[llvm] GlobalISel: Use G_UADDE when narrowing G_UMULH (PR #97194)

[Jay Foad via llvm-commits]

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@@ -5755,15 +5758,15 @@ void LegalizerHelper::multiplyRegisters(SmallVectorImpl<Register> &DstRegs,
     // Add all factors and accumulate all carries into CarrySum.
     if (DstIdx != DstParts - 1) {
       MachineInstrBuilder Uaddo =
-          B.buildUAddo(NarrowTy, LLT::scalar(1), Factors[0], Factors[1]);
+          B.buildUAddo(NarrowTy, S1, Factors[0], Factors[1]);
       FactorSum = Uaddo.getReg(0);
-      CarrySum = B.buildZExt(NarrowTy, Uaddo.getReg(1)).getReg(0);
+      CarrySum = Zero;
       for (unsigned i = 2; i < Factors.size(); ++i) {
-        MachineInstrBuilder Uaddo =
-            B.buildUAddo(NarrowTy, LLT::scalar(1), FactorSum, Factors[i]);
-        FactorSum = Uaddo.getReg(0);
-        MachineInstrBuilder Carry = B.buildZExt(NarrowTy, Uaddo.getReg(1));
-        CarrySum = B.buildAdd(NarrowTy, CarrySum, Carry).getReg(0);
+        auto Uadde =
+            B.buildUAdde(NarrowTy, S1, FactorSum, Factors[i], Uaddo.getReg(1));
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jayfoad wrote:

No, this one should stay as a uaddo. The carry output from the original uaddo (outside the loop) should go into CarrySum once, but you're adding it into FactorSum every time around this loop.

https://github.com/llvm/llvm-project/pull/97194