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I'm getting a performance discrepancy issue when trying to copy FFI
structure values around.  If I copy each member from a structure
separately the performance is about 4 or 5 times faster than just
assigning the structure all at once.  I'm actually having this problem
as part of a larger application but I have stripped it down to the
following test case:

local ffi = require'ffi'

   typedef struct {
      uint64_t a;
      uint64_t b;
      uint64_t c;
      double d;
      double e;
      double f;
   } foo;

local function copyfoo(a, b)
   a.a = b.a
   a.b = b.b
   a.c = b.c
   a.d = b.d
   a.e = b.e
   a.f = b.f

buffer ="foo[?]", 1000001)
bar ="foo")

for x=1, 40, 1 do
   for y=1, 1000000, 1 do
      buffer[y] = bar
      --copyfoo(buffer[y], bar)

If I switch out that "buffer[y] = bar" assignment for the "copyfoo"
call then it runs considerably faster.  It makes me wonder why the JIT
can't just do what it's doing in the copyfoo function.

$ time luajit ffitest.lua
real	0m2.965s
user	0m2.950s
sys	0m0.010s

Using copyfoo:
$ time luajit ffitest.lua
real	0m0.682s
user	0m0.670s
sys	0m0.010s

Did I mess something up or what's going on here?