[RISCV] Prefer vmv.s.x for build_vector a, undef, ..., undef (llvm#136164)

preames · IanWood1 · commit b86f0c724b33 · 2025-05-06T08:50:57.000-07:00
If we have a build vector which could be either a splat or a scalar
insert, prefer the scalar insert. At high LMUL, this reduces vector
register pressure (locally, the use will likely still be aligned), and
the amount of work performed for the splat.
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -4208,8 +4208,22 @@ static SDValue lowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG,
   if (SDValue Splat = cast<BuildVectorSDNode>(Op)->getSplatValue()) {
     if (auto Gather = matchSplatAsGather(Splat, VT, DL, DAG, Subtarget))
       return Gather;
-    unsigned Opc = VT.isFloatingPoint() ? RISCVISD::VFMV_V_F_VL
-                                        : RISCVISD::VMV_V_X_VL;
+
+    // Prefer vmv.s.x/vfmv.s.f if legal to reduce work and register
+    // pressure at high LMUL.
+    if (all_of(Op->ops().drop_front(),
+               [](const SDUse &U) { return U.get().isUndef(); })) {
+      unsigned Opc =
+          VT.isFloatingPoint() ? RISCVISD::VFMV_S_F_VL : RISCVISD::VMV_S_X_VL;
+      if (!VT.isFloatingPoint())
+        Splat = DAG.getNode(ISD::ANY_EXTEND, DL, XLenVT, Splat);
+      Splat = DAG.getNode(Opc, DL, ContainerVT, DAG.getUNDEF(ContainerVT),
+                          Splat, VL);
+      return convertFromScalableVector(VT, Splat, DAG, Subtarget);
+    }
+
+    unsigned Opc =
+        VT.isFloatingPoint() ? RISCVISD::VFMV_V_F_VL : RISCVISD::VMV_V_X_VL;
     if (!VT.isFloatingPoint())
       Splat = DAG.getNode(ISD::ANY_EXTEND, DL, XLenVT, Splat);
     Splat =
diff --git a/llvm/test/CodeGen/RISCV/rvv/fixed-vectors-binop-splats.ll b/llvm/test/CodeGen/RISCV/rvv/fixed-vectors-binop-splats.ll
@@ -187,7 +187,7 @@ define <2 x i8> @v2i8(i8 %x, i8 %y) {
 ; CHECK-LABEL: v2i8:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 2, e8, mf8, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v9, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v9, 0
 ; CHECK-NEXT:    ret
@@ -203,7 +203,7 @@ define <4 x i8> @v4i8(i8 %x, i8 %y) {
 ; CHECK-LABEL: v4i8:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 4, e8, mf4, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v9, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v9, 0
 ; CHECK-NEXT:    ret
@@ -219,7 +219,7 @@ define <8 x i8> @v8i8(i8 %x, i8 %y) {
 ; CHECK-LABEL: v8i8:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 8, e8, mf2, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v9, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v9, 0
 ; CHECK-NEXT:    ret
@@ -235,7 +235,7 @@ define <16 x i8> @v16i8(i8 %x, i8 %y) {
 ; CHECK-LABEL: v16i8:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 16, e8, m1, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v9, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v9, 0
 ; CHECK-NEXT:    ret
