[PATCH] D90479: [WIP][ValueTracking] ComputeKnownBits - minimum leading/trailing zero bits in LSHR/SHL (PR44526)

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:1302
     computeKnownBitsFromShiftOperator(I, DemandedElts, Known, Known2, Depth, Q,
                                       KZF, KOF);
     break;
----------------
We could also do AShr... not really useful for known zeros, but maybe for known ones.


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Comment at: llvm/test/Transforms/InstCombine/and2.ll:155
+; CHECK-NEXT:    ret i8 [[SH]]
 ;
   %sh = lshr i8 1, %x
----------------
RKSimon wrote:
> These tests look like they might even need to be scrapped? And we just limit the transform to Shl instead of logical shifts in general.
I suspect we might want to continue to favor icmp+zext over lshr.


Repository:
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https://reviews.llvm.org/D90479