[RISCV] Combine vmerge_vl allones -> vmv_v_v, vmv_v_v splat(x) -> vmv_v_x (#170539)

lukel97 · web-flow · commit 6584e47101b0 · 2025-12-04T19:53:28.000+08:00
An upcoming patch aims to remove the last use of
@llvm.experimental.vp.splat in RISCVCodegenPrepare by replacing it with
a vp_merge of a regular splat.

A vp_merge will get lowered to vmerge_vl, and if we combine vmerge_vl of
a splat to vmv_v_x we can get the same behaviour as the vp.splat
intrinsic.

This adds the two combines needed. It was easier to do the combines on
_vl nodes rather than on vp_merge itself, since the types are already
legal for _vl nodes.
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -21872,6 +21872,44 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
       return N->getOperand(0);
     break;
   }
+  case RISCVISD::VMERGE_VL: {
+    // vmerge_vl allones, x, y, passthru, vl -> vmv_v_v passthru, x, vl
+    SDValue Mask = N->getOperand(0);
+    SDValue True = N->getOperand(1);
+    SDValue Passthru = N->getOperand(3);
+    SDValue VL = N->getOperand(4);
+
+    // Fixed vectors are wrapped in scalable containers, unwrap them.
+    using namespace SDPatternMatch;
+    SDValue SubVec;
+    if (sd_match(Mask, m_InsertSubvector(m_Undef(), m_Value(SubVec), m_Zero())))
+      Mask = SubVec;
+
+    if (!isOneOrOneSplat(Mask))
+      break;
+
+    return DAG.getNode(RISCVISD::VMV_V_V_VL, SDLoc(N), N->getSimpleValueType(0),
+                       Passthru, True, VL);
+  }
+  case RISCVISD::VMV_V_V_VL: {
+    // vmv_v_v passthru, splat(x), vl -> vmv_v_x passthru, x, vl
+    SDValue Passthru = N->getOperand(0);
+    SDValue Src = N->getOperand(1);
+    SDValue VL = N->getOperand(2);
+
+    // Fixed vectors are wrapped in scalable containers, unwrap them.
+    using namespace SDPatternMatch;
+    SDValue SubVec;
+    if (sd_match(Src, m_InsertSubvector(m_Undef(), m_Value(SubVec), m_Zero())))
+      Src = SubVec;
+
+    SDValue SplatVal = DAG.getSplatValue(Src);
+    if (!SplatVal)
+      break;
+    MVT VT = N->getSimpleValueType(0);
+    return lowerScalarSplat(Passthru, SplatVal, VL, VT, SDLoc(N), DAG,
+                            Subtarget);
+  }
   case RISCVISD::VSLIDEDOWN_VL:
   case RISCVISD::VSLIDEUP_VL:
     if (N->getOperand(1)->isUndef())
diff --git a/llvm/test/CodeGen/RISCV/rvv/fixed-vectors-vpmerge.ll b/llvm/test/CodeGen/RISCV/rvv/fixed-vectors-vpmerge.ll
@@ -1353,3 +1353,48 @@ define <32 x double> @vpmerge_vf_v32f64(double %a, <32 x double> %vb, <32 x i1>
   %v = call <32 x double> @llvm.vp.merge.v32f64(<32 x i1> %m, <32 x double> %va, <32 x double> %vb, i32 %evl)
   ret <32 x double> %v
 }
+
+define <4 x i32> @splat_v4i32(i32 %x, i32 zeroext %evl) {
+; CHECK-LABEL: splat_v4i32:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli zero, a1, e32, m1, ta, ma
+; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    ret
+  %head = insertelement <4 x i32> poison, i32 %x, i32 0
+  %splat = shufflevector <4 x i32> %head, <4 x i32> poison, <4 x i32> zeroinitializer
+  %v = call <4 x i32> @llvm.vp.merge(<4 x i1> splat (i1 true), <4 x i32> %splat, <4 x i32> poison, i32 %evl)
+  ret <4 x i32> %v
+}
+
+define <4 x float> @splat_v4f32(float %x, i32 zeroext %evl) {
+; CHECK-LABEL: splat_v4f32:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli zero, a0, e32, m1, ta, ma
+; CHECK-NEXT:    vfmv.v.f v8, fa0
+; CHECK-NEXT:    ret
+  %head = insertelement <4 x float> poison, float %x, i32 0
+  %splat = shufflevector <4 x float> %head, <4 x float> poison, <4 x i32> zeroinitializer
