Cmm code of `id` function referring to `breakpoint`?

[Ömer Sinan Ağacan]

That's because of the CSE (common subexpression elimination) pass. Here's an
example:

    module Lib where

    foo :: a -> a
    foo x = x

    bar :: a -> a
    bar x = x

Build with -O -ddump-stg and you'll see something like:

    Lib.foo :: forall a. a -> a
    [GblId, Arity=1, Caf=NoCafRefs, Str=<S,1*U>, Unf=OtherCon []] =
        [] \r [x_s1bB] x_s1bB;

    Lib.bar :: forall a. a -> a
    [GblId, Arity=1, Caf=NoCafRefs, Str=<S,1*U>, Unf=OtherCon []] =
        [] \r [eta_B1] Lib.foo eta_B1;

Without -O or with -fno-cse this does not happen.

This is quite unexpected, but maybe not harmful.

Ömer

Shao, Cheng <cheng.shao at tweag.io>, 6 Şub 2019 Çar, 08:35 tarihinde şunu yazdı:
>
> Hi devs,
>
> I just found that the Cmm code of `GHC.Base.id` refers to `breakpoint`
> in the same module, however, in the Haskell source of `GHC.Base`, the
> definition of `id` and `breakpoint` are totally unrelated:
>
> ```
> id                      :: a -> a
> id x                    =  x
>
> breakpoint :: a -> a
> breakpoint r = r
> ```
>
> And here's the pretty-printed Cmm code:
>
> ```
> base_GHCziBase_id_entry() //  [R2]
>           { []
>           }
>       {offset
>         chwa: // global
>             R2 = R2;
>             call base_GHCziBase_breakpoint_entry(R2) args: 8, res: 0, upd: 8;
>       }
> base_GHCziBase_breakpoint_entry() //  [R2]
>           { []
>           }
>       {offset
>         chvW: // global
>             R1 = R2;
>             call stg_ap_0_fast(R1) args: 8, res: 0, upd: 8;
>       }
> ```
>
> This looks suspicious. I'm curious if this is intended behavior of ghc.
>
> Regards,
> Shao Cheng
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