[llvm] [VPlan] Lower safe uniform load to unconditional scalar load (Alt. approach) (PR #217564)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -5852,3 +5823,36 @@ void VPlanTransforms::convertToStridedAccesses(VPlan &Plan,
     }
   }
 }
+
+void VPlanTransforms::lowerSafeUniformLoads(VPlan &Plan,
+                                            const TargetLibraryInfo &TLI) {
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
+           vp_depth_first_deep(Plan.getEntry()))) {
+    for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
+      auto *UniformGather = dyn_cast<VPWidenLoadRecipe>(&R);
+      if (!UniformGather ||
+          !vputils::isUniformAcrossVFsAndUFs(UniformGather->getAddr()))
+        continue;
+
+      auto &LI = cast<LoadInst>(UniformGather->getIngredient());
+      VPValue *Addr = UniformGather->getAddr();
+      Value *Underlying = Addr->getUnderlyingValue();
+      if (!Underlying ||
+          !isDereferenceableAndAlignedPointer(
+              Underlying, LI.getType(), LI.getAlign(),
+              SimplifyQuery(Plan.getDataLayout(), &TLI, /*DT=*/nullptr,
+                            /*AC=*/nullptr, &LI)))
+        continue;
+      DebugLoc DbgLoc = UniformGather->getDebugLoc();
+      auto *ScalarLoad =
+          new VPReplicateRecipe(&LI, Addr, /*IsSingleScalar=*/true,
+                                /*Mask=*/nullptr, {}, *UniformGather, DbgLoc);
+      ScalarLoad->insertBefore(UniformGather);
+      auto *Broadcast =
+          new VPInstruction(VPInstruction::Broadcast, {ScalarLoad});
+      Broadcast->insertBefore(UniformGather);
----------------
artagnon wrote:

```suggestion
      VPBuilder Builder(UniformGather);
      VPSingleDefRecipe *ScalarLoad =
          Builder.createSingleScalarOp(Instruction::Load, Addr,
                                /*Mask=*/nullptr, {}, *UniformGather, DbgLoc, &LI);
      VPInstruction *Broadcast =
          Builder.createNaryOp(VPInstruction::Broadcast, ScalarLoad, DbgLoc);
```

https://github.com/llvm/llvm-project/pull/217564