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typedef union TKey {
 struct {
   struct Node *next;  /* for chaining */
 } nk;
 TValue tvk;
} TKey;

Why TKey has a member 'tvk'?

It allows TKey to be efficiently stored.

See, on Windows this'll print "16 bytes".
 struct {
   double n;
   int tt;
   void *next;
 } temp;
 printf("%d bytes", temp);

Whereas this will print "24 bytes":
 struct {
   struct { double n; int tt; } tvalue;
   void *next;
 } temp;
Because sizeof(temp.tvalue) = 16 (as doubles need to be 8 byte aligned),
making the temp structure be layed out in memory like this:
 Double (0..7 bytes)
 Tag (8..11 bytes)
 <tvalue alignment padding> (9..16 bytes)
 Node *next (17..20 bytes)
 <temp alignment padding> (17..20 bytes)

So Lua effectively declares this:
 union {
   struct { double n; int tt; void *next; } nk;
   TValue tvk;
 } temp;
nk is the first way, tvk allows accessing nk like a TValue.

In ltable.c, I found two macros: gval and key2tval.
Why there is one key but two values?

table["hello"] = "greeting"
key2tval == "hello"
gval == "greeting"

- Alex