@@ -252,7 +252,7 @@ define <32 x i8> @v32i8(i8 %x, i8 %y) {
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    li a2, 32
 ; CHECK-NEXT:    vsetvli zero, a2, e8, m2, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v10, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v10, 0
 ; CHECK-NEXT:    ret
@@ -269,7 +269,7 @@ define <64 x i8> @v64i8(i8 %x, i8 %y) {
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    li a2, 64
 ; CHECK-NEXT:    vsetvli zero, a2, e8, m4, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v12, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v12, 0
 ; CHECK-NEXT:    ret
@@ -300,7 +300,7 @@ define <2 x i16> @v2i16(i16 %x, i16 %y) {
 ; CHECK-LABEL: v2i16:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 2, e16, mf4, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v9, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v9, 0
 ; CHECK-NEXT:    ret
@@ -316,7 +316,7 @@ define <4 x i16> @v4i16(i16 %x, i16 %y) {
 ; CHECK-LABEL: v4i16:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 4, e16, mf2, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v9, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v9, 0
 ; CHECK-NEXT:    ret
@@ -332,7 +332,7 @@ define <8 x i16> @v8i16(i16 %x, i16 %y) {
 ; CHECK-LABEL: v8i16:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 8, e16, m1, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v9, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v9, 0
 ; CHECK-NEXT:    ret
@@ -348,7 +348,7 @@ define <16 x i16> @v16i16(i16 %x, i16 %y) {
 ; CHECK-LABEL: v16i16:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 16, e16, m2, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v10, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v10, 0
 ; CHECK-NEXT:    ret
@@ -365,7 +365,7 @@ define <32 x i16> @v32i16(i16 %x, i16 %y) {
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    li a2, 32
 ; CHECK-NEXT:    vsetvli zero, a2, e16, m4, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v12, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v12, 0
 ; CHECK-NEXT:    ret
@@ -396,7 +396,7 @@ define <2 x i32> @v2i32(i32 %x, i32 %y) {
 ; CHECK-LABEL: v2i32:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 2, e32, mf2, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v9, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v9, 0
 ; CHECK-NEXT:    ret
@@ -412,7 +412,7 @@ define <4 x i32> @v4i32(i32 %x, i32 %y) {
 ; CHECK-LABEL: v4i32:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 4, e32, m1, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v9, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v9, 0
 ; CHECK-NEXT:    ret
@@ -428,7 +428,7 @@ define <8 x i32> @v8i32(i32 %x, i32 %y) {
 ; CHECK-LABEL: v8i32:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 8, e32, m2, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v10, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v10, 0
