://reviews.llvm.org/D68672#1714990


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D69217

Files:
  llvm/lib/IR/ConstantRange.cpp
  llvm/unittests/IR/ConstantRangeTest.cpp

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