Hi Norman,

I vaguely remember that we "finish" such unterminated code blocks by
jumping to the block again.

That is for code like this:

    myswitch2 (bits32 n) {
      foreign "C" D();
    }

We produce code like this:

    {
      cg: call "ccall" arg hints:  []  result hints:  [] D();
          goto cg;
    }

Instead of blowing up the compiler at compile time or the program at
runtime.

For switch statements I think blocks are just syntactic sugar. E.g. if
you write

case n: { <code> }

it's treated as if you wrote

case n: jmp codeBlock;
...

codeBlock:
    <code>

And since your blocks don't terminate we get the behaviour you are seeing.
But I haven't looked at any of the code related to this so it's possible
I got it wrong.

Cheers
Andreas


Am 12/01/2022 um 01:02 schrieb Norman Ramsey:
For testing purposes, I created the following Cmm program:

     myswitch (bits32 n) {
       switch [0 .. 4] n {
         case 0, 1: { foreign "C" A(); }
         case 2: { foreign "C" B(); }
         case 4: { foreign "C" C(); }
         default: { foreign "C" D(); }
       }
       return (666);
     }

In the original C-- specification, it's pretty clear that when, say,
the call to foreign function `A` terminates, the switch statement is
supposed to finish and function `myswitch` is supposed to return 