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More uniform implementation for tail calls
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'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).
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@roberto-ieru
roberto-ieru committed Oct 29, 2021
1 parent 3699446 commit 1fce5be
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Showing 3 changed files with 58 additions and 44 deletions.
81 changes: 52 additions & 29 deletions ldo.c
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Original file line number Diff line number Diff line change
Expand Up @@ -475,30 +475,6 @@ void luaD_poscall (lua_State *L, CallInfo *ci, int nres) {
#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 */
Expand All @@ -513,7 +489,7 @@ l_sinline CallInfo *prepCallInfo (lua_State *L, StkId func, int nret,
/*
** 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;
Expand All @@ -530,7 +506,50 @@ l_sinline CallInfo *precallC (lua_State *L, StkId func, int nresults,
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 */
}
}
}


Expand All @@ -543,11 +562,14 @@ l_sinline CallInfo *precallC (lua_State *L, StkId func, int nresults,
** 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;
Expand All @@ -564,7 +586,8 @@ CallInfo *luaD_precall (lua_State *L, StkId func, int nresults) {
}
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 */
}
}
}
Expand Down
2 changes: 1 addition & 1 deletion ldo.h
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Original file line number Diff line number Diff line change
Expand Up @@ -58,7 +58,7 @@ LUAI_FUNC int luaD_protectedparser (lua_State *L, ZIO *z, const char *name,
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);
Expand Down
19 changes: 5 additions & 14 deletions lvm.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1643,6 +1643,7 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
}
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;
Expand All @@ -1656,24 +1657,14 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
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 */
Expand Down

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