optimization: use a new upvalue table for the ctxs holder to make LuaJIT JIT compiler happy to generate more efficient machine code. (openresty#360)

Now `ngx.ctx` is 80% faster than the old implementation.

Author: doujiang

lib/resty/core/ctx.lua

@@ -53,6 +53,12 @@ local _M = {
 }
 
 
+-- use a new ctxs table to make LuaJIT JIT compiler happy to generate more
+-- efficient machine code.
+local ctxs = {}
+registry.ngx_lua_ctx_tables = ctxs
+
+
 local get_ctx_table
 do
  local in_ssl_phase = ffi.new("int[1]")
@@ -70,7 +76,6 @@ do
  error("no request ctx found")
  end
 
- local ctxs = registry.ngx_lua_ctx_tables
  if ctx_ref < 0 then
  ctx_ref = ssl_ctx_ref[0]
  if ctx_ref > 0 and ctxs[ctx_ref] then
@@ -129,7 +134,6 @@ local function set_ctx_table(ctx)
  error("no request ctx found")
  end
 
- local ctxs = registry.ngx_lua_ctx_tables
  if ctx_ref < 0 then
  ctx_ref = ref_in_table(ctxs, ctx)
  ngx_lua_ffi_set_ctx_ref(r, ctx_ref)