[dev.simd] cmd/compile: exclude simd vars from merge local

It looks like mergelocals pass's liveness analysis does not handle simd
variables well.

The added test forces two vectors to spill in a way that does not work
with mergelocals: if the added check is removed, then `v` and `m` will
be marked merged and spilled to the same location, failing the test.

Change-Id: Ife4e4e939565d817fc24f7180cb791a5084dd191
Reviewed-on: https://go-review.googlesource.com/c/go/+/687375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/func.go

@@ -850,6 +850,13 @@ func (f *Func) NewLocal(pos src.XPos, typ *types.Type) *ir.Name {
 // items larger than what CanSSA would allow (approximateky, we disallow things
 // marked as open defer slots so as to avoid complicating liveness
 // analysis.
+//
+// TODO: make SIMD variables mergible.
+//
+//		Right now this check excludes SIMD vars because sometimes two live SIMD
+//		vectors will be put into the same partition by mergelocals, we need to figure
+//		out why because these vectors are big and should be merged when possible.
+//	 Details in CL 687375.
 func IsMergeCandidate(n *ir.Name) bool {
 	if base.Debug.MergeLocals == 0 ||
 		base.Flag.N != 0 ||
@@ -857,6 +864,7 @@ func IsMergeCandidate(n *ir.Name) bool {
 		n.Type().Size() <= int64(3*types.PtrSize) ||
 		n.Addrtaken() ||
 		n.NonMergeable() ||
+		n.Type().IsSIMD() ||
 		n.OpenDeferSlot() {
 		return false
 	}

src/simd/simd_test.go

@@ -364,3 +364,29 @@ func TestSlicesFloat64(t *testing.T) {
 		}
 	}
 }
+
+// TODO: try to reduce this test to be smaller.
+func TestMergeLocals(t *testing.T) {
+	testMergeLocalswrapper(t, simd.Int64x4.Add)
+}
+
+//go:noinline
+func forceSpill() {}
+
+func testMergeLocalswrapper(t *testing.T, op func(simd.Int64x4, simd.Int64x4) simd.Int64x4) {
+	t.Helper()
+	s0 := []int64{0, 1, 2, 3}
+	s1 := []int64{-1, 0, -1, 0}
+	want := []int64{-1, 1, 1, 3}
+	v := simd.LoadInt64x4Slice(s0)
+	m := simd.LoadInt64x4Slice(s1)
+	forceSpill()
+	got := make([]int64, 4)
+	gotv := op(v, m)
+	gotv.StoreSlice(got)
+	for i := range len(want) {
+		if !(got[i] == want[i]) {
+			t.Errorf("Result at %d incorrect: want %v, got %v", i, want[i], got[i])
+		}
+	}
+}