[PATCH] D128911: Emit table lookup from TargetLowering::expandCTTZ()

[Shubham Narlawar via Phabricator via llvm-commits]

gsocshubham added inline comments.


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Comment at: llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp:8025
+  // Emit Table Lookup if ISD::CTLZ and ISD::CTPOP are not legal.
+  if (NumBitsPerElt == 32 && !VT.isVector() &&
+      TLI.isOperationExpand(ISD::CTPOP, VT) && !isOperationLegal(ISD::CTLZ, VT))
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craig.topper wrote:
> Should this be checking CTTZ_ZERO_UNDEF? The zero case is not handled correctly by the table lookup. For CTTZ we need a select. CodeGenPrepare rewrites llvm.cttz(i32 %x, i1 false) into a branch around llvm.cttz(i32 %x, i1 true) on some targets. So the difference might be hard to test.
Can you point me to target and test where above scenario occurs? I will update it accordingly.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D128911/new/

https://reviews.llvm.org/D128911