[llvm] [LV] Keep duplicate recipes in VPExpressionRecipe (PR #156976)

[Sander de Smalen via llvm-commits]

================
@@ -2797,25 +2796,42 @@ VPExpressionRecipe::VPExpressionRecipe(
       R->removeFromParent();
   }
 
+  // Keep track of how many instances of each recipe occur in the recipe list
+  SmallMapVector<VPSingleDefRecipe *, unsigned, 4> ExpressionRecipeCounts;
+  for (auto *R : ExpressionRecipes) {
+    auto *F = ExpressionRecipeCounts.find(R);
+    if (F == ExpressionRecipeCounts.end())
+      ExpressionRecipeCounts.insert(std::make_pair(R, 1));
+    else
+      F->second++;
+  }
+
   // Internalize all external operands to the expression recipes. To do so,
   // create new temporary VPValues for all operands defined by a recipe outside
   // the expression. The original operands are added as operands of the
   // VPExpressionRecipe itself.
   for (auto *R : ExpressionRecipes) {
+    auto *F = ExpressionRecipeCounts.find(R);
+    F->second--;
     for (const auto &[Idx, Op] : enumerate(R->operands())) {
       auto *Def = Op->getDefiningRecipe();
       if (Def && ExpressionRecipesAsSetOfUsers.contains(Def))
         continue;
       addOperand(Op);
-      LiveInPlaceholders.push_back(new VPValue());
-      R->setOperand(Idx, LiveInPlaceholders.back());
+      auto *Tmp = new VPValue();
+      LiveInPlaceholders.push_back(Tmp);
+      // Only modify this recipe's operands if it's the last time it occurs in
+      // the recipe list
+      if (F->second == 0)
+        R->setOperand(Idx, Tmp);
----------------
sdesmalen-arm wrote:

If you were to keep a map of `Op -> new VPValue()` nodes, then you can just do:

```
LiveInPlaceholders.push_back(new VPValue());
MyMap[Op] = LiveInPlaceholders.back();
```

and then have a loop at the end to replace all values of `Op` by their LiveInPlaceholder (i.e. `new VPValue()`) values for each `R` in `ExpressionRecipes`.

That would also fix the potential issue I mentioned above, where same input operand is shared by multiple recipes passed to the expression.

https://github.com/llvm/llvm-project/pull/156976