Port closure optimizations

[saghul]

Ref: bellard/quickjs@ae7219b

[bnoordhuis]

Nice work.

[bnoordhuis]

This field creates a 6 byte padding hole on 64 bits architectures because of the alignment requirement of the next field. Can maybe be eliminated with some reshuffling.

[saghul]

I'll take a look!

[saghul]

I did some AI assisted suffling of fields :-) Can you PTAL?

[saghul]

FWIW, got this from struct_layout.py:

struct ::JSFunctionBytecode [136 Bytes]
     : [JSGCObjectHeader : 24] header
    0:   [int : 4] ref_count                              -- {cache-line 0}
   --- 1 Bytes padding ---
    5:   [unsigned char : 1] dummy1                       -- {cache-line 0}
    6:   [unsigned short : 2] dummy2                      -- {cache-line 0}
     :   [list_head : 16] link
    8:     [list_head* : 8] prev                          -- {cache-line 0}
   16:     [list_head* : 8] next                          -- {cache-line 0}
   --- 8 Bytes padding ---
   32: [unsigned char* : 8] byte_code_buf                 -- {cache-line 0}
   40: [int : 4] byte_code_len                            -- {cache-line 0}
   44: [unsigned int : 4] func_name                       -- {cache-line 0}
   48: [JSVarDef* : 8] vardefs                            -- {cache-line 0}
   56: [JSClosureVar* : 8] closure_var                    -- {cache-line 0}
   64: [unsigned short : 2] arg_count                     -- {cache-line 1}
   66: [unsigned short : 2] var_count                     -- {cache-line 1}
   68: [unsigned short : 2] defined_arg_count             -- {cache-line 1}
   70: [unsigned short : 2] stack_size                    -- {cache-line 1}
   72: [unsigned short : 2] var_ref_count                 -- {cache-line 1}
   74: [unsigned short : 2] closure_var_count             -- {cache-line 1}
   76: [int : 4] cpool_count                              -- {cache-line 1}
   80: [JSContext* : 8] realm                             -- {cache-line 1}
   88: [JSValue* : 8] cpool                               -- {cache-line 1}
   96: [unsigned int : 4] filename                        -- {cache-line 1}
  100: [int : 4] line_num                                 -- {cache-line 1}
  104: [int : 4] col_num                                  -- {cache-line 1}
  108: [int : 4] source_len                               -- {cache-line 1}
  112: [int : 4] pc2line_len                              -- {cache-line 1}
   --- 4 Bytes padding ---
  120: [unsigned char* : 8] pc2line_buf                   -- {cache-line 1}
  128: [char* : 8] source                                 -- {cache-line 2}

Maybe it can be further improved to avoid the 8 byte hole?