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

[Ramkumar Ramachandra 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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artagnon wrote:

We are iterating backwards through the VPBB, but there is no way to narrow certain Widen-like recipes, as they don't have an underlying value (eg. WidenCast might not have an underlying value). The VPI/Store handling in chainUsesScalarValues is also necessary.

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