Calling an unknown function from low-level Cmm

14 Feb 2020

      Hi all,

I’m trying to understand how to properly call an unknown function from low-level Cmm code. If I’m just applying a function to a state token, it’s easy; I can just do

    R1 = io;
    jump stg_ap_v_fast [R1];

since the calling convention is consistent in that case. But what if my function takes actual arguments? I can’t do

    R1 = fun;
    R2 = arg;
    jump stg_ap_p_fast [R1, R2];

because if the calling convention doesn’t pass any arguments in registers, that would be wrong. I could check if NO_ARG_REGS is defined and generate different code in that situation, but that seems extreme. One option I think would work would be to do

    R1 = fun;
    Sp_adj(-2);
    Sp(1) = arg;
    jump RET_LBL(stg_ap_p) [R1];

but that seems wasteful if I have the argument in a register already anyway. Am I missing something?

Thanks,
Alexis

Alexis King

Simon Peyton Jones

Alexis King

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