Re: log/log10 in Lua 5.2.0-alpha

[Jerome Vuarand <jerome.vuarand@...>]

2011/5/20 François Perrad <francois.perrad@gadz.org>:
>
> when I see the implementation of math.log and math.log10 in LuaJIT2 and in
> Lua 5.1 :
>
>     LuaJIT2
>         LJLIB_ASM_(math_log)            LJLIB_REC(math_unary IRFPM_LOG)
>         LJLIB_ASM_(math_log10)          LJLIB_REC(math_unary IRFPM_LOG10)
>
>     Lua 5.1.4
>         static int math_log (lua_State *L) {
>           lua_pushnumber(L, log(luaL_checknumber(L, 1)));
>           return 1;
>         }
>         static int math_log10 (lua_State *L) {
>           lua_pushnumber(L, log10(luaL_checknumber(L, 1)));
>           return 1;
>         }
>
> I don't understand the rational of the removal of math.log10 and the
> addition of optional second parameter in math.log
>
>         Lua 5.2.0-alpha : math.log (x [, base])
>         static int math_log (lua_State *L) {
>           lua_Number x = luaL_checknumber(L, 1);
>           lua_Number res;
>           if (lua_isnoneornil(L, 2))
>             res = l_tg(log)(x);
>           else {
>             lua_Number base = luaL_checknumber(L, 2);
>             if (base == 10.0) res = l_tg(log10)(x);
>             else res = l_tg(log)(x)/l_tg(log)(base);
>           }
>           lua_pushnumber(L, res);
>           return 1;
>         }
>
> It's a general implementation of log, but all opportunities of optimization
> are lost.

Isn't LuaJIT smart enough to recognize that in a given code path
math.log is always called with no base or a 10 base, and plug the
right optimization in those cases ? I'm no expert, but I thought such
optimizations were what tracing JITs were designed to allow.