[llvm] [LV][EVL] Attach a new metadata on EVL vectorized loops (PR #131000)

[Luke Lau via llvm-commits]

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@@ -1013,6 +1013,32 @@ void VPlan::execute(VPTransformState *State) {
     Value *Val = State->get(PhiR->getBackedgeValue(), NeedsScalar);
     cast<PHINode>(Phi)->addIncoming(Val, VectorLatchBB);
   }
+
+  // Check if it's EVL-vectorized and mark the corresponding metadata.
+  // Note that we could have done this during the codegen of
+  // ExplictVectorLength, but the enclosing vector loop was not in a good shape
+  // for us to attach the metadata.
+  bool IsEVLVectorized = llvm::any_of(*Header, [](const VPRecipeBase &Recipe) {
+    // Looking for the ExplictVectorLength VPInstruction.
+    if (const auto *VI = dyn_cast<VPInstruction>(&Recipe))
+      return VI->getOpcode() == VPInstruction::ExplicitVectorLength;
+    return false;
+  });
+  if (IsEVLVectorized) {
+    // VPTransformState::CurrentParentLoop has already been reset
+    // at this moment.
+    Loop *L = State->LI->getLoopFor(VectorLatchBB);
+    assert(L);
+    LLVMContext &Context = State->Builder.getContext();
+    MDNode *LoopID = L->getLoopID();
+    auto *IsEVLVectorizedMD = MDNode::get(
+        Context,
+        {MDString::get(Context, "llvm.loop.isvectorized.withevl"),
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lukel97 wrote:

Nit, should we align the terminology with how it's represented via tail folding, something like `llvm.loop.isvectorized.tailfoldingstyle = evl`?

https://github.com/llvm/llvm-project/pull/131000