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Almost Inline ASM in Haskell With Foreign Import Prim

With help from Reid Barton in questions here and here I discovered it’s pretty easy to call assembly from GHC haskell with minimal overhead, so I cleaned up an example of this technique and posted it here:

This is especially useful if you want to return multiple values from a foreign procedure, where otherwise with the traditional FFI approach you would have to do some allocation and stuff the values into a struct or something. I find the above more understandable in any case.

Here’s an example of the dumped ASM from the Main in the example above:

    call newCAF
    addq $8,%rsp
    testq %rax,%rax
    je _c73k
    movq $stg_bh_upd_frame_info,-16(%rbp)
    movq %rax,-8(%rbp)
    movq $block_info,-24(%rbp)
    movl $4,%edi
    movl $3,%esi
    movl $2,%r14d
    movl $1,%ebx
    addq $-24,%rbp
    jmp sipRound_s_x3
    movq $104,904(%r13)
    movq $block_info,-32(%rbp)
    movq %r14,-24(%rbp)
    movq %rsi,-16(%rbp)
    movq %rdi,-8(%rbp)
    movq %rbx,(%rbp)
    addq $-32,%rbp

You can see we just prepare argument registers, do whatever with the stack pointer, do a jump, and then push the return values onto the stack. For my purposes this was almost too much overhead to make this worthwhile (you can look at notes in the code).

I thought about sketching out a ghc proposal about a way to formalize this, maybe make it safer, and maybe somehow more efficient but I don’t have the time right now and don’t really have the expertise to know if this is even a good idea or how it could work.


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