[PATCH] D45173: [InstCombine] Recognize idioms for ctpop and ctlz

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

Does this trigger on the compiler-rt implementation of __popcountsi2/__clzsi2?  I guess it technically isn't a problem if it does, given the LLVM backend currently doesn't generate calls to them, but it might be worth adding a backend testcase to make sure we don't generate an infinite loop.



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Comment at: lib/Transforms/AggressiveInstCombine/BitCountCombine.cpp:156
+  if (!match(&In, m_Sub(m_ConstantInt(BW), m_Intrinsic<Intrinsic::ctpop>())))
+    return nullptr;
+  // Get the argument of the ctpop.
----------------
You might want to match patterns which don't include the subtraction; it could easily get combined away before you get here (if someone computes "ctlz(n)-1", or "cltz(x)-ctlz(y)", etc.).


Repository:
  rL LLVM

https://reviews.llvm.org/D45173