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I have been checking out integer-gmp source code to understand how foreign primops can be implemented in terms of cmm as documented on GHC Primops page . I am aware of techniques to implement them using llvm hack or fvia-C/gcc - this is more of a learning experience for me to understand this third approach that interger-gmp library uses. So, I looked up CMM tutorial on MSFT page (pdf link) , went through GHC CMM page , and still there are some unanswered questions (hard to keep all those concepts in head without digging into CMM which is what I am doing now). There is this code fragment from integer-bmp cmm file : integer_cmm_int2Integerzh (W_ val) { W_ s, p; /* to avoid aliasing */ ALLOC_PRIM_N (SIZEOF_StgArrWords + WDS(1), integer_cmm_int2Integerzh, val); p = Hp - SIZEOF_StgArrWords; SET_HDR(p, stg_ARR_WORDS_info, CCCS); StgArrWords_bytes(p) = SIZEOF_W; /* mpz_set_si is inlined here, makes things simpler */ if (%lt(val,0)) { s = -1; Hp(0) = -val; } else { if (%gt(val,0)) { s = 1; Hp(0) = val; } else { s = 0; } } /* returns (# size :: Int#, data :: ByteArray# #) */ return (s,p); } As defined in ghc cmm header : W_ is alias for word. ALLOC_PRIM_N is a function for allocating memory on the heap for primitive object. Sp(n) and Hp(n) are defined as below (comments are

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