arm_exidx_search.hpp

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// SPDX-FileCopyrightText: 2026 Jarosław Pelczar <jarek@jpelczar.com>
//
// SPDX-License-Identifier: BSD-2-Clause
 
#pragma once
 
#include "address_space.hpp"
#include <cstdint>
 
namespace microfmt {
 
class MICROFMT_API_CLASS exidx_table_searcher {
public:
  [[nodiscard]] static constexpr uintptr_t decode_prel31(uintptr_t entry_addr, uint32_t prel31) noexcept {
    // PREL31 is a 31-bit signed offset relative to the address of the prel31
    // word itself. Arithmetic right shift sign-extends from bit 30 to
    // 32-bit/64-bit int32_t.
    int32_t signed_offset = static_cast<int32_t>(prel31 << 1) >> 1;
    return detail::add_address_offset(entry_addr, signed_offset);
  }
 
  [[nodiscard]] static bool find_exidx_entry(address_space_ref space, uintptr_t table_base, size_t num_entries,
                                             uintptr_t target_pc, uint32_t &out_unwind_data) noexcept {
    if (num_entries == 0 || table_base == 0)
      return false;
 
    size_t low = 0;
    size_t high = num_entries;
    size_t match_index = num_entries;
 
    // Iterative binary search: find the highest function address <= target_pc
    while (low < high) {
      size_t mid = low + (high - low) / 2;
      uintptr_t entry_addr = table_base + (mid * 8); // Each entry is 2 words (8 bytes)
 
      uint32_t prel31 = 0;
      if (!space.read_bytes(entry_addr, &prel31, 4)) {
        return false;
      }
 
      uintptr_t fn_addr = decode_prel31(entry_addr, prel31);
 
      if (fn_addr <= target_pc) {
        match_index = mid; // Candidate found, try looking for a closer (higher)
                           // function start
        low = mid + 1;
      } else {
        high = mid;
      }
    }
 
    if (match_index == num_entries) {
      return false; // Target PC precedes all entries in the table
    }
 
    // Read the second word (unwind descriptor or pointer to .ARM.extab)
    uintptr_t matched_entry_addr = table_base + (match_index * 8);
    uint32_t unwind_data = 0;
    if (!space.read_bytes(matched_entry_addr + 4, &unwind_data, 4)) {
      return false;
    }
 
    // Check for EXIDX_CANTUNWIND sentinel value (0x1)
    if (unwind_data == 0x1) {
      return false;
    }
 
    out_unwind_data = unwind_data;
    return true;
  }
};
 
} // namespace microfmt