cmd/compile: add a test for writebarrier pass with single-block loop

The old writebarrier implementation fails to handle single-block
loop where a memory Phi value depends on the write barrier store
in the same block. The new implementation (CL 36834) doesn't have
this problem. Add a test to ensure it.

Fix #19067.

Change-Id: Iab13c6817edc12be8a048d18699b4450fa7ed712
Reviewed-on: https://go-review.googlesource.com/36940Reviewed-by: David Chase <drchase@google.com>

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

@@ -27,3 +27,27 @@ func TestWriteBarrierStoreOrder(t *testing.T) {
 	writebarrier(fun.f)
 	CheckFunc(fun.f)
 }
+
+func TestWriteBarrierPhi(t *testing.T) {
+	// Make sure writebarrier phase works for single-block loop, where
+	// a Phi op takes the store in the same block as argument.
+	// See issue #19067.
+	c := testConfig(t)
+	ptrType := &TypeImpl{Size_: 8, Ptr: true, Name: "testptr"} // dummy for testing
+	fun := Fun(c, "entry",
+		Bloc("entry",
+			Valu("start", OpInitMem, TypeMem, 0, nil),
+			Valu("sb", OpSB, TypeInvalid, 0, nil),
+			Valu("sp", OpSP, TypeInvalid, 0, nil),
+			Goto("loop")),
+		Bloc("loop",
+			Valu("phi", OpPhi, TypeMem, 0, nil, "start", "wb"),
+			Valu("v", OpConstNil, ptrType, 0, nil),
+			Valu("addr", OpAddr, ptrType, 0, nil, "sb"),
+			Valu("wb", OpStore, TypeMem, 8, ptrType, "addr", "v", "phi"), // has write barrier
+			Goto("loop")))
+
+	CheckFunc(fun.f)
+	writebarrier(fun.f)
+	CheckFunc(fun.f)
+}