Haskell -> C FFI performance - c

Question

This is the dual question of Performance considerations of Haskell FFI / C?: I would like to call a C function with as small an overhead as possible.
To set the scene, I have the following C function:
typedef struct
{
uint64_t RESET;
} INPUT;
typedef struct
{
uint64_t VGA_HSYNC;
uint64_t VGA_VSYNC;
uint64_t VGA_DE;
uint8_t VGA_RED;
uint8_t VGA_GREEN;
uint8_t VGA_BLUE;
} OUTPUT;
void Bounce(const INPUT* input, OUTPUT* output);
Let's run it from C and time it, with gcc -O3:
int main (int argc, char **argv)
{
INPUT input;
input.RESET = 0;
OUTPUT output;
int cycles = 0;
for (int j = 0; j < 60; ++j)
{
for (;; ++cycles)
{
Bounce(&input, &output);
if (output.VGA_HSYNC == 0 && output.VGA_VSYNC == 0) break;
}
for (;; ++cycles)
{
Bounce(&input, &output);
if (output.VGA_DE) break;
}
}
printf("%d cycles\n", cycles);
}
Running it for 25152001 cycles takes ~400 ms:
$ time ./Bounce
25152001 cycles
real 0m0.404s
user 0m0.403s
sys 0m0.001s
Now let's write some Haskell code to set up FFI (note that Bool's Storable instance really does use a full int):
data INPUT = INPUT
{ reset :: Bool
}
data OUTPUT = OUTPUT
{ vgaHSYNC, vgaVSYNC, vgaDE :: Bool
, vgaRED, vgaGREEN, vgaBLUE :: Word64
}
deriving (Show)
foreign import ccall unsafe "Bounce" topEntity :: Ptr INPUT -> Ptr OUTPUT -> IO ()
instance Storable INPUT where ...
instance Storable OUTPUT where ...
And let's do what I believe to be functionally equivalent to our C code from before:
main :: IO ()
main = alloca $ \inp -> alloca $ \outp -> do
poke inp $ INPUT{ reset = False }
let loop1 n = do
topEntity inp outp
out#OUTPUT{..} <- peek outp
let n' = n + 1
if not vgaHSYNC && not vgaVSYNC then loop2 n' else loop1 n'
loop2 n = do
topEntity inp outp
out <- peek outp
let n' = n + 1
if vgaDE out then return n' else loop2 n'
loop3 k n
| k < 60 = do
n <- loop1 n
loop3 (k + 1) n
| otherwise = return n
n <- loop3 (0 :: Int) (0 :: Int)
printf "%d cycles" n
I build it with GHC 8.6.5, using -O3, and I get.. more than 3 seconds!
$ time ./.stack-work/dist/x86_64-linux/Cabal-2.4.0.1/build/sim-ffi/sim-ffi
25152001 cycles
real 0m3.468s
user 0m3.146s
sys 0m0.280s
And it's not a constant overhead at startup, either: if I run for 10 times the cycles, I get roughly 3.5 seconds from C and 34 seconds from Haskell.
What can I do to reduce the Haskell -> C FFI overhead?

Answer 1

I managed to reduce the overhead so that the 25 M calls now finish in 1.2 seconds. The changes were:
Make loop1, loop2 and loop3 strict in the n argument (using BangPatterns)
Add an INLINE pragma to peek in OUTPUT's Storable instance
Point #1 is silly, of course, but that's what I get for not profiling earlier. That change alone gets me to 1.5 seconds....
Point #2, however, makes a ton of sense and is generally applicable. It also addresses the comment from #Thomas M. DuBuisson:
Do you ever need the Haskell structure in haskell? If you can just keep it as a pointer to memory and have a few test functions such as vgaVSYNC :: Ptr OUTPUT -> IO Bool then that will save a log of copying, allocation, GC work on every call.
In the eventual full program, I do need to look at all the fields of OUTPUT. However, with peek inlined, GHC is happy to do the case-of-case transformation, so I can see in Core that now there is no OUTPUT value allocated; the output of peek is consumed directly.