[PATCH] D37514: [PowerPC] support ZERO_EXTEND in tryBitPermutation

[Hal Finkel via Phabricator via llvm-commits]

hfinkel added inline comments.


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Comment at: lib/Target/PowerPC/PPCISelDAGToDAG.cpp:1079
+      for (i = 32; i < NumBits; ++i)
+        if (!(*LHSBits)[i].isZero()) break;
+      if (i != NumBits)
----------------
hfinkel wrote:
> Why are you checking that the upper bits are zero? This is a zext node, so I'd think you should just force them all to zero (like the code for AND does for bits that are zero).
Or, to put it another way, if you do it this way, then you should have a comment that reads something like, "We'll look through zext nodes here, but only if they're provably redundant." If we do this, however, we should explain why.


https://reviews.llvm.org/D37514