+  %v = call <4 x float> @llvm.vp.merge(<4 x i1> splat (i1 true), <4 x float> %splat, <4 x float> poison, i32 %evl)
+  ret <4 x float> %v
+}
+
+define <4 x i32> @splat_v4i32_const(i32 zeroext %evl) {
+; CHECK-LABEL: splat_v4i32_const:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli zero, a0, e32, m1, ta, ma
+; CHECK-NEXT:    vmv.v.i v8, 1
+; CHECK-NEXT:    ret
+  %v = call <4 x i32> @llvm.vp.merge(<4 x i1> splat (i1 true), <4 x i32> splat (i32 1), <4 x i32> poison, i32 %evl)
+  ret <4 x i32> %v
+}
+
+define <4 x float> @splat_v4f32_const(i32 zeroext %evl) {
+; CHECK-LABEL: splat_v4f32_const:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    lui a1, 270976
+; CHECK-NEXT:    vsetvli zero, a0, e32, m1, ta, ma
+; CHECK-NEXT:    vmv.v.x v8, a1
+; CHECK-NEXT:    ret
+  %v = call <4 x float> @llvm.vp.merge(<4 x i1> splat (i1 true), <4 x float> splat (float 42.0), <4 x float> poison, i32 %evl)
+  ret <4 x float> %v
+}
diff --git a/llvm/test/CodeGen/RISCV/rvv/vpmerge-sdnode.ll b/llvm/test/CodeGen/RISCV/rvv/vpmerge-sdnode.ll
@@ -1663,3 +1663,48 @@ define <vscale x 8 x double> @vpmerge_vf_nxv8f64(double %a, <vscale x 8 x double
   %v = call <vscale x 8 x double> @llvm.vp.merge.nxv8f64(<vscale x 8 x i1> %m, <vscale x 8 x double> %va, <vscale x 8 x double> %vb, i32 %evl)
   ret <vscale x 8 x double> %v
 }
+
+define <vscale x 2 x i32> @splat_nxv2i32(i32 %x, i32 zeroext %evl) {
+; CHECK-LABEL: splat_nxv2i32:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli zero, a1, e32, m1, ta, ma
+; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    ret
+  %head = insertelement <vscale x 2 x i32> poison, i32 %x, i32 0
+  %splat = shufflevector <vscale x 2 x i32> %head, <vscale x 2 x i32> poison, <vscale x 2 x i32> zeroinitializer
+  %v = call <vscale x 2 x i32> @llvm.vp.merge(<vscale x 2 x i1> splat (i1 true), <vscale x 2 x i32> %splat, <vscale x 2 x i32> poison, i32 %evl)
+  ret <vscale x 2 x i32> %v
+}
+
+define <vscale x 2 x float> @splat_nxv2f32(float %x, i32 zeroext %evl) {
+; CHECK-LABEL: splat_nxv2f32:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli zero, a0, e32, m1, ta, ma
+; CHECK-NEXT:    vfmv.v.f v8, fa0
+; CHECK-NEXT:    ret
+  %head = insertelement <vscale x 2 x float> poison, float %x, i32 0
+  %splat = shufflevector <vscale x 2 x float> %head, <vscale x 2 x float> poison, <vscale x 2 x i32> zeroinitializer
+  %v = call <vscale x 2 x float> @llvm.vp.merge(<vscale x 2 x i1> splat (i1 true), <vscale x 2 x float> %splat, <vscale x 2 x float> poison, i32 %evl)
+  ret <vscale x 2 x float> %v
+}
+
+define <vscale x 2 x i32> @splat_nxv2i32_const(i32 zeroext %evl) {
+; CHECK-LABEL: splat_nxv2i32_const:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli zero, a0, e32, m1, ta, ma
+; CHECK-NEXT:    vmv.v.i v8, 1
+; CHECK-NEXT:    ret
+  %v = call <vscale x 2 x i32> @llvm.vp.merge(<vscale x 2 x i1> splat (i1 true), <vscale x 2 x i32> splat (i32 1), <vscale x 2 x i32> poison, i32 %evl)
+  ret <vscale x 2 x i32> %v
+}
+
+define <vscale x 2 x float> @splat_nxv2f32_const(i32 zeroext %evl) {
+; CHECK-LABEL: splat_nxv2f32_const:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    lui a1, 270976
+; CHECK-NEXT:    vsetvli zero, a0, e32, m1, ta, ma
+; CHECK-NEXT:    vmv.v.x v8, a1
+; CHECK-NEXT:    ret
+  %v = call <vscale x 2 x float> @llvm.vp.merge(<vscale x 2 x i1> splat (i1 true), <vscale x 2 x float> splat (float 42.0), <vscale x 2 x float> poison, i32 %evl)
+  ret <vscale x 2 x float> %v
+}
