[llvm] [VPlan] Add VPInstruction to unpack vector values to scalars. (PR #155670)

[Ramkumar Ramachandra via llvm-commits]

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@@ -3755,6 +3764,48 @@ void VPlanTransforms::materializeBuildVectors(VPlan &Plan) {
           });
     }
   }
+
+  // Create explicit VPInstructions to convert vectors to scalars.
+  for (VPBasicBlock *VPBB :
+       concat<VPBasicBlock *>(VPBBsOutsideLoopRegion, VPBBsInsideLoopRegion)) {
+    for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
+      if (isa<VPReplicateRecipe, VPInstruction, VPScalarIVStepsRecipe>(&R))
+        continue;
+      for (VPValue *Def : R.definedValues()) {
+        if (vputils::isSingleScalar(Def) || vputils::onlyFirstLaneUsed(Def))
+          continue;
+
+        if (VPBB->getParent() != Plan.getVectorLoopRegion())
+          continue;
+
+        auto UsesVectorOrInsideReplicateRegion = [LoopRegion](VPUser *U) {
+          VPRegionBlock *ParentRegion =
+              cast<VPRecipeBase>(U)->getParent()->getParent();
+          return ParentRegion && ParentRegion != LoopRegion;
+        };
+
+        if (none_of(Def->users(),
+                    [Def, &UsesVectorOrInsideReplicateRegion](VPUser *U) {
+                      return !UsesVectorOrInsideReplicateRegion(U) &&
+                             U->usesScalars(Def);
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artagnon wrote:

Quick note: usesScalars can be a bit weak; I put up chainUsesScalarValues to fix this, but I'm not sure if it's the best idea.

https://github.com/llvm/llvm-project/pull/155670