Dead code and node reuse in generated code when using async

[arnetheduck]

compiling https://github.com/nim-lang/Nim/blob/master/tests/async/tasync_traceback.nim, the async code gets transformed into a state machine - in this state machine, there are two issues with the generated tree:

1. there's an extra call to complete_rqmfICbsG8zaMmva4T4xIg after the last goto BeforeRet_ that will never be hit

2. the snippet (*(*colonenvP_).colonup_).retFuture1 is generated by a single node instance appearing twice in the tree - at this point the tree is no longer a tree (the node has two parents, so it's more of a graph), and the node ID can suddenly be found in two branches of the tree - this makes it difficult to use for things that depend on it being unique, such as dead code elimination or lazy code generation (unlike the C gen, nlvm uses it this)!

{   result = (tyObject_FutureBasecolonObjectType__cnXnCCtV9cjKaEq9alHheOFg*)0;
    colonenvP_ = (tyObject_Env_libslashpureslashasyncmacrodotnim__fhbkBamT1XdhWaGDl4D4JA*) ClE_0;
    switch ((*colonenvP_).colonstate_) {
    case -1: goto BeforeRet_;
    case 0: goto STATE0;
    case 1: goto STATE1;
    }
    STATE0: ;
    (*colonenvP_).future3 = b_POGshUH6CHI2ak8lki3jtg_2();
    (*colonenvP_).colonstate_ = ((NI) 1);
    result = &(*colonenvP_).future3->Sup;
    goto BeforeRet_;
    STATE1: ;
    T1_ = (NI)0;
    T1_ = read_4NOxxpXfTo1DJap5zqepHw((*colonenvP_).future3);
    complete_rqmfICbsG8zaMmva4T4xIg((*(*colonenvP_).colonup_).retFuture1, T1_);
    (*colonenvP_).colonstate_ = ((NI) -1);
    result = NIM_NIL;
    goto BeforeRet_;
    complete_rqmfICbsG8zaMmva4T4xIg((*(*colonenvP_).colonup_).retFuture1, (*colonenvP_).result1);
    (*colonenvP_).colonstate_ = ((NI) -1);
    }BeforeRet_: ;
    return result;
}

using Nim 673f5f3

[Araq]

Is this still an issue with the newer async transformation?

[dom96]

Surely you mean newer closure iterator transformation.

[Araq]

Well yes.