[llvm] [SVE] Wide active lane mask (PR #76514)

[Momchil Velikov via llvm-commits]

================
@@ -19791,15 +19815,63 @@ static SDValue performIntrinsicCombine(SDNode *N,
           EVT::getVectorVT(*DAG.getContext(), PromVT.getVectorElementType(),
                            VT.getVectorElementCount());
 
-      Res = DAG.getNode(ISD::INTRINSIC_WO_CHAIN, DL, WhileVT, ID,
-                        N->getOperand(1), N->getOperand(2));
+      SDValue Res = DAG.getNode(ISD::INTRINSIC_WO_CHAIN, DL, WhileVT, ID,
+                                N->getOperand(1), N->getOperand(2));
       Res = DAG.getNode(ISD::SIGN_EXTEND, DL, PromVT, Res);
       Res = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, ExtVT, Res,
                         DAG.getConstant(0, DL, MVT::i64));
       Res = DAG.getNode(ISD::TRUNCATE, DL, VT, Res);
+
+      return Res;
     }
-    return Res;
+
+    if (!Subtarget->hasSVE2p1() && !Subtarget->hasSME2())
+      return SDValue();
+
+    if (!N->hasNUsesOfValue(2, 0))
+      return SDValue();
+
+    auto It = N->use_begin();
+    SDNode *Lo = *It++;
+    SDNode *Hi = *It;
+
+    const uint64_t HalfSize = VT.getVectorMinNumElements() / 2;
+    uint64_t OffLo, OffHi;
+    if (Lo->getOpcode() != ISD::EXTRACT_SUBVECTOR ||
+        Lo->getOperand(1)->getOpcode() != ISD::Constant ||
+        ((OffLo = Lo->getConstantOperandVal(1)) != 0 && OffLo != HalfSize) ||
+        Hi->getOpcode() != ISD::EXTRACT_SUBVECTOR ||
+        Hi->getOperand(1)->getOpcode() != ISD::Constant ||
+        ((OffHi = Hi->getConstantOperandVal(1)) != 0 && OffHi != HalfSize))
+      return SDValue();
----------------
momchil-velikov wrote:

Alternatively, we can perhaps move the logic from here https://github.com/llvm/llvm-project/blob/3a82a1c3f6bdc9259cc4641f66fc76d1e171e382/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp#L7542
to CodeGenPrepare and do the expansion in LLVM IR instead of DAG (and it'll work for GlobalISel too, maybe now it doesn't).

I'd rather not emit LLVM IR that results in terrible code in the first place, though.


https://github.com/llvm/llvm-project/pull/76514