Is "cml_cont" of CmmCall used in practice?

[Michal Terepeta]

On Sun, Mar 18, 2018 at 6:38 AM Shao, Cheng <cheng.shao at tweag.io> wrote:

> Hi all,
>
> Is the "cml_cont" field of the CmmCall variant is really used in practice?
> I traversed the output of raw Cmm produced by ghc compiling the whole base
> package, but the value of cml_cont is always Nothing.
>
> Regards,
> Shao Cheng
>


Hi,

I'm not a GHC expert, so please don't trust everything I say ;)

That being said, I think `cml_cont` is used a lot. If you look at the
`compiler/codeGen` directory (that's what turns STG to cmm), you'll
see that `MkGraph.mkCallReturnsTo` is called a few times. That's the
function that will construct a `CmmCall` with the continuation block.

When dumping cmm, you'll often see all those `returns to` notes. For
instance, compiling:

```
foo :: Int -> Int
foo x =
  case x of
    42 -> 111111
    _  -> 000000
```

results in:

```
       [...]
       c2cN: // global
           I64[Sp - 8] = c2cI;
           R1 = R2;
           Sp = Sp - 8;
           if (R1 & 7 != 0) goto c2cI; else goto c2cJ;

       // Evaluate the parameter.
       c2cJ: // global
           call (I64[R1])(R1) returns to c2cI, args: 8, res: 8, upd: 8;
           //                 ^^^^^^^^^^^^^^^
           //                 this specifies the continuation block
           //                 see also PprCmm.pprNode

       // Now check if it's 42.
       c2cI: // global
           if (I64[R1 + 7] == 42) goto c2cU; else goto c2cT;
       c2cU: // global
       [...]
```

As far as I understand it, this allows the code above to jump to the
`x` closure (to evalutae it), and have the closure jump back to the
continuation block (note that it's address is stored before we jump to
closure). AFAICS this particular code is created by
`StgCmmExpr.emitEnter`.

Hope this helps!

- Michal
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