[llvm] [ConstraintElim] Strengthen flags at the closest dominator of all uses. (PR #220918)

[Yingwei Zheng via llvm-commits]

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@@ -75,6 +75,26 @@ static Instruction *getContextInstForUse(Use &U) {
   return UserI;
 }
 
+/// Returns the closest program point dominating all uses of \p I, or nullptr if
+/// \p I has no uses.
+static Instruction *findCommonDominatorOfUses(Instruction &I,
+                                              DominatorTree &DT) {
+  Instruction *CommonDom = nullptr;
+  unsigned NumUses = 0;
+  for (Use &U : I.uses()) {
+    // Conservatively use original instruction, if there are too many uses.
+    if (++NumUses == 16)
+      return &I;
+    Instruction *UserI = getContextInstForUse(U);
+    CommonDom =
+        CommonDom ? DT.findNearestCommonDominator(CommonDom, UserI) : UserI;
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dtcxzyw wrote:

Does it trigger an assertion when one of the users is unreachable?


https://github.com/llvm/llvm-project/pull/220918