[DSE] Add predicated vector length store support for masked store elimination

In isMaskedStoreOverwrite we process two stores that fully overwrite one
another, here we add support for predicated vector length stores so that
DSE will eliminate this variant of masked stores.

Author: mcberg2021

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -269,6 +269,42 @@ static OverwriteResult isMaskedStoreOverwrite(const Instruction *KillingI,
       return OW_Unknown;
     return OW_Complete;
   }
+  if (KillingII->getIntrinsicID() == Intrinsic::vp_store) {
+    // Operands {0        , 1     , 2   , 3 }
+    //          {StoredVal, VecPtr, Mask, VL}
+    // Types.
+    VectorType *KillingTy =
+        cast<VectorType>(KillingII->getArgOperand(0)->getType());
+    VectorType *DeadTy = cast<VectorType>(DeadII->getArgOperand(0)->getType());
+    if (KillingTy->getScalarSizeInBits() != DeadTy->getScalarSizeInBits())
+      return OW_Unknown;
+    // Element count.
+    if (KillingTy->getElementCount() != DeadTy->getElementCount())
+      return OW_Unknown;
+    // Pointers.
+    Value *KillingPtr = KillingII->getArgOperand(1)->stripPointerCasts();
+    Value *DeadPtr = DeadII->getArgOperand(1)->stripPointerCasts();
+    if (KillingPtr != DeadPtr && !AA.isMustAlias(KillingPtr, DeadPtr))
+      return OW_Unknown;
+    // Masks.
+    // TODO: check that KillingII's mask is a superset of the DeadII's mask.
+    if (KillingII->getArgOperand(2) != DeadII->getArgOperand(2))
+      return OW_Unknown;
+    // Lengths.
+    if (KillingII->getArgOperand(3) != DeadII->getArgOperand(3))
+      return OW_Unknown;
+    AAMDNodes KillingAA = KillingII->getAAMetadata();
+    AAMDNodes DeadAA = DeadII->getAAMetadata();
+    // There must be scoped noalias metadata on both stores.
+    if (!KillingAA.Scope || !DeadAA.Scope ||
+        !KillingAA.NoAlias || !DeadAA.NoAlias)
+      return OW_Unknown;
+    // Check that both stores have the same scope and noalias metadata.
+    if (KillingAA.Scope != DeadAA.Scope ||
+        KillingAA.NoAlias != DeadAA.NoAlias)
+      return OW_Unknown;
+    return OW_Complete;
+  }
   return OW_Unknown;
 }
 

llvm/test/Transforms/DeadStoreElimination/dead-vp.store.ll

@@ -0,0 +1,34 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes=dse -S < %s | FileCheck %s
+target triple = "riscv64-unknown-linux-gnu"
+
+; Test predicated vector length masked stores for elimination
+
+define void @foo(ptr %a, i32 %vl, <vscale x 8 x i32> %v1, <vscale x 8 x i32> %v2) {
+;
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:    [[VP_OP:%.*]] = call <vscale x 8 x i32> @llvm.vp.add.nxv8i32(<vscale x 8 x i32> [[V1:%.*]], <vscale x 8 x i32> [[V2:%.*]], <vscale x 8 x i1> splat (i1 true), i32 [[VL:%.*]])
+; CHECK-NEXT:    call void @llvm.vp.store.nxv8i32.p0(<vscale x 8 x i32> [[VP_OP]], ptr nonnull [[A:%.*]], <vscale x 8 x i1> splat (i1 true), i32 [[VL]]), !alias.scope [[META0:![0-9]+]], !noalias [[META5:![0-9]+]]
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.vp.store.nxv8i32.p0(<vscale x 8 x i32> %v1, ptr nonnull %a, <vscale x 8 x i1> shufflevector (<vscale x 8 x i1> insertelement (<vscale x 8 x i1> poison, i1 true, i32 0), <vscale x 8 x i1> poison, <vscale x 8 x i32> zeroinitializer), i32 %vl), !tbaa !16, !alias.scope !34, !noalias !37
+  %vp.op = call <vscale x 8 x i32> @llvm.vp.add.nxv8i32(<vscale x 8 x i32> %v1, <vscale x 8 x i32> %v2, <vscale x 8 x i1> shufflevector (<vscale x 8 x i1> insertelement (<vscale x 8 x i1> poison, i1 true, i32 0), <vscale x 8 x i1> poison, <vscale x 8 x i32> zeroinitializer), i32 %vl)
+  call void @llvm.vp.store.nxv8i32.p0(<vscale x 8 x i32> %vp.op, ptr nonnull %a, <vscale x 8 x i1> shufflevector (<vscale x 8 x i1> insertelement (<vscale x 8 x i1> poison, i1 true, i32 0), <vscale x 8 x i1> poison, <vscale x 8 x i32> zeroinitializer), i32 %vl), !alias.scope !34, !noalias !37
+  ret void
+}
+
+declare <vscale x 8 x i32> @llvm.vp.add.nxv8i32(<vscale x 8 x i32>, <vscale x 8 x i32>, <vscale x 8 x i1>, i32)
+declare void @llvm.vp.store.nxv8i32.p0(<vscale x 8 x i32>, ptr nocapture, <vscale x 8 x i1>, i32)
+
+!11 = !{!"omnipotent char", !12, i64 0}
+!12 = !{!"Simple C/C++ TBAA"}
+!13 = !{!"int", !11, i64 0}
+!16 = !{!13, !13, i64 0}
+!28 = distinct !{!28, !"LVerDomain"}
+!30 = distinct !{!30, !"LVerDomain"}
+!34 = !{!35, !36}
+!35 = distinct !{!35, !28}
+!36 = distinct !{!36, !30}
+!37 = !{!38, !39}
+!38 = distinct !{!38, !28}
+!39 = distinct !{!39, !28}