feat(asm): emit the arm64 stack-split guard and morestack block

asm/guard_test.go

@@ -100,3 +100,46 @@ func TestStackGuardGOObj(t *testing.T) {
 		t.Fatal("object lacks the GOOBJ magic")
 	}
 }
+
+// The arm64 stack-split guard, pinned from `go tool asm` (Go 1.27, arm64):
+// the guard classes, the auto-NOSPLIT leaf behaviour and the morestack
+// block.  Relocation fields are masked: the toolchain's object leaves them
+// zero for the linker, the gasm image resolves file-internal references.
+func TestStackGuardBytesARM64(t *testing.T) {
+	for _, tt := range []struct {
+		name string
+		src  string
+		want string
+	}{
+		{"leafsmall", "TEXT \u00b7leafsmall(SB), $16-0\n\tRET\n",
+			"fe0f1ef8fd831ff8fd2300d1fd630091ff830091c0035fd6"},
+		{"leafmed", "TEXT \u00b7leafmed(SB), $256-0\n\tRET\n",
+			"900b40f9f14302d13f0210eb09010054f44304d19dfa3fa99f020091fd2300d1fd230491ff430491c0035fd6e3031eaa00000000f3ffff17"},
+		{"leafbig", "TEXT \u00b7leafbig(SB), $8192-0\n\tRET\n",
+			"900b40f91bf283d2f1031beba30100543f0210eb690100541b0284d2f4031bcb9dfa3fa99f020091fd2300d11b0184d2fd031b8b1b0284d2ff031b8bc0035fd6e3031eaa00000000eeffff17"},
+		{"callsmall", "TEXT \u00b7callsmall(SB), $16-0\n\tCALL \u00b7other(SB)\n\tRET\nTEXT \u00b7other(SB), NOSPLIT, $0\n\tRET\n",
+			"900b40f9ff6330eb09010054fe0f1ef8fd831ff8fd2300d100000000fd835ff8fe0742f8c0035fd6e3031eaa00000000f4ffff17"},
+		{"nosplit", "TEXT \u00b7nosplit(SB), NOSPLIT, $16-0\n\tRET\n",
+			"fe0f1ef8fd831ff8fd2300d1fd630091ff830091c0035fd6"},
+	} {
+		f, errs := parser.Parse("g_arm64.s", tt.src)
+		if len(errs) > 0 {
+			t.Fatalf("%s: parse: %v", tt.name, errs)
+		}
+		img, err := AssembleFileARM64(f)
+		if err != nil {
+			t.Fatalf("%s: assemble: %v", tt.name, err)
+		}
+		fn := img.Funcs[0]
+		code := append([]byte(nil), img.Code[fn.Offset:fn.Offset+fn.Size]...)
+		for _, r := range fn.Relocs {
+			for j := r.Off; j < r.Off+4 && j < len(code); j++ {
+				code[j] = 0
+			}
+		}
+		got := hex.EncodeToString(code)
+		if got != tt.want {
+			t.Errorf("%s:\n  got  %s\n  want %s", tt.name, got, tt.want)
+		}
+	}
+}