[llvm] [SCEV] Collect and merge loop guards through PHI nodes with multiple incoming values (PR #113915)

[Florian Hahn via llvm-commits]

================
@@ -15217,7 +15221,83 @@ bool ScalarEvolution::matchURem(const SCEV *Expr, const SCEV *&LHS,
 
 ScalarEvolution::LoopGuards
 ScalarEvolution::LoopGuards::collect(const Loop *L, ScalarEvolution &SE) {
+  BasicBlock *Header = L->getHeader();
+  BasicBlock *Pred = L->getLoopPredecessor();
   LoopGuards Guards(SE);
+  SmallPtrSet<const BasicBlock *, 8> VisitedBlocks;
+  collectFromBlock(SE, Guards, Header, Pred, VisitedBlocks);
+  return Guards;
+}
+
+void ScalarEvolution::LoopGuards::collectFromPHI(
+    ScalarEvolution &SE, ScalarEvolution::LoopGuards &Guards,
+    const PHINode &Phi, SmallPtrSetImpl<const BasicBlock *> &VisitedBlocks,
+    SmallDenseMap<const BasicBlock *, LoopGuards> &IncomingGuards,
+    unsigned Depth) {
+  if (!SE.isSCEVable(Phi.getType()))
+    return;
+
+  using MinMaxPattern = std::pair<const SCEVConstant *, SCEVTypes>;
+  auto GetMinMaxConst = [&](unsigned IncomingIdx) -> MinMaxPattern {
+    const BasicBlock *InBlock = Phi.getIncomingBlock(IncomingIdx);
+    if (!VisitedBlocks.insert(InBlock).second)
+      return {nullptr, scCouldNotCompute};
+    if (!IncomingGuards.contains(InBlock)) {
+      LoopGuards G(SE);
+      collectFromBlock(SE, G, Phi.getParent(), InBlock, VisitedBlocks,
+                       Depth + 1);
+      IncomingGuards.try_emplace(InBlock, std::move(G));
+    }
+    const LoopGuards &G = IncomingGuards.at(InBlock);
----------------
fhahn wrote:

nit: Could try to avoid the repeated lookup by using `IncomingGuards.insert()`

https://github.com/llvm/llvm-project/pull/113915