[PATCH] D65380: [InstCombine] Shift amount reassociation: shl-trunc-shl pattern

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: spatel, nikic, xbolva00.
lebedev.ri added a project: LLVM.
Herald added a subscriber: hiraditya.

Currently `reassociateShiftAmtsOfTwoSameDirectionShifts()` only handles two shifts one after another.
If the shifts are `shl`, we still can easily perform the fold, with no extra legality checks:
https://rise4fun.com/Alive/OQbM

If we have right-shift however, we won't be able to make it any simpler than it already is.

After this the only thing missing here is constant-folding: (`NewShAmt >= bitwidth(X)`)

- If it's a logical shift, then constant-fold to `0` regardless whether there is `trunc` or not.
- If it's a `ashr`, then a splat of original signbit

https://rise4fun.com/Alive/KQE
https://rise4fun.com/Alive/i0V


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D65380

Files:
  llvm/include/llvm/IR/Constants.h
  llvm/include/llvm/IR/PatternMatch.h
  llvm/lib/IR/Constants.cpp
  llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
  llvm/test/Transforms/InstCombine/shift-amount-reassociation-with-truncation-shl.ll

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