Simon Pilgrim d96529af3c [DAG] Attempt shl narrowing in SimplifyDemandedBits (REAPPLIED) ...

If a shl node leaves the upper half bits zero / undemanded, then see if we can profitably perform this with a half-width shl and a free trunc/zext.

Followup to D146121

Reapplied - moved after the ShrinkDemandedOp call; reuse the existing KnownBits result; ensure that we only attempt this if all the upper bits are demanded; 547dc46122 should address the remaining regressions that were noticed in the previous commit.

Differential Revision: https://reviews.llvm.org/D155472

2023-10-29 15:38:46 +00:00

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