jvmFlagLookup.hpp

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#ifndef SHARE_RUNTIME_FLAGS_JVMFLAGLOOKUP_HPP
#define SHARE_RUNTIME_FLAGS_JVMFLAGLOOKUP_HPP

#include "runtime/globals_extension.hpp"
#include "runtime/flags/jvmFlag.hpp"

// This is a hashtable that maps from (const char*) to (JVMFlag*) to speed up
// the processing of JVM command-line arguments at runtime.
//
// With constexpr, this table is generated at C++ compile time so there's
// no set up cost at runtime.
class JVMFlagLookup {
  static constexpr int NUM_BUCKETS = 277;
  short _buckets[NUM_BUCKETS];
  short _table[NUM_JVMFlagsEnum];
  u2    _hashes[NUM_JVMFlagsEnum];

  // Cannot use strlen() -- it's not constexpr.
  static constexpr size_t string_len(const char* s) {
    size_t len = 0;
    while (*s != 0) {
      len++;
      s++;
    }
    return len;
  }

  // This is executed at build-time only, so it doesn't matter if we walk
  // the string twice.
  static constexpr unsigned int hash_code(const char* s) {
    return hash_code(s, string_len(s));
  }

  static constexpr unsigned int hash_code(const char* s, size_t len) {
    unsigned int h = 0;
    while (len -- > 0) {
      h = 31*h + (unsigned int) *s;
      s++;
    }
    return h;
  }

  JVMFlag* find_impl(const char* flag_name, size_t length) const;

public:
  constexpr JVMFlagLookup();
  static JVMFlag* find(const char* flag_name, size_t length);
};

#endif // SHARE_RUNTIME_FLAGS_JVMFLAGLOOKUP_HPP