 ; CHECK-NEXT:    ret
@@ -444,7 +444,7 @@ define <16 x i32> @v16i32(i32 %x, i32 %y) {
 ; CHECK-LABEL: v16i32:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 16, e32, m4, ta, ma
-; CHECK-NEXT:    vmv.v.x v8, a0
+; CHECK-NEXT:    vmv.s.x v8, a0
 ; CHECK-NEXT:    vadd.vx v12, v8, a1
 ; CHECK-NEXT:    vrgather.vi v8, v12, 0
 ; CHECK-NEXT:    ret
@@ -509,7 +509,7 @@ define <2 x i64> @v2i64(i64 %x, i64 %y) {
 ; RV64-LABEL: v2i64:
 ; RV64:       # %bb.0:
 ; RV64-NEXT:    vsetivli zero, 2, e64, m1, ta, ma
-; RV64-NEXT:    vmv.v.x v8, a0
+; RV64-NEXT:    vmv.s.x v8, a0
 ; RV64-NEXT:    vadd.vx v9, v8, a1
 ; RV64-NEXT:    vrgather.vi v8, v9, 0
 ; RV64-NEXT:    ret
@@ -542,7 +542,7 @@ define <4 x i64> @v4i64(i64 %x, i64 %y) {
 ; RV64-LABEL: v4i64:
 ; RV64:       # %bb.0:
 ; RV64-NEXT:    vsetivli zero, 4, e64, m2, ta, ma
-; RV64-NEXT:    vmv.v.x v8, a0
+; RV64-NEXT:    vmv.s.x v8, a0
 ; RV64-NEXT:    vadd.vx v10, v8, a1
 ; RV64-NEXT:    vrgather.vi v8, v10, 0
 ; RV64-NEXT:    ret
@@ -575,7 +575,7 @@ define <8 x i64> @v8i64(i64 %x, i64 %y) {
 ; RV64-LABEL: v8i64:
 ; RV64:       # %bb.0:
 ; RV64-NEXT:    vsetivli zero, 8, e64, m4, ta, ma
-; RV64-NEXT:    vmv.v.x v8, a0
+; RV64-NEXT:    vmv.s.x v8, a0
 ; RV64-NEXT:    vadd.vx v12, v8, a1
 ; RV64-NEXT:    vrgather.vi v8, v12, 0
 ; RV64-NEXT:    ret
@@ -591,7 +591,7 @@ define <4 x half> @v4f16(half %x, half %y) {
 ; CHECK-LABEL: v4f16:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 4, e16, mf2, ta, ma
-; CHECK-NEXT:    vfmv.v.f v8, fa0
+; CHECK-NEXT:    vfmv.s.f v8, fa0
 ; CHECK-NEXT:    vfadd.vf v9, v8, fa1
 ; CHECK-NEXT:    vrgather.vi v8, v9, 0
 ; CHECK-NEXT:    ret
@@ -607,7 +607,7 @@ define <2 x float> @v2f32(float %x, float %y) {
 ; CHECK-LABEL: v2f32:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 2, e32, mf2, ta, ma
-; CHECK-NEXT:    vfmv.v.f v8, fa0
+; CHECK-NEXT:    vfmv.s.f v8, fa0
 ; CHECK-NEXT:    vfadd.vf v9, v8, fa1
 ; CHECK-NEXT:    vrgather.vi v8, v9, 0
 ; CHECK-NEXT:    ret
diff --git a/llvm/test/CodeGen/RISCV/rvv/fixed-vectors-buildvec-of-binop.ll b/llvm/test/CodeGen/RISCV/rvv/fixed-vectors-buildvec-of-binop.ll
@@ -439,7 +439,7 @@ define void @buggy(i32 %0) #0 {
 ; RV32-LABEL: buggy:
 ; RV32:       # %bb.0: # %entry
 ; RV32-NEXT:    vsetivli zero, 4, e32, m1, ta, ma
-; RV32-NEXT:    vmv.v.x v8, a0
+; RV32-NEXT:    vmv.s.x v8, a0
 ; RV32-NEXT:    vadd.vv v8, v8, v8
 ; RV32-NEXT:    vor.vi v8, v8, 1
 ; RV32-NEXT:    vrgather.vi v9, v8, 0
@@ -450,7 +450,7 @@ define void @buggy(i32 %0) #0 {
 ; RV64:       # %bb.0: # %entry
 ; RV64-NEXT:    slli a0, a0, 1
 ; RV64-NEXT:    vsetivli zero, 4, e32, m1, ta, ma
-; RV64-NEXT:    vmv.v.x v8, a0
+; RV64-NEXT:    vmv.s.x v8, a0
 ; RV64-NEXT:    vor.vi v8, v8, 1
 ; RV64-NEXT:    vrgather.vi v9, v8, 0
 ; RV64-NEXT:    vse32.v v9, (zero)
diff --git a/llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shuffle-vslide1up.ll b/llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shuffle-vslide1up.ll
