[AArch64] Fix selection of G_UNMERGE <2 x s16>

Differential revision: https://reviews.llvm.org/D106007

Author: jroelofs

llvm/lib/Target/AArch64/GISel/AArch64InstructionSelector.cpp

@@ -3988,6 +3988,13 @@ bool AArch64InstructionSelector::selectUnmergeValues(MachineInstr &I,
   } else {
     // No. We have to perform subregister inserts. For each insert, create an
     // implicit def and a subregister insert, and save the register we create.
+    const TargetRegisterClass *RC =
+        getMinClassForRegBank(*RBI.getRegBank(SrcReg, MRI, TRI),
+                              WideTy.getScalarSizeInBits() * NumElts);
+    unsigned SubReg = 0;
+    bool Found = getSubRegForClass(RC, TRI, SubReg);
+    (void)Found;
+    assert(Found && "expected to find last operand's subeg idx");
     for (unsigned Idx = 0; Idx < NumInsertRegs; ++Idx) {
       Register ImpDefReg = MRI.createVirtualRegister(&AArch64::FPR128RegClass);
       MachineInstr &ImpDefMI =
@@ -4001,7 +4008,7 @@ bool AArch64InstructionSelector::selectUnmergeValues(MachineInstr &I,
                    TII.get(TargetOpcode::INSERT_SUBREG), InsertReg)
                .addUse(ImpDefReg)
                .addUse(SrcReg)
-               .addImm(AArch64::dsub);
+               .addImm(SubReg);
 
       constrainSelectedInstRegOperands(ImpDefMI, TII, TRI, RBI);
       constrainSelectedInstRegOperands(InsMI, TII, TRI, RBI);

llvm/test/CodeGen/AArch64/GlobalISel/select-unmerge.mir

@@ -11,6 +11,10 @@
     ret <4 x float> %a
   }
 
+  define <2 x half> @test_v2s16_unmerge(<2 x half> %a) {
+    ret <2 x half> %a
+  }
+
   define <4 x half> @test_v4s16_unmerge(<4 x half> %a) {
     ret <4 x half> %a
   }
@@ -87,6 +91,49 @@ body:             |
     RET_ReallyLR implicit $q0
 ...
 ---
+name:            test_v2s16_unmerge
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: fpr }
+  - { id: 1, class: fpr }
+  - { id: 2, class: fpr }
+  - { id: 3, class: fpr }
+  - { id: 4, class: fpr }
+  - { id: 5, class: fpr }
+body:             |
+  bb.1 (%ir-block.0):
+    liveins: $s0
+    ; CHECK-LABEL: name: test_v2s16_unmerge
+
+    ; CHECK: [[COPY:%[0-9]+]]:fpr32 = COPY $s0
+    %0:fpr(<2 x s16>) = COPY $s0
+
+    ; Since 2 * 16 != 128, we need to widen using implicit defs.
+    ; Note that we expect to reuse one of the INSERT_SUBREG results, as CPYi16
+    ; expects a lane > 0.
+    ; CHECK: [[DEF:%[0-9]+]]:fpr128 = IMPLICIT_DEF
+    ; CHECK: [[INSERT_SUBREG:%[0-9]+]]:fpr128 = INSERT_SUBREG [[DEF]], [[COPY]], %subreg.ssub
+    ; CHECK: [[COPY1:%[0-9]+]]:fpr16 = COPY [[INSERT_SUBREG]].hsub
+    ; CHECK: [[CPYi16_:%[0-9]+]]:fpr16 = CPYi16 [[INSERT_SUBREG]], 1
+    %2:fpr(s16), %3:fpr(s16) = G_UNMERGE_VALUES %0(<2 x s16>)
+
+    ; CHECK: [[DEF1:%[0-9]+]]:fpr128 = IMPLICIT_DEF
+    ; CHECK: [[INSERT_SUBREG1:%[0-9]+]]:fpr128 = INSERT_SUBREG [[DEF1]], [[COPY1]], %subreg.hsub
+    ; CHECK: [[DEF2:%[0-9]+]]:fpr128 = IMPLICIT_DEF
+    ; CHECK: [[INSERT_SUBREG2:%[0-9]+]]:fpr128 = INSERT_SUBREG [[DEF2]], [[CPYi16_]], %subreg.hsub
+    ; CHECK: [[INSvi16lane:%[0-9]+]]:fpr128 = INSvi16lane [[INSERT_SUBREG1]], 1, [[INSERT_SUBREG2]], 0
+    ; CHECK: [[COPY2:%[0-9]+]]:fpr32 = COPY [[INSvi16lane]].ssub
+    %1:fpr(<2 x s16>) = G_BUILD_VECTOR %2(s16), %3(s16)
+
+    ; CHECK: $s0 = COPY [[COPY2]]
+    $s0 = COPY %1(<2 x s16>)
+
+    ; CHECK: RET_ReallyLR implicit $s0
+    RET_ReallyLR implicit $s0
+...
+---
 name:            test_v4s16_unmerge
 alignment:       4
 legalized:       true