feat(asm): encode the arm64 FP immediate moves

Assisted-by: GLM 5.3 Flash

asm/arm64_encode_test.go

@@ -2069,3 +2069,53 @@ func TestArm64ConRnRejections(t *testing.T) {
 		}
 	}
 }
+
+// TestArm64FPImmediate pins the FP immediate moves against `go tool asm`
+// words: the FMOV (immediate) instruction for the 8-bit encodable values and
+// the FMOV-from-ZR move for zero, plus the rejections the toolchain raises
+// (integer immediates to FP registers, FMOVQ immediates) and the values that
+// only the toolchain's $f64 pool reaches.
+func TestArm64FPImmediate(t *testing.T) {
+	got := arm64Words(t,
+		"\tFMOVS $(4.0), F0\n"+
+			"\tFMOVD $(4.0), F0\n"+
+			"\tFMOVS $(0.265625), F1\n"+
+			"\tFMOVD $(0.1796875), F2\n"+
+			"\tFMOVD $(28.0), F4\n"+
+			"\tFMOVD $(-4.0), F6\n"+
+			"\tFMOVD $0, F0\n"+
+			"\tFMOVD $(0.0), F5\n")
+	want := []uint32{
+		0x1e221000, // FMOV S0, #4.0
+		0x1e621000, // FMOV D0, #4.0
+		0x1e2a3001, // FMOV S1, #0.265625
+		0x1e68f002, // FMOV D2, #0.1796875
+		0x1e679004, // FMOV D4, #28.0
+		0x1e721006, // FMOV D6, #-4.0
+		0x9e6703e0, // FMOV D0, XZR
+		0x9e6703e5, // FMOV D5, XZR
+		0xd65f03c0, // RET
+	}
+	if len(got) != len(want) {
+		t.Fatalf("word count = %d, want %d", len(got), len(want))
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Errorf("word %d = %08x, want %08x", i, got[i], want[i])
+		}
+	}
+	for _, src := range []string{
+		"\tFMOVD\t$5, F0\n",
+		"\tFMOVS\t$4, F0\n",
+		"\tFMOVQ\t$(4.0), F0\n",
+		"\tFMOVD\t$(2.0), F0\n",
+	} {
+		f, errs := parser.Parse("test_arm64.s", "#include \"textflag.h\"\n\nTEXT ·f(SB), NOSPLIT, $0-0\n"+src+"\tRET\n")
+		if len(errs) > 0 {
+			continue // a parse rejection is a rejection
+		}
+		if _, err := AssembleFileARM64(f); err == nil {
+			t.Errorf("expected rejection for %q, got nil", strings.TrimSpace(src))
+		}
+	}
+}