[LLVM][CodeGen][SVE] Improve custom lowering for EXTRACT_SUBVECTOR.

We can extract any legal fixed length vector from a scalable vector
by using VECTOR_SPLICE. I've also taken the time to simplify the
code a little.

Author: paulwalker-arm

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -13897,45 +13897,52 @@ AArch64TargetLowering::LowerEXTRACT_VECTOR_ELT(SDValue Op,
 
 SDValue AArch64TargetLowering::LowerEXTRACT_SUBVECTOR(SDValue Op,
                                                       SelectionDAG &DAG) const {
-  assert(Op.getValueType().isFixedLengthVector() &&
+  EVT VT = Op.getValueType();
+  assert(VT.isFixedLengthVector() &&
          "Only cases that extract a fixed length vector are supported!");
-
   EVT InVT = Op.getOperand(0).getValueType();
-  unsigned Idx = Op.getConstantOperandVal(1);
-  unsigned Size = Op.getValueSizeInBits();
 
   // If we don't have legal types yet, do nothing
-  if (!DAG.getTargetLoweringInfo().isTypeLegal(InVT))
+  if (!isTypeLegal(InVT))
     return SDValue();
 
-  if (InVT.isScalableVector()) {
-    // This will be matched by custom code during ISelDAGToDAG.
-    if (Idx == 0 && isPackedVectorType(InVT, DAG))
+  if (InVT.is128BitVector()) {
+    assert(VT.is64BitVector() && "Extracting unexpected vector type!");
+    unsigned Idx = Op.getConstantOperandVal(1);
+
+    // This will get lowered to an appropriate EXTRACT_SUBREG in ISel.
+    if (Idx == 0)
       return Op;
 
-    return SDValue();
+    // If this is extracting the upper 64-bits of a 128-bit vector, we match
+    // that directly.
+    if (Idx * InVT.getScalarSizeInBits() == 64 && Subtarget->isNeonAvailable())
+      return Op;
   }
 
-  // This will get lowered to an appropriate EXTRACT_SUBREG in ISel.
-  if (Idx == 0 && InVT.getSizeInBits() <= 128)
-    return Op;
-
-  // If this is extracting the upper 64-bits of a 128-bit vector, we match
-  // that directly.
-  if (Size == 64 && Idx * InVT.getScalarSizeInBits() == 64 &&
-      InVT.getSizeInBits() == 128 && Subtarget->isNeonAvailable())
-    return Op;
-
-  if (useSVEForFixedLengthVectorVT(InVT, !Subtarget->isNeonAvailable())) {
+  if (InVT.isScalableVector() ||
+      useSVEForFixedLengthVectorVT(InVT, !Subtarget->isNeonAvailable())) {
     SDLoc DL(Op);
+    SDValue Vec = Op.getOperand(0);
+    SDValue Idx = Op.getOperand(1);
 
-    EVT ContainerVT = getContainerForFixedLengthVector(DAG, InVT);
-    SDValue NewInVec =
-        convertToScalableVector(DAG, ContainerVT, Op.getOperand(0));
+    EVT PackedVT = getPackedSVEVectorVT(InVT.getVectorElementType());
+    if (PackedVT != InVT) {
+      // Pack input into the bottom part of an SVE register and try again.
+      SDValue Container = DAG.getNode(ISD::INSERT_SUBVECTOR, DL, PackedVT,
+                                      DAG.getUNDEF(PackedVT), Vec,
+                                      DAG.getVectorIdxConstant(0, DL));
+      return DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, VT, Container, Idx);
+    }
+
+    // This will get matched by custom code during ISelDAGToDAG.
+    if (isNullConstant(Idx))
+      return Op;
 
-    SDValue Splice = DAG.getNode(ISD::VECTOR_SPLICE, DL, ContainerVT, NewInVec,
-                                 NewInVec, DAG.getConstant(Idx, DL, MVT::i64));
-    return convertFromScalableVector(DAG, Op.getValueType(), Splice);
+    assert(InVT.isScalableVector() && "Unexpected vector type!");
+    // Move requested subvector to the start of the vector and try again.
+    SDValue Splice = DAG.getNode(ISD::VECTOR_SPLICE, DL, InVT, Vec, Vec, Idx);
+    return convertFromScalableVector(DAG, VT, Splice);
   }
 
   return SDValue();

