[llvm] [InstCombine] Add CTLZ -> CTTZ simplification (PR #164733)

[Mihail Mihov via llvm-commits]

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@@ -0,0 +1,101 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt < %s -S -passes=instcombine | FileCheck %s
+
+; ctpop(~i & (i - 1)) -> bitwidth - cttz(i, false)
+define i8 @ctlz_to_sub_bw_cttz(i8 %a0) {
+; CHECK-LABEL: define i8 @ctlz_to_sub_bw_cttz(
+; CHECK-SAME: i8 [[A0:%.*]]) {
+; CHECK-NEXT:    [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[A0]], i1 false)
+; CHECK-NEXT:    [[CLZ:%.*]] = sub nuw nsw i8 8, [[TMP1]]
+; CHECK-NEXT:    ret i8 [[CLZ]]
+;
+  %dec = add i8 %a0, -1
+  %not = xor i8 %a0, -1
+  %and = and i8 %dec, %not
+  %clz = tail call i8 @llvm.ctlz.i8(i8 %and, i1 false)
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MihailMihov wrote:

I have added the test and [here](https://alive2.llvm.org/ce/z/trKk-J) is the Alive2 for it, if needed

https://github.com/llvm/llvm-project/pull/164733