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  1. MariaDB Server
  2. MDEV-27712

Reduce the size of Lex_length_and_dec_st from 16 to 8

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      Description

      We want to use Bison stack more efficiently and pass more structures through the stack instead of using Lex members.

      In particular, we want to get rid of Lex->charset. This change is really overdue.

      To be able to do this, we want to pass through the Bison stack a structure with data type handler and its basic attributes, consisting of the following four members:

      struct {
        const Type_handler *handler;
        CHARSET_INFO *charset;
        Lex_length_and_dec_st length_and_dec;
      };
      

      This structure should fit into the Bison variable limitation (see sql_yacc.yy):

      /* avoid unintentional %union size increases, it's what a parser stack made of */
      static_assert(sizeof(YYSTYPE) == sizeof(void*)*2+8, "%union size check");
      

      Currently this structure would not fit because Lex_length_and_dec_st consists of two pointers and therefore occupies 16 bytes (on 64bit platforms):

      struct Lex_length_and_dec_st
      {
      private:
        const char *m_length;
        const char *m_dec;
      public:
      

      Let's reduce the size of Lex_length_and_dec_st from current 16 bytes as follows:

      struct Lex_length_and_dec_st
      {
      private:
        uint32 m_length;
        uint8  m_dec;
        bool   m_has_explicit_length:1;
        bool   m_has_explicit_dec:1;
      public:
      

      After this change, the future structure consisting of the four fields (mentioned above) will occupy 24 bytes:

      struct {
        const Type_handler *handler;          // 8 bytes
        CHARSET_INFO *charset;                // 8 bytes
        Lex_length_and_dec_st length_and_dec; // 8 bytes
      };
      

      Note, this task is only about changing Lex_length_and_dec_st. The new structure (with handler, charset, length and dec) will be added later, under terms of a separate task.

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              Assignee:
              bar Alexander Barkov
              Reporter:
              bar Alexander Barkov
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                Updated:
                Resolved:

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