[llvm] [CombinerHelper]: Use undef for handling divisors of one (PR #91037)

[Matt Arsenault via llvm-commits]

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@@ -5111,27 +5113,30 @@ MachineInstr *CombinerHelper::buildUDivUsingMul(MachineInstr &MI) {
 
     // Magic algorithm doesn't work for division by 1. We need to emit a select
     // at the end.
-    // TODO: Use undef values for divisor of 1.
-    if (!Divisor.isOne()) {
-
-      // UnsignedDivisionByConstantInfo doesn't work correctly if leading zeros
-      // in the dividend exceeds the leading zeros for the divisor.
-      UnsignedDivisionByConstantInfo magics =
-          UnsignedDivisionByConstantInfo::get(
-              Divisor, std::min(KnownLeadingZeros, Divisor.countl_zero()));
-
-      Magic = std::move(magics.Magic);
-
-      assert(magics.PreShift < Divisor.getBitWidth() &&
-             "We shouldn't generate an undefined shift!");
-      assert(magics.PostShift < Divisor.getBitWidth() &&
-             "We shouldn't generate an undefined shift!");
-      assert((!magics.IsAdd || magics.PreShift == 0) && "Unexpected pre-shift");
-      PreShift = magics.PreShift;
-      PostShift = magics.PostShift;
-      SelNPQ = magics.IsAdd;
+    if (Divisor.isOne()) {
+      PreShifts.push_back(MIB.buildUndef(ScalarShiftAmtTy).getReg(0));
+      MagicFactors.push_back(MIB.buildUndef(ScalarTy).getReg(0));
+      NPQFactors.push_back(MIB.buildUndef(ScalarTy).getReg(0));
+      PostShifts.push_back(MIB.buildUndef(ScalarShiftAmtTy).getReg(0));
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arsenm wrote:

call buildUndef once per type 

https://github.com/llvm/llvm-project/pull/91037