[llvm] [VPlan] Introduce chainUsesScalarValues (PR #158377)

[Luke Lau via llvm-commits]

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@@ -1322,15 +1322,11 @@ static void narrowToSingleScalarRecipes(VPlan &Plan) {
         continue;
       }
 
-      // Skip recipes that aren't single scalars or don't have only their
-      // scalar results used. In the latter case, we would introduce extra
-      // broadcasts.
+      // Only consider recipes that are single scalars whose scalar value is
+      // used by the final leaves in a recipe-chain walk: if the final leaves
+      // don't use the scalar value, it could introduce extra broadcasts.
       if (!vputils::isSingleScalar(RepOrWidenR) ||
-          !all_of(RepOrWidenR->users(), [RepOrWidenR](const VPUser *U) {
-            return U->usesScalars(RepOrWidenR) ||
-                   match(cast<VPRecipeBase>(U),
-                         m_ExtractLastElement(m_VPValue()));
-          }))
+          !vputils::chainUsesScalarValues(RepOrWidenR))
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lukel97 wrote:

Aren't we already iterating backwards through the VPBB? My understanding is that because we're narrowing the uses before the defs anyway we wouldn't need to look through the chain.

Are we seeing the the diff because we're now checking `isVectorToScalar`?

https://github.com/llvm/llvm-project/pull/158377