llvm/test/CodeGen/AArch64/sve-extract-fixed-from-scalable-vector.ll

@@ -143,15 +143,8 @@ define <4 x float> @extract_v4f32_nxv16f32_12(<vscale x 16 x float> %arg) {
 define <2 x float> @extract_v2f32_nxv16f32_2(<vscale x 16 x float> %arg) {
 ; CHECK-LABEL: extract_v2f32_nxv16f32_2:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    str x29, [sp, #-16]! // 8-byte Folded Spill
-; CHECK-NEXT:    addvl sp, sp, #-1
-; CHECK-NEXT:    .cfi_escape 0x0f, 0x0c, 0x8f, 0x00, 0x11, 0x10, 0x22, 0x11, 0x08, 0x92, 0x2e, 0x00, 0x1e, 0x22 // sp + 16 + 8 * VG
-; CHECK-NEXT:    .cfi_offset w29, -16
-; CHECK-NEXT:    ptrue p0.s
-; CHECK-NEXT:    st1w { z0.s }, p0, [sp]
-; CHECK-NEXT:    ldr d0, [sp, #8]
-; CHECK-NEXT:    addvl sp, sp, #1
-; CHECK-NEXT:    ldr x29, [sp], #16 // 8-byte Folded Reload
+; CHECK-NEXT:    ext z0.b, z0.b, z0.b, #8
+; CHECK-NEXT:    // kill: def $d0 killed $d0 killed $z0
 ; CHECK-NEXT:    ret
   %ext = call <2 x float> @llvm.vector.extract.v2f32.nxv16f32(<vscale x 16 x float> %arg, i64 2)
   ret <2 x float> %ext
@@ -274,15 +267,8 @@ define <4 x i3> @extract_v4i3_nxv32i3_16(<vscale x 32 x i3> %arg) {
 define <2 x i32> @extract_v2i32_nxv16i32_2(<vscale x 16 x i32> %arg) {
 ; CHECK-LABEL: extract_v2i32_nxv16i32_2:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    str x29, [sp, #-16]! // 8-byte Folded Spill
-; CHECK-NEXT:    addvl sp, sp, #-1
-; CHECK-NEXT:    .cfi_escape 0x0f, 0x0c, 0x8f, 0x00, 0x11, 0x10, 0x22, 0x11, 0x08, 0x92, 0x2e, 0x00, 0x1e, 0x22 // sp + 16 + 8 * VG
-; CHECK-NEXT:    .cfi_offset w29, -16
-; CHECK-NEXT:    ptrue p0.s
-; CHECK-NEXT:    st1w { z0.s }, p0, [sp]
-; CHECK-NEXT:    ldr d0, [sp, #8]
-; CHECK-NEXT:    addvl sp, sp, #1
-; CHECK-NEXT:    ldr x29, [sp], #16 // 8-byte Folded Reload
+; CHECK-NEXT:    ext z0.b, z0.b, z0.b, #8
+; CHECK-NEXT:    // kill: def $d0 killed $d0 killed $z0
 ; CHECK-NEXT:    ret
   %ext = call <2 x i32> @llvm.vector.extract.v2i32.nxv16i32(<vscale x 16 x i32> %arg, i64 2)
   ret <2 x i32> %ext
@@ -314,16 +300,9 @@ define <4 x half> @extract_v4f16_nxv2f16_0(<vscale x 2 x half> %arg) {
 ; CHECK-NEXT:    addvl sp, sp, #-1
 ; CHECK-NEXT:    .cfi_escape 0x0f, 0x0c, 0x8f, 0x00, 0x11, 0x10, 0x22, 0x11, 0x08, 0x92, 0x2e, 0x00, 0x1e, 0x22 // sp + 16 + 8 * VG
 ; CHECK-NEXT:    .cfi_offset w29, -16
-; CHECK-NEXT:    cntd x8
 ; CHECK-NEXT:    ptrue p0.d
-; CHECK-NEXT:    addpl x9, sp, #6
-; CHECK-NEXT:    subs x8, x8, #4
-; CHECK-NEXT:    csel x8, xzr, x8, lo
-; CHECK-NEXT:    st1h { z0.d }, p0, [sp, #3, mul vl]
-; CHECK-NEXT:    cmp x8, #0
-; CHECK-NEXT:    csel x8, x8, xzr, lo
-; CHECK-NEXT:    lsl x8, x8, #1
-; CHECK-NEXT:    ldr d0, [x9, x8]
+; CHECK-NEXT:    st1h { z0.d }, p0, [sp]
+; CHECK-NEXT:    ldr d0, [sp]
 ; CHECK-NEXT:    addvl sp, sp, #1
 ; CHECK-NEXT:    ldr x29, [sp], #16 // 8-byte Folded Reload
 ; CHECK-NEXT:    ret
@@ -338,17 +317,12 @@ define <4 x half> @extract_v4f16_nxv2f16_4(<vscale x 2 x half> %arg) {
 ; CHECK-NEXT:    addvl sp, sp, #-1
 ; CHECK-NEXT:    .cfi_escape 0x0f, 0x0c, 0x8f, 0x00, 0x11, 0x10, 0x22, 0x11, 0x08, 0x92, 0x2e, 0x00, 0x1e, 0x22 // sp + 16 + 8 * VG
 ; CHECK-NEXT:    .cfi_offset w29, -16
-; CHECK-NEXT:    cntd x8
-; CHECK-NEXT:    mov w9, #4 // =0x4
 ; CHECK-NEXT:    ptrue p0.d
-; CHECK-NEXT:    subs x8, x8, #4
-; CHECK-NEXT:    csel x8, xzr, x8, lo
-; CHECK-NEXT:    st1h { z0.d }, p0, [sp, #3, mul vl]
-; CHECK-NEXT:    cmp x8, #4
-; CHECK-NEXT:    csel x8, x8, x9, lo
-; CHECK-NEXT:    addpl x9, sp, #6
-; CHECK-NEXT:    lsl x8, x8, #1
-; CHECK-NEXT:    ldr d0, [x9, x8]
+; CHECK-NEXT:    ptrue p1.h
+; CHECK-NEXT:    st1h { z0.d }, p0, [sp]
+; CHECK-NEXT:    ld1h { z0.h }, p1/z, [sp]
+; CHECK-NEXT:    ext z0.b, z0.b, z0.b, #8
+; CHECK-NEXT:    // kill: def $d0 killed $d0 killed $z0
 ; CHECK-NEXT:    addvl sp, sp, #1
 ; CHECK-NEXT:    ldr x29, [sp], #16 // 8-byte Folded Reload
 ; CHECK-NEXT:    ret