[PATCH] D32605: Recognize CTLZ builtin

[Evgeny Stupachenko via Phabricator via llvm-commits]

evstupac added a comment.

In https://reviews.llvm.org/D32605#750202, @joerg wrote:

>   if the CPU has no direct lowering for the intrinsic, this transform is beneficial only if the resulting intrinsic can be constant folded


Why?
What about converting loop to countable?
What about InstCombine optimizations (not sure how useful they are, but still)?

  // fold (srl (ctlz x), "5") -> x  iff x has one bit set (the low bit).
  // select_cc seteq X, 0, sizeof(X), ctlz(X) -> ctlz(X) 
  // select_cc seteq X, 0, sizeof(X), ctlz_zero_undef(X) -> ctlz(X)
  // select_cc seteq X, 0, sizeof(X), cttz(X) -> cttz(X)
  // select_cc seteq X, 0, sizeof(X), cttz_zero_undef(X) -> cttz(X)

.....
What about clear range of CTLZ(X):  0 <= CTLZ(X) <= bitwidth(X)?


Repository:
  rL LLVM

https://reviews.llvm.org/D32605