Dump/undump reuse strings

A repeated string in a dump is represented as an index to its first
occurence, instead of another copy of the string.

lapi.c

@@ -1107,16 +1107,37 @@ LUA_API int lua_load (lua_State *L, lua_Reader reader, void *data,
 }
 
 
+/*
+** Dump a function, calling 'writer' to write its parts. Because the
+** writer can use the stack in unkown ways, this function should not
+** push things on the stack, but it must anchor an auxiliary table
+** used by 'luaU_dump'. To do so, it creates the table, anchors the
+** function that is on the stack in the table, and substitutes the
+** table for the function in the stack.
+*/
+
 LUA_API int lua_dump (lua_State *L, lua_Writer writer, void *data, int strip) {
   int status;
+  StkId fstk;  /* pointer to function */
   TValue *o;
   lua_lock(L);
   api_checknelems(L, 1);
-  o = s2v(L->top.p - 1);
-  if (isLfunction(o))
-    status = luaU_dump(L, getproto(o), writer, data, strip);
-  else
+  fstk = L->top.p - 1;
+  o = s2v(fstk);
+  if (!isLfunction(o))
     status = 1;
+  else {
+    LClosure *f = clLvalue(o);
+    ptrdiff_t fidx = savestack(L, fstk);  /* function index */
+    Table *h = luaH_new(L);  /* auxiliary table used by 'luaU_dump' */
+    sethvalue2s(L, L->top.p, h);  /* anchor it (luaH_set may call GC) */
+    L->top.p++;  /* (assume extra slot) */
+    luaH_set(L, h, o, o);  /* anchor function into table */
+    setobjs2s(L, fstk, L->top.p - 1);  /* move table over function */
+    L->top.p--;  /* stack back to initial size */
+    status = luaU_dump(L, f->p, writer, data, strip, h);
+    setclLvalue2s(L, restorestack(L, fidx), f);  /* put function back */
+  }
   lua_unlock(L);
   return status;
 }

ldump.c

@@ -14,8 +14,10 @@
 
 #include "lua.h"
 
+#include "lgc.h"
 #include "lobject.h"
 #include "lstate.h"
+#include "ltable.h"
 #include "lundump.h"
 
 
@@ -25,6 +27,8 @@ typedef struct {
   void *data;
   int strip;
   int status;
+  Table *h;  /* table to track saved strings */
+  lua_Integer nstr;  /* counter to number saved strings */
 } DumpState;
 
 
@@ -85,14 +89,33 @@ static void dumpInteger (DumpState *D, lua_Integer x) {
 }
 
 
-static void dumpString (DumpState *D, const TString *s) {
+/*
+** Dump a String. First dump its "size": size==0 means NULL;
+** size==1 is followed by an index and means "reuse saved string with
+** that index"; size>=2 is followed by the string contents with real
+** size==size-2 and means that string, which will be saved with
+** the next available index.
+*/
+static void dumpString (DumpState *D, TString *s) {
   if (s == NULL)
     dumpSize(D, 0);
   else {
+    const TValue *idx = luaH_getstr(D->h, s);
+    if (ttisinteger(idx)) {  /* string already saved? */
+      dumpSize(D, 1);  /* reuse a saved string */
+      dumpInt(D, ivalue(idx));  /* index of saved string */
+    }
+    else {  /* must write and save the string */
+      TValue key, value;  /* to save the string in the hash */
       size_t size = tsslen(s);
-    const char *str = getstr(s);
-    dumpSize(D, size + 1);
-    dumpVector(D, str, size);
+      dumpSize(D, size + 2);
+      dumpVector(D, getstr(s), size);
+      D->nstr++;  /* one more saved string */
+      setsvalue(D->L, &key, s);  /* the string is the key */
+      setivalue(&value, D->nstr);  /* its index is the value */
+      luaH_finishset(D->L, D->h, &key, idx, &value);  /* h[s] = nstr */
+      /* integer value does not need barrier */
+    }
   }
 }
 