@@ -250,7 +250,7 @@ define <4 x double> @vslide1up_4xf64(<4 x double> %v, double %b) {
 ; CHECK-LABEL: vslide1up_4xf64:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetivli zero, 4, e64, m2, ta, ma
-; CHECK-NEXT:    vfmv.v.f v10, fa0
+; CHECK-NEXT:    vfmv.s.f v10, fa0
 ; CHECK-NEXT:    vslideup.vi v10, v8, 3
 ; CHECK-NEXT:    vmv.v.v v8, v10
 ; CHECK-NEXT:    ret
diff --git a/llvm/test/CodeGen/RISCV/rvv/fixed-vectors-unaligned.ll b/llvm/test/CodeGen/RISCV/rvv/fixed-vectors-unaligned.ll
@@ -500,7 +500,7 @@ define void @masked_load_v2i32_align1(ptr %a, <2 x i32> %m, ptr %res_ptr) nounwi
 ; RV32-SLOW-NEXT:    or a4, a6, a5
 ; RV32-SLOW-NEXT:    or a3, a4, a3
 ; RV32-SLOW-NEXT:    vsetvli zero, zero, e32, mf2, ta, ma
-; RV32-SLOW-NEXT:    vmv.v.x v8, a3
+; RV32-SLOW-NEXT:    vmv.s.x v8, a3
 ; RV32-SLOW-NEXT:  .LBB8_2: # %else
 ; RV32-SLOW-NEXT:    andi a2, a2, 2
 ; RV32-SLOW-NEXT:    beqz a2, .LBB8_4
@@ -544,7 +544,7 @@ define void @masked_load_v2i32_align1(ptr %a, <2 x i32> %m, ptr %res_ptr) nounwi
 ; RV64-SLOW-NEXT:    or a4, a6, a5
 ; RV64-SLOW-NEXT:    or a3, a4, a3
 ; RV64-SLOW-NEXT:    vsetvli zero, zero, e32, mf2, ta, ma
-; RV64-SLOW-NEXT:    vmv.v.x v8, a3
+; RV64-SLOW-NEXT:    vmv.s.x v8, a3
 ; RV64-SLOW-NEXT:  .LBB8_2: # %else
 ; RV64-SLOW-NEXT:    andi a2, a2, 2
 ; RV64-SLOW-NEXT:    beqz a2, .LBB8_4
diff --git a/llvm/test/CodeGen/RISCV/rvv/fold-scalar-load-crash.ll b/llvm/test/CodeGen/RISCV/rvv/fold-scalar-load-crash.ll
@@ -9,8 +9,8 @@ define i32 @test(i32 %size, ptr %add.ptr, i64 %const) {
 ; RV32:       # %bb.0: # %entry
 ; RV32-NEXT:    addi a3, a2, 1
 ; RV32-NEXT:    th.lbib a4, (a1), -1, 0
-; RV32-NEXT:    vsetivli zero, 8, e8, mf2, ta, ma
-; RV32-NEXT:    vmv.v.x v8, a4
+; RV32-NEXT:    vsetivli zero, 8, e8, m1, ta, ma
+; RV32-NEXT:    vmv.s.x v8, a4
 ; RV32-NEXT:    vmv.s.x v9, zero
 ; RV32-NEXT:    vsetvli zero, a3, e8, mf2, tu, ma
 ; RV32-NEXT:    vslideup.vx v8, v9, a2
@@ -35,8 +35,8 @@ define i32 @test(i32 %size, ptr %add.ptr, i64 %const) {
 ; RV64:       # %bb.0: # %entry
 ; RV64-NEXT:    addi a3, a2, 1
 ; RV64-NEXT:    th.lbib a4, (a1), -1, 0
-; RV64-NEXT:    vsetivli zero, 8, e8, mf2, ta, ma
-; RV64-NEXT:    vmv.v.x v8, a4
+; RV64-NEXT:    vsetivli zero, 8, e8, m1, ta, ma
+; RV64-NEXT:    vmv.s.x v8, a4
 ; RV64-NEXT:    vmv.s.x v9, zero
 ; RV64-NEXT:    vsetvli zero, a3, e8, mf2, tu, ma
 ; RV64-NEXT:    vslideup.vx v8, v9, a2
diff --git a/llvm/test/CodeGen/RISCV/rvv/pr125306.ll b/llvm/test/CodeGen/RISCV/rvv/pr125306.ll
@@ -60,7 +60,7 @@ define <2 x i32> @main(ptr %0) {
 ; CHECK-NEXT:    vslide1down.vx v8, v8, zero
 ; CHECK-NEXT:    vslide1down.vx v10, v10, zero
 ; CHECK-NEXT:    vmin.vv v8, v10, v8
-; CHECK-NEXT:    vmv.v.x v10, a0
+; CHECK-NEXT:    vmv.s.x v10, a0
 ; CHECK-NEXT:    vslide1down.vx v11, v11, zero
 ; CHECK-NEXT:    vmin.vx v10, v10, a2
 ; CHECK-NEXT:    vmin.vx v10, v10, a1
