[llvm] [SCEV] Unify and optimize constant folding (NFC) (PR #101473)

[Philip Reames via llvm-commits]

================
@@ -831,6 +831,49 @@ static bool hasHugeExpression(ArrayRef<const SCEV *> Ops) {
   });
 }
 
+/// Performs a number of common optimizations on the passed \p Ops. If the
+/// whole expression reduces down to a single operand, it will be returned.
+///
+/// The following optimizations are performed:
+///  * Fold constants using the \p Fold function.
+///  * Remove identity constants satisfying \p IsIdentity.
+///  * If a constant satisfies \p IsAbsorber, return it.
+///  * Sort operands by complexity.
+template <typename FoldT, typename IsIdentityT, typename IsAbsorberT>
+static const SCEV *
+constantFoldAndGroupOps(ScalarEvolution &SE, LoopInfo &LI, DominatorTree &DT,
+                        SmallVectorImpl<const SCEV *> &Ops, FoldT Fold,
+                        IsIdentityT IsIdentity, IsAbsorberT IsAbsorber) {
+  const SCEVConstant *Folded = nullptr;
+  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;
+    }
+    ++Idx;
+  }
+
+  if (Ops.empty()) {
+    assert(Folded && "Must have folded value");
+    return Folded;
+  }
+
+  if (Folded && IsAbsorber(Folded->getAPInt()))
+    return Folded;
+
+  GroupByComplexity(Ops, &LI, DT);
+  if (Folded && !IsIdentity(Folded->getAPInt()))
----------------
preames wrote:

This insert may be reasonable expensive.  An alternative would be to keep the folded value in the first constant position of Ops, and then remove it after the loop if it were identity.  This does make GroupByComplexity do slightly more work, so it's not clear which approach is faster.  

https://github.com/llvm/llvm-project/pull/101473