[PATCH] D63812: [InstCombine] Shift amount reassociation (PR42391)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


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Comment at: lib/Transforms/InstCombine/InstCombineShifts.cpp:51
+  // All good, we can do this fold.
+  return BinaryOperator::Create(ShiftOpcode, X, NewShAmt);
+}
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nikic wrote:
> Would it be possible to preserve nowrap/exact flags that exist on both shifts?
* `exact` on both `lshr` => `exact` https://rise4fun.com/Alive/plHk
* `exact` on both `ashr` => `exact` https://rise4fun.com/Alive/QDAA
* `nuw` on both `shl` => `nuw` https://rise4fun.com/Alive/5Uk
* `nsw` on both `shl` => `nsw` https://rise4fun.com/Alive/0plg
So basically if the same flag is set on both original shifts -> set it on new shift.
Don't think we can do anything with non-matching flags on `shl`.


Repository:
  rL LLVM

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  https://reviews.llvm.org/D63812/new/

https://reviews.llvm.org/D63812