@@ -211,13 +234,15 @@ static void dumpHeader (DumpState *D) {
 ** dump Lua function as precompiled chunk
 */
 int luaU_dump(lua_State *L, const Proto *f, lua_Writer w, void *data,
-              int strip) {
+              int strip, Table *h) {
   DumpState D;
   D.L = L;
   D.writer = w;
   D.data = data;
   D.strip = strip;
   D.status = 0;
+  D.h = h;
+  D.nstr = 0;
   dumpHeader(&D);
   dumpByte(&D, f->sizeupvalues);
   dumpFunction(&D, f, NULL);

lstrlib.c

@@ -239,6 +239,7 @@ static int str_dump (lua_State *L) {
   if (l_unlikely(lua_dump(L, writer, &state, strip) != 0))
     return luaL_error(L, "unable to dump given function");
   luaL_pushresult(&state.B);
+  lua_assert(lua_isfunction(L, 1));  /* lua_dump kept that value */
   return 1;
 }
 

lundump.c

@@ -21,6 +21,7 @@
 #include "lmem.h"
 #include "lobject.h"
 #include "lstring.h"
+#include "ltable.h"
 #include "lundump.h"
 #include "lzio.h"
 
@@ -34,6 +35,8 @@ typedef struct {
   lua_State *L;
   ZIO *Z;
   const char *name;
+  Table *h;  /* list for string reuse */
+  lua_Integer nstr;  /* number of strings in the list */
 } LoadState;
 
 
@@ -110,10 +113,16 @@ static lua_Integer loadInteger (LoadState *S) {
 static TString *loadStringN (LoadState *S, Proto *p) {
   lua_State *L = S->L;
   TString *ts;
+  TValue sv;
   size_t size = loadSize(S);
   if (size == 0)  /* no string? */
     return NULL;
-  else if (--size <= LUAI_MAXSHORTLEN) {  /* short string? */
+  else if (size == 1) {  /* previously saved string? */
+    int idx = loadInt(S);  /* get its index */
+    const TValue *stv = luaH_getint(S->h, idx);
+    return tsvalue(stv);
+  }
+  else if (size -= 2, size <= LUAI_MAXSHORTLEN) {  /* short string? */
     char buff[LUAI_MAXSHORTLEN];
     loadVector(S, buff, size);  /* load string into buffer */
     ts = luaS_newlstr(L, buff, size);  /* create string */
@@ -126,6 +135,10 @@ static TString *loadStringN (LoadState *S, Proto *p) {
     L->top.p--;  /* pop string */
   }
   luaC_objbarrier(L, p, ts);
+  S->nstr++;  /* add string to list of saved strings */
+  setsvalue(L, &sv, ts);
+  luaH_setint(L, S->h, S->nstr, &sv);
+  luaC_objbarrierback(L, obj2gco(S->h), ts);
   return ts;
 }
 
@@ -323,11 +336,16 @@ LClosure *luaU_undump(lua_State *L, ZIO *Z, const char *name) {
   cl = luaF_newLclosure(L, loadByte(&S));
   setclLvalue2s(L, L->top.p, cl);
   luaD_inctop(L);
+  S.h = luaH_new(L);  /* create list of saved strings */
+  S.nstr = 0;
+  sethvalue2s(L, L->top.p, S.h);  /* anchor it */
+  luaD_inctop(L);
   cl->p = luaF_newproto(L);
   luaC_objbarrier(L, cl, cl->p);
   loadFunction(&S, cl->p, NULL);
   lua_assert(cl->nupvalues == cl->p->sizeupvalues);
   luai_verifycode(L, cl->p);
+  L->top.p--;  /* pop table */
   return cl;
 }
 

lundump.h

@@ -31,6 +31,6 @@ LUAI_FUNC LClosure* luaU_undump (lua_State* L, ZIO* Z, const char* name);
 
 /* dump one chunk; from ldump.c */
 LUAI_FUNC int luaU_dump (lua_State* L, const Proto* f, lua_Writer w,
-                         void* data, int strip);
+                         void* data, int strip, Table *h);
 
 #endif