[AArch64] Fix SelectionDAG infinite loop for v1i64 SCALAR_TO_VECTOR

A consequence of r347274 is that SCALAR_TO_VECTOR can be converted into
BUILD_VECTOR by SimplifyDemandedBits, but LowerBUILD_VECTOR can turn
BUILD_VECTOR into SCALAR_TO_VECTOR so we get an infinite loop.

Fix this by making LowerBUILD_VECTOR not do this transformation for those
vectors that would get transformed back, i.e. BUILD_VECTOR of a single-element
constant vector. Doing that means we get a DUP, which we then need to recognise
in ISel as a copy.

llvm-svn: 347456

3 files changed, 32 insertions, 1 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index 92223c8e897..0df4c05eb1c 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -7260,7 +7260,10 @@ SDValue AArch64TargetLowering::LowerBUILD_VECTOR(SDValue Op,
     return DAG.getUNDEF(VT);
   }
 
-  if (isOnlyLowElement) {
+  // Convert BUILD_VECTOR where all elements but the lowest are undef into
+  // SCALAR_TO_VECTOR, except for when we have a single-element constant vector
+  // as SimplifyDemandedBits will just turn that back into BUILD_VECTOR.
+  if (isOnlyLowElement && !(NumElts == 1 && isa<ConstantSDNode>(Value))) {
     LLVM_DEBUG(dbgs() << "LowerBUILD_VECTOR: only low element used, creating 1 "
                          "SCALAR_TO_VECTOR node\n");
     return DAG.getNode(ISD::SCALAR_TO_VECTOR, dl, VT, Value);
diff --git a/llvm/lib/Target/AArch64/AArch64InstrInfo.td b/llvm/lib/Target/AArch64/AArch64InstrInfo.td
index 3c949f5d6f6..84aedaafffe 100644
--- a/llvm/lib/Target/AArch64/AArch64InstrInfo.td
+++ b/llvm/lib/Target/AArch64/AArch64InstrInfo.td
@@ -4349,6 +4349,12 @@ def DUPv8i16lane : SIMDDup16FromElement<1, ".8h", v8i16, V128>;
 def DUPv8i8lane  : SIMDDup8FromElement <0, ".8b", v8i8, V64>;
 def DUPv16i8lane : SIMDDup8FromElement <1, ".16b", v16i8, V128>;
 
+// DUP from a 64-bit register to a 64-bit register is just a copy
+def : Pat<(v1i64 (AArch64dup (i64 GPR64:$Rn))),
+          (COPY_TO_REGCLASS GPR64:$Rn, FPR64)>;
+def : Pat<(v1f64 (AArch64dup (f64 FPR64:$Rn))),
+          (COPY_TO_REGCLASS FPR64:$Rn, FPR64)>;
+
 def : Pat<(v2f32 (AArch64dup (f32 FPR32:$Rn))),
           (v2f32 (DUPv2i32lane
             (INSERT_SUBREG (v4i32 (IMPLICIT_DEF)), FPR32:$Rn, ssub),
diff --git a/llvm/test/CodeGen/AArch64/arm64-build-vector.ll b/llvm/test/CodeGen/AArch64/arm64-build-vector.ll
index d7fe9c6d68b..d268f761c9a 100644
--- a/llvm/test/CodeGen/AArch64/arm64-build-vector.ll
+++ b/llvm/test/CodeGen/AArch64/arm64-build-vector.ll
@@ -53,3 +53,25 @@ define void @widen_f16_build_vector(half* %addr) {
   store <2 x half> <half 0xH33EE, half 0xH33EE>, <2 x half>* %1, align 2
   ret void
 }
+
+; Check that a single element vector is constructed with a mov
+define <1 x i64> @single_element_vector_i64(<1 x i64> %arg) {
+; CHECK-LABEL: single_element_vector_i64
+; CHECK: orr w[[GREG:[0-9]+]], wzr, #0x1
+; CHECK: fmov d[[DREG:[0-9]+]], x[[GREG]]
+; CHECK: add d0, d0, d[[DREG]]
+; CHECK: ret
+entry:
+  %add = add <1 x i64> %arg, <i64 1>
+  ret <1 x i64> %add
+}
+
+define <1 x double> @single_element_vector_double(<1 x double> %arg) {
+; CHECK-LABEL: single_element_vector_double
+; CHECK: fmov d[[DREG:[0-9]+]], #1.00000000
+; CHECK: fadd d0, d0, d[[DREG]]
+; CHECK: ret
+entry:
+  %add = fadd <1 x double> %arg, <double 1.0>
+  ret <1 x double> %add
+}