[PATCH] D73200: GlobalISel: Add computeKnownBitsForTargetInstr

Mon Mar 23 02:10:43 PDT 2020

foad added inline comments.

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Comment at: llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp:500-507
+  // Okay, we know that the sign bit in Mask is set.  Use CLZ to determine
+  // the number of identical bits in the top of the input value.
+  Mask = ~Mask;
+  Mask <<= Mask.getBitWidth() - TyBits;

-  // TODO: Handle target instructions
-  // TODO: Fall back to known bits
-  return 1;
+  // Return # leading zeros.  We use 'min' here in case Val was zero before
+  // shifting.  We don't want to return '64' as for an i32 "0".
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I've simplified the SelectionDAG version of this code a bit in 7cdbf1ed4b94259a3aad2a7575e928fa61b0e57e.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D73200/new/

https://reviews.llvm.org/D73200