Re: Performance implications of large FFI constants
- From: Peter Colberg <peter@xxxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Wed, 26 Sep 2012 11:28:17 -0400
On Wed, Sep 26, 2012 at 12:05:58PM +0200, Mike Pall wrote:
> local ffi = require("ffi")
> ffi.cdef[[
> struct whatever {
> static const int FOO = 42;
> static const int BAR = 99;
> };
> ]]
> local W = ffi.new("struct whatever")
> print(W.FOO, W.BAR)
>
> Yes, these are compiled to actual constants, just like enum
> constants (and unlike Lua table fields). Except that enum constants
> are global in C, even if defined inside a struct. Note that it's
> slightly more efficient to use an instance of the dummy struct (W)
> as a namespace than using the ctype.
Could one define non-integral constants more efficiently as well,
without resorting to a Lua table on top of a C library namespace?
In the MPICH2 implementation of MPI Version 2, there are dozens
of constants that are cast to an opaque handle type:
#define MPI_CHAR ((MPI_Datatype)0x4c000101)
#define MPI_SHORT ((MPI_Datatype)0x4c000203)
#define MPI_INT ((MPI_Datatype)0x4c000405)
#define MPI_COMM_WORLD ((MPI_Comm)0x44000000)
#define MPI_COMM_SELF ((MPI_Comm)0x44000001)
Currently I generate a Lua table for these constants:
ffi.cdef[[
typedef struct MPI_Comm *__ptr32 MPI_Comm;
typedef struct MPI_Datatype *__ptr32 MPI_Datatype;
]]
-- load MPI library into global namespace, too,
-- to avoid a segmentation fault in MPI_Init
local mpi = ffi.load("mpi", true)
local defs = {
MPI_CHAR = ffi.cast("MPI_Datatype", 0x4c000101),
MPI_SHORT = ffi.cast("MPI_Datatype", 0x4c000203),
MPI_INT = ffi.cast("MPI_Datatype", 0x4c000405),
MPI_COMM_WORLD = ffi.cast("MPI_Comm", 0x44000000),
MPI_COMM_SELF = ffi.cast("MPI_Comm", 0x44000001),
}
return setmetatable(defs, {__index = mpi})
The reason I would like to have the constants with correct type,
rather than casting from an enum constant before a function call,
is to allow consistent bindings for different implementations.
In the OpenMPI implementation, the ABI uses global variables:
extern struct ompi_predefined_communicator_t ompi_mpi_comm_world;
extern struct ompi_predefined_communicator_t ompi_mpi_comm_self;
#define MPI_COMM_WORLD ((MPI_Comm) ((void *) &(ompi_mpi_comm_world)))
#define MPI_COMM_SELF ((MPI_Comm) ((void *) &(ompi_mpi_comm_self)))
Which currently I translate to:
ffi.cdef[[
typedef struct MPI_Comm *MPI_Comm;
extern struct MPI_Comm MPI_COMM_WORLD asm("ompi_mpi_comm_world");
extern struct MPI_Comm MPI_COMM_SELF asm("ompi_mpi_comm_self");
]]
local mpi = ffi.load("mpi", true)
local defs = {
MPI_COMM_WORLD = ffi.cast("MPI_Comm", mpi.MPI_COMM_WORLD),
MPI_COMM_SELF = ffi.cast("MPI_Comm", mpi.MPI_COMM_SELF),
}
return setmetatable(defs, {__index = mpi})
Is there a better way of binding the above with LuaJIT?
Thanks,
Peter
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