[llvm] [SCEV] Avoid erase+insert in constant folding (PR #101642)

[Philip Reames via llvm-commits]

================
@@ -844,34 +844,39 @@ static const SCEV *
 constantFoldAndGroupOps(ScalarEvolution &SE, LoopInfo &LI, DominatorTree &DT,
                         SmallVectorImpl<const SCEV *> &Ops, FoldT Fold,
                         IsIdentityT IsIdentity, IsAbsorberT IsAbsorber) {
-  const SCEVConstant *Folded = nullptr;
+  bool HasConst = false;
   for (unsigned Idx = 0; Idx < Ops.size();) {
     const SCEV *Op = Ops[Idx];
     if (const auto *C = dyn_cast<SCEVConstant>(Op)) {
-      if (!Folded)
-        Folded = C;
-      else
-        Folded = cast<SCEVConstant>(
-            SE.getConstant(Fold(Folded->getAPInt(), C->getAPInt())));
-      Ops.erase(Ops.begin() + Idx);
-      continue;
+      if (!HasConst) {
+        // Move the constant to the start.
+        std::swap(Ops[0], Ops[Idx]);
+        HasConst = true;
+      } else {
+        Ops[0] = SE.getConstant(
+            Fold(cast<SCEVConstant>(Ops[0])->getAPInt(), C->getAPInt()));
+        Ops.erase(Ops.begin() + Idx);
+        // Revisit this index.
+        continue;
+      }
     }
     ++Idx;
   }
 
-  if (Ops.empty()) {
-    assert(Folded && "Must have folded value");
-    return Folded;
-  }
+  if (Ops.size() == 1)
+    return Ops[0];
 
-  if (Folded && IsAbsorber(Folded->getAPInt()))
-    return Folded;
+  if (HasConst && IsIdentity(cast<SCEVConstant>(Ops[0])->getAPInt())) {
+    Ops.erase(Ops.begin());
+    HasConst = false;
+  }
 
-  GroupByComplexity(Ops, &LI, DT);
-  if (Folded && !IsIdentity(Folded->getAPInt()))
-    Ops.insert(Ops.begin(), Folded);
+  if (Ops.size() == 1 ||
+      (HasConst && IsAbsorber(cast<SCEVConstant>(Ops[0])->getAPInt())))
+    return Ops[0];
 
-  return Ops.size() == 1 ? Ops[0] : nullptr;
+  GroupByComplexity(MutableArrayRef(Ops).drop_front(HasConst), &LI, DT);
----------------
preames wrote:

Is this correct?  This seems to unconditionally drop any remaining constant term from the result.  The prior code inserted the folder value back into the set after grouping.  This seems to not do that?

https://github.com/llvm/llvm-project/pull/101642