How to use C-land variable from Cmm-land?

[Yuras Shumovich]

Hi,

I'm working on that issue as an exercise/playground while studding the
GHC internals: http://hackage.haskell.org/trac/ghc/ticket/693

First I tried just to replace "ccall lockClosure(mvar "ptr")" with
GET_INFO(mvar) in stg_takeMVarzh and stg_putMVarzh and got 60% speedup
(see the test case at the end.)

Then I changed lockClosure to read header info directly when
enabled_capabilities == 1. The speedup was significantly lower, <20%

I tried to hack stg_putMVarzh directly:

    if (enabled_capabilities == 1) {
        info = GET_INFO(mvar);
    } else {
        ("ptr" info) = ccall lockClosure(mvar "ptr");
    }

But got no speedup at all.
The generated asm (amd64):

        movl $enabled_capabilities,%eax
        cmpq $1,%rax
        je .Lcgq
.Lcgp:
        movq %rbx,%rdi
        subq $8,%rsp
        movl $0,%eax
        call lockClosure
        addq $8,%rsp
.Lcgr:
        cmpq $stg_MVAR_CLEAN_info,%rax
        jne .Lcgu
{...}
.Lcgq:
        movq (%rbx),%rax
        jmp .Lcgr


It moves enabled_capabilities into %eax and then compares 1 with %rax.
It looks wrong for me: the highest part of %rax remains uninitialized.
(And actually it ignores the highest 32 bits of enabled_capabilities.
That is ok here, but generally wrong.)
The compiler knows that enabled_capabilities is 64bit
(it complains when trying to assign it to bits32 register.)

What do I miss? Any help?

Thanks,
Yuras


-- The test case

import Control.Concurrent
import Control.Monad

main :: IO ()
main = do
  var <- newMVar ()
  replicateM_ 100000000 $ do
    takeMVar var
    putMVar var ()