[llvm] [ConstraintElim] Link values to decompositions during queries if needed. (PR #224623)

[Florian Hahn via llvm-commits]

================
@@ -1812,9 +1833,39 @@ static std::optional<bool> checkCondition(CmpInst::Predicate Pred, Value *A,
     return std::nullopt;
   };
 
+  // Retry the query after adding additional facts for A == decompose(A) and B
+  // == decompose(B), if needed.
+  auto TryWithLinkedDecomposition =
+      [&](const ConstraintTy &C) -> std::optional<bool> {
+    if (C.empty())
+      return std::nullopt;
+
+    auto &CS = Info.getCS(C.IsSigned);
+    unsigned NumVars = Info.getValue2Index(C.IsSigned).size();
+    unsigned NumPushed = 0;
+    for (Value *V : {A, B}) {
+      RowTy Row =
+          getDecompositionLinkRow(V, C, Info, CheckInst->getDataLayout());
+      RowTy Negated = ConstraintSystem::negateOrEqual(Row);
+      if (Row.empty() || Negated.empty())
+        continue;
+      NumPushed += CS.addRow(Row, NumVars);
+      NumPushed += CS.addRow(Negated, NumVars);
----------------
fhahn wrote:

Yes that's a good point. It could benefit `doesHold` as well. I am not sure about integrating into `getConstraint` directly, as constructing the link row is not free (we need to decompose again, which trigger more queries). 

Perhaps we can share the re-try logic with `doesHold` though. I'll check if I can find cases in practice and do this as follow-up

https://github.com/llvm/llvm-project/pull/224623