[llvm] [KnownBits] Make abdu and abds optimal (PR #89081)

[Simon Pilgrim via llvm-commits]

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@@ -232,41 +232,53 @@ KnownBits KnownBits::smin(const KnownBits &LHS, const KnownBits &RHS) {
 }
 
 KnownBits KnownBits::abdu(const KnownBits &LHS, const KnownBits &RHS) {
-  // abdu(LHS,RHS) = sub(umax(LHS,RHS), umin(LHS,RHS)).
-  KnownBits UMaxValue = umax(LHS, RHS);
-  KnownBits UMinValue = umin(LHS, RHS);
-  KnownBits MinMaxDiff = computeForAddSub(/*Add=*/false, /*NSW=*/false,
-                                          /*NUW=*/true, UMaxValue, UMinValue);
+  // If we know which argument is larger, return (sub LHS, RHS) or
+  // (sub RHS, LHS) directly.
+  if (LHS.getMinValue().uge(RHS.getMaxValue()))
+    return computeForAddSub(/*Add=*/false, /*NSW=*/false, /*NUW=*/false, LHS,
+                            RHS);
+  if (RHS.getMinValue().uge(LHS.getMaxValue()))
+    return computeForAddSub(/*Add=*/false, /*NSW=*/false, /*NUW=*/false, RHS,
+                            LHS);
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RKSimon wrote:

So should we provide an internal version of computeForAddSub "computeForAddSub_internal?" that doesn't clear out conflicts and have computeForAddSub wrap that?

https://github.com/llvm/llvm-project/pull/89081