[llvm] [ConstraintElim] Use cond from header as upper bound on IV in exit BB. (PR #94610)

[Yingwei Zheng via llvm-commits]

================
@@ -1031,6 +1031,27 @@ void State::addInfoForInductions(BasicBlock &BB) {
   WorkList.push_back(FactOrCheck::getConditionFact(
       DTN, CmpInst::ICMP_SLT, PN, B,
       ConditionTy(CmpInst::ICMP_SLE, StartValue, B)));
+
+  assert(!StepOffset.isNegative() && "induction must be increasing");
+  // Try to add condition from header to the unique exit block, if there is one.
+  // When exiting either with EQ or NE, we know that the induction value must be
+  // u<= B, as a different exit may exit earlier.
+  ConditionTy Precond;
+  if (!MonotonicallyIncreasingUnsigned)
+    Precond = {CmpInst::ICMP_ULE, StartValue, B};
+  if (Pred == CmpInst::ICMP_EQ) {
+    BasicBlock *EB = cast<BranchInst>(BB.getTerminator())->getSuccessor(0);
+    if (L->getUniqueExitBlock() == EB) {
+      WorkList.emplace_back(FactOrCheck::getConditionFact(
+          DT.getNode(EB), CmpInst::ICMP_ULE, A, B, Precond));
+    }
+  }
+  if (Pred == CmpInst::ICMP_NE) {
+    BasicBlock *EB = cast<BranchInst>(BB.getTerminator())->getSuccessor(1);
+    if (L->getUniqueExitBlock() == EB)
+      WorkList.emplace_back(FactOrCheck::getConditionFact(
+          DT.getNode(EB), CmpInst::ICMP_ULE, A, B, Precond));
+  }
----------------
dtcxzyw wrote:

```suggestion
  if (ICmpInst::isEquality(Pred)) {
  ConditionTy Precond;
  if (!MonotonicallyIncreasingUnsigned)
    Precond = {CmpInst::ICMP_ULE, StartValue, B};
  BasicBlock *EB = cast<BranchInst>(BB.getTerminator())->getSuccessor(Pred == CmpInst::ICMP_NE);
    if (L->getUniqueExitBlock() == EB)
      WorkList.emplace_back(FactOrCheck::getConditionFact(
          DT.getNode(EB), CmpInst::ICMP_ULE, A, B, Precond));
  }
```

It should be simpler :)

https://github.com/llvm/llvm-project/pull/94610