[llvm] [RISCV] Emit VP strided load in mgather combine. NFCI (PR #98112)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-risc-v

Author: Luke Lau (lukel97)

<details>
<summary>Changes</summary>

This combine is a duplication of the transform in RISCVGatherScatterLowering but at the SelectionDAG level, so similarly to #<!-- -->98111 we can replace the use of riscv_masked_strided_load with a VP strided load.

Unlike #<!-- -->98111 we don't require #<!-- -->97800 or #<!-- -->97798 since it only operates on fixed vectors with a non-zero stride.


---
Full diff: https://github.com/llvm/llvm-project/pull/98112.diff


1 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVISelLowering.cpp (+10-9) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 7972b9abc456c..e488b9f327582 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -17060,15 +17060,16 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
         SDValue BasePtr = DAG.getNode(ISD::ADD, DL, PtrVT, MGN->getBasePtr(),
                                       DAG.getConstant(Addend, DL, PtrVT));
 
-        SDVTList VTs = DAG.getVTList({VT, MVT::Other});
-        SDValue IntID =
-          DAG.getTargetConstant(Intrinsic::riscv_masked_strided_load, DL,
-                                XLenVT);
-        SDValue Ops[] =
-          {MGN->getChain(), IntID, MGN->getPassThru(), BasePtr,
-           DAG.getConstant(StepNumerator, DL, XLenVT), MGN->getMask()};
-        return DAG.getMemIntrinsicNode(ISD::INTRINSIC_W_CHAIN, DL, VTs,
-                                       Ops, VT, MGN->getMemOperand());
+        SDValue EVL = DAG.getElementCount(DL, Subtarget.getXLenVT(),
+                                          VT.getVectorElementCount());
+        SDValue StridedLoad =
+            DAG.getStridedLoadVP(VT, DL, MGN->getChain(), BasePtr,
+                                 DAG.getConstant(StepNumerator, DL, XLenVT),
+                                 MGN->getMask(), EVL, MGN->getMemOperand());
+        SDValue VPSelect = DAG.getNode(ISD::VP_SELECT, DL, VT, MGN->getMask(),
+                                       StridedLoad, MGN->getPassThru(), EVL);
+        return DAG.getMergeValues({VPSelect, SDValue(StridedLoad.getNode(), 1)},
+                                  DL);
       }
     }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/98112