feat: add exception table implementation with binary search and initialization

1 files changed, 55 insertions, 0 deletions

diff --git a/na/arch/riscv/include/arch/extable.h b/na/arch/riscv/include/arch/extable.h
new file mode 100644
index 0000000..eb2daac
--- /dev/null
+++ b/na/arch/riscv/include/arch/extable.h
@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* Copyright (C) 2026 Richard Qin */
+
+#pragma once
+
+#ifndef __ASSEMBLER__
+
+#include <lume/types.h>
+
+/*
+ * Exception table entry.
+ * insn: offset from this struct to the faulting instruction.
+ * fixup: offset from this struct to the fixup code.
+ * 
+ * Using relative offsets avoids absolute relocations and saves space.
+ */
+struct ExceptionTableEntry {
+    int32 insn;
+    int32 fixup;
+};
+
+/* Search the exception table for a given PC. Returns fixup address, or 0 if not found. */
+uint64 search_extable(uint64 pc);
+
+/* Sorts the exception table at boot for binary search O(log N). */
+void extable_init();
+
+#endif /* __ASSEMBLER__ */
+
+/*
+ * EXTABLE macro for assembly usage.
+ * Appends an entry to the .extable section.
+ */
+#ifdef __ASSEMBLER__
+
+#define EXTABLE(insn, fixup) \
+    .pushsection .extable, "a"; \
+    .balign 4; \
+    .long insn - .; \
+    .long fixup - .; \
+    .popsection
+
+#else /* !__ASSEMBLER__ */
+
+/* Stringified macro for inline assembly */
+#define _ASM_EXTABLE(insn, fixup) \
+    ".pushsection .extable, \"a\"\n" \
+    ".balign 4\n" \
+    ".long " #insn " - .\n" \
+    ".long " #fixup " - .\n" \
+    ".popsection\n"
+
+#define EXTABLE(insn, fixup) _ASM_EXTABLE(insn, fixup)
+
+#endif /* __ASSEMBLER__ */