More uniform implementation for tail calls
'luaD_pretailcall' mimics 'luaD_precall', handling call metamethods
and calling C functions directly. That makes the code in the
interpreter loop simpler.
This commit also goes back to emulating the tail call in 'luaD_precall'
with a goto, as C compilers may not do proper tail calls and the C
stack can overflow much sooner than the Lua stack (which grows as the
metamethod is added to it).
diff --git a/ldo.c b/ldo.c
index 0ac12e7..d0edc8b 100644
--- a/ldo.c
+++ b/ldo.c
@@ -475,30 +475,6 @@
#define next_ci(L) (L->ci->next ? L->ci->next : luaE_extendCI(L))
-/*
-** Prepare a function for a tail call, building its call info on top
-** of the current call info. 'narg1' is the number of arguments plus 1
-** (so that it includes the function itself).
-*/
-void luaD_pretailcall (lua_State *L, CallInfo *ci, StkId func, int narg1) {
- Proto *p = clLvalue(s2v(func))->p;
- int fsize = p->maxstacksize; /* frame size */
- int nfixparams = p->numparams;
- int i;
- for (i = 0; i < narg1; i++) /* move down function and arguments */
- setobjs2s(L, ci->func + i, func + i);
- checkstackGC(L, fsize);
- func = ci->func; /* moved-down function */
- for (; narg1 <= nfixparams; narg1++)
- setnilvalue(s2v(func + narg1)); /* complete missing arguments */
- ci->top = func + 1 + fsize; /* top for new function */
- lua_assert(ci->top <= L->stack_last);
- ci->u.l.savedpc = p->code; /* starting point */
- ci->callstatus |= CIST_TAIL;
- L->top = func + narg1; /* set top */
-}
-
-
l_sinline CallInfo *prepCallInfo (lua_State *L, StkId func, int nret,
int mask, StkId top) {
CallInfo *ci = L->ci = next_ci(L); /* new frame */
@@ -513,7 +489,7 @@
/*
** precall for C functions
*/
-l_sinline CallInfo *precallC (lua_State *L, StkId func, int nresults,
+l_sinline int precallC (lua_State *L, StkId func, int nresults,
lua_CFunction f) {
int n; /* number of returns */
CallInfo *ci;
@@ -530,7 +506,50 @@
lua_lock(L);
api_checknelems(L, n);
luaD_poscall(L, ci, n);
- return NULL;
+ return n;
+}
+
+
+/*
+** Prepare a function for a tail call, building its call info on top
+** of the current call info. 'narg1' is the number of arguments plus 1
+** (so that it includes the function itself). Return the number of
+** results, if it was a C function, or -1 for a Lua function.
+*/
+int luaD_pretailcall (lua_State *L, CallInfo *ci, StkId func,
+ int narg1, int delta) {
+ retry:
+ switch (ttypetag(s2v(func))) {
+ case LUA_VCCL: /* C closure */
+ return precallC(L, func, LUA_MULTRET, clCvalue(s2v(func))->f);
+ case LUA_VLCF: /* light C function */
+ return precallC(L, func, LUA_MULTRET, fvalue(s2v(func)));
+ case LUA_VLCL: { /* Lua function */
+ Proto *p = clLvalue(s2v(func))->p;
+ int fsize = p->maxstacksize; /* frame size */
+ int nfixparams = p->numparams;
+ int i;
+ ci->func -= delta; /* restore 'func' (if vararg) */
+ for (i = 0; i < narg1; i++) /* move down function and arguments */
+ setobjs2s(L, ci->func + i, func + i);
+ checkstackGC(L, fsize);
+ func = ci->func; /* moved-down function */
+ for (; narg1 <= nfixparams; narg1++)
+ setnilvalue(s2v(func + narg1)); /* complete missing arguments */
+ ci->top = func + 1 + fsize; /* top for new function */
+ lua_assert(ci->top <= L->stack_last);
+ ci->u.l.savedpc = p->code; /* starting point */
+ ci->callstatus |= CIST_TAIL;
+ L->top = func + narg1; /* set top */
+ return -1;
+ }
+ default: { /* not a function */
+ func = luaD_tryfuncTM(L, func); /* try to get '__call' metamethod */
+ /* return luaD_pretailcall(L, ci, func, narg1 + 1, delta); */
+ narg1++;
+ goto retry; /* try again */
+ }
+ }
}
@@ -543,11 +562,14 @@
** original function position.
*/
CallInfo *luaD_precall (lua_State *L, StkId func, int nresults) {
+ retry:
switch (ttypetag(s2v(func))) {
case LUA_VCCL: /* C closure */
- return precallC(L, func, nresults, clCvalue(s2v(func))->f);
+ precallC(L, func, nresults, clCvalue(s2v(func))->f);
+ return NULL;
case LUA_VLCF: /* light C function */
- return precallC(L, func, nresults, fvalue(s2v(func)));
+ precallC(L, func, nresults, fvalue(s2v(func)));
+ return NULL;
case LUA_VLCL: { /* Lua function */
CallInfo *ci;
Proto *p = clLvalue(s2v(func))->p;
@@ -564,7 +586,8 @@
}
default: { /* not a function */
func = luaD_tryfuncTM(L, func); /* try to get '__call' metamethod */
- return luaD_precall(L, func, nresults); /* try again with metamethod */
+ /* return luaD_precall(L, func, nresults); */
+ goto retry; /* try again with metamethod */
}
}
}
diff --git a/ldo.h b/ldo.h
index 9fb772f..911e67f 100644
--- a/ldo.h
+++ b/ldo.h
@@ -58,7 +58,7 @@
LUAI_FUNC void luaD_hook (lua_State *L, int event, int line,
int fTransfer, int nTransfer);
LUAI_FUNC void luaD_hookcall (lua_State *L, CallInfo *ci);
-LUAI_FUNC void luaD_pretailcall (lua_State *L, CallInfo *ci, StkId func, int n);
+LUAI_FUNC int luaD_pretailcall (lua_State *L, CallInfo *ci, StkId func, int narg1, int delta);
LUAI_FUNC CallInfo *luaD_precall (lua_State *L, StkId func, int nResults);
LUAI_FUNC void luaD_call (lua_State *L, StkId func, int nResults);
LUAI_FUNC void luaD_callnoyield (lua_State *L, StkId func, int nResults);
diff --git a/lvm.c b/lvm.c
index 49ed3dd..2ec3440 100644
--- a/lvm.c
+++ b/lvm.c
@@ -1643,6 +1643,7 @@
}
vmcase(OP_TAILCALL) {
int b = GETARG_B(i); /* number of arguments + 1 (function) */
+ int n; /* number of results when calling a C function */
int nparams1 = GETARG_C(i);
/* delta is virtual 'func' - real 'func' (vararg functions) */
int delta = (nparams1) ? ci->u.l.nextraargs + nparams1 : 0;
@@ -1656,24 +1657,14 @@
lua_assert(L->tbclist < base); /* no pending tbc variables */
lua_assert(base == ci->func + 1);
}
- while (!ttisfunction(s2v(ra))) { /* not a function? */
- ra = luaD_tryfuncTM(L, ra); /* try '__call' metamethod */
- b++; /* there is now one extra argument */
- }
- if (!ttisLclosure(s2v(ra))) { /* C function? */
- luaD_precall(L, ra, LUA_MULTRET); /* call it */
- updatetrap(ci);
- updatestack(ci); /* stack may have been relocated */
+ if ((n = luaD_pretailcall(L, ci, ra, b, delta)) < 0) /* Lua function? */
+ goto startfunc; /* execute the callee */
+ else { /* C function? */
ci->func -= delta; /* restore 'func' (if vararg) */
- luaD_poscall(L, ci, cast_int(L->top - ra)); /* finish caller */
+ luaD_poscall(L, ci, n); /* finish caller */
updatetrap(ci); /* 'luaD_poscall' can change hooks */
goto ret; /* caller returns after the tail call */
}
- else { /* Lua function */
- ci->func -= delta; /* restore 'func' (if vararg) */
- luaD_pretailcall(L, ci, ra, b); /* prepare call frame */
- goto startfunc; /* execute the callee */
- }
}
vmcase(OP_RETURN) {
int n = GETARG_B(i) - 1; /* number of results */
