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.s...@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
> _______________________________________________
> ghc-devs mailing list
> ghc-devs@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
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