[llvm] [InstCombine] Fold `X udiv Y` to `X lshr cttz(Y)` if Y is a power of 2 (PR #121386)

[Yingwei Zheng via llvm-commits]

================
@@ -1294,3 +1298,100 @@ entry:
   %div = sdiv i32 %add, %add2
   ret i32 %div
 }
+
+define i8 @udiv_if_power_of_two(i8 %x, i8 %y) {
+; CHECK-LABEL: @udiv_if_power_of_two(
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    [[TMP0:%.*]] = tail call range(i8 0, 9) i8 @llvm.ctpop.i8(i8 [[Y:%.*]])
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i8 [[TMP0]], 1
+; CHECK-NEXT:    br i1 [[TMP1]], label [[BB1:%.*]], label [[BB3:%.*]]
+; CHECK:       bb1:
+; CHECK-NEXT:    [[TMP2:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[Y]], i1 true)
+; CHECK-NEXT:    [[TMP3:%.*]] = lshr i8 [[X:%.*]], [[TMP2]]
+; CHECK-NEXT:    br label [[BB3]]
+; CHECK:       bb3:
+; CHECK-NEXT:    [[_0_SROA_0_0:%.*]] = phi i8 [ [[TMP3]], [[BB1]] ], [ 0, [[START:%.*]] ]
+; CHECK-NEXT:    ret i8 [[_0_SROA_0_0]]
+;
+start:
+  %0 = tail call i8 @llvm.ctpop.i8(i8 %y)
----------------
dtcxzyw wrote:

Please use named values.

https://github.com/llvm/llvm-project/pull/121386