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[InstCombine] dropRedundantMaskingOfLeftShiftInput(): pat. c/d/e with…
… mask (PR42563) Summary: If we have a pattern `(x & (-1 >> maskNbits)) << shiftNbits`, we already know (have a fold) that will drop the `& (-1 >> maskNbits)` mask iff `(shiftNbits-maskNbits) s>= 0` (i.e. `shiftNbits u>= maskNbits`). So even if `(shiftNbits-maskNbits) s< 0`, we can still fold, we will just need to apply a **constant** mask afterwards: ``` Name: c, normal+mask %t0 = lshr i32 -1, C1 %t1 = and i32 %t0, %x %r = shl i32 %t1, C2 => %n0 = shl i32 %x, C2 %n1 = i32 ((-(C2-C1))+32) %n2 = zext i32 %n1 to i64 %n3 = lshr i64 -1, %n2 %n4 = trunc i64 %n3 to i32 %r = and i32 %n0, %n4 ``` https://rise4fun.com/Alive/gslRa Naturally, old `%masked` will have to be one-use. This is not valid for pattern f - where "masking" is done via `ashr`. https://bugs.llvm.org/show_bug.cgi?id=42563 Reviewers: spatel, nikic, xbolva00 Reviewed By: spatel Subscribers: hiraditya, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D67725 llvm-svn: 372630
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