feat(lint): flag writes to the platform-reserved register

lint/reserved.go

@@ -0,0 +1,56 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package lint
+
+import (
+	"fmt"
+	"strings"
+
+	"sourcedock.dev/petrbalvin/gasm-devkit/arch"
+	"sourcedock.dev/petrbalvin/gasm-devkit/ast"
+)
+
+// reservedRegister returns the platform-reserved general-purpose register of
+// the architecture, or "" when it has none.  arm64 reserves R18 for platform
+// use: the OS may own it (the Windows TEB, the Darwin el0 TLS), the Go
+// toolchain never allocates it, and the Go assembler offers no spelling that
+// even addresses it, so a kernel writing R18 cannot be assembled by the tool
+// it must ship with.  riscv64, loong64 and amd64 reserve no register beyond
+// the ones the Go ABI fixes, which the register-clobber audit covers.
+func reservedRegister(a arch.Arch) string {
+	if a == arch.ARM64 {
+		return "R18"
+	}
+	return ""
+}
+
+// scanReservedRegisterWrites flags instructions whose destination is the
+// platform-reserved register.  Only the Plan 9 destination (the last
+// operand) is checked: reads such as MOVD (R18), R0 are legitimate address
+// uses, and multi-register stores (STP) keep the rule silent rather than
+// guess.  Like the label heuristics, the check is suppressed in macro-using
+// files, where an opaque macro may save and restore the register.
+func scanReservedRegisterWrites(t *ast.Text, a arch.Arch) []Diagnostic {
+	res := reservedRegister(a)
+	if res == "" {
+		return nil
+	}
+	var out []Diagnostic
+	for _, s := range t.Body {
+		in, ok := s.(*ast.Instr)
+		if !ok || len(in.Operands) == 0 {
+			continue
+		}
+		dst := in.Operands[dstIndex(in)]
+		if r := gprName(dst, a); r != "" && strings.EqualFold(r, res) {
+			out = append(out, Diagnostic{
+				Pos:      dst.Pos,
+				Severity: Warning,
+				Code:     CodeReservedRegister,
+				Message:  fmt.Sprintf("write to %s, the platform-reserved register: the OS may own it and the Go assembler cannot spell it", res),
+			})
+		}
+	}
+	return out
+}