Hugo Musso Gualandi <hgualandi@inf.puc-rio.br> 于2020年12月23日周三 上午4:21写道：
>
> > I think using a pointer to hold the string data would be better then
> > putting the data into a continuous TString structure.
>
> One advantage of putting the string data in a contiguous structure is that it avoids having to go through an additional layer of pointer indirection to read the string contents.
>
> But to be honest, I don't know how much that matters in practice. That's an interesting question.
In my opinion, the string object in lua VM core is only a reference.
We seldom use long-string as a key of the table,
and to the short-string key, lua doesn't need to read the string
contents. It only compares the pointers of TString.
A smaller GCObject is more (cache) friendly to the collector because
it always traverses the objects.
With C99 is possible declare a struct with 0 size, like this:
typedef struct TString {
CommonHeader;
lu_byte extra; /* reserved words for short strings; "has hash" for longs */
lu_byte shrlen; /* length for short strings */
unsigned int hash;
union {
size_t lnglen; /* length for long strings */
struct TString *hnext; /* linked list for hash table */
} u;
char contents[];
} TString;
Which effectively removes the "contents" field from the structure itself and places it immediately afterwards in memory.
regards,
Ranier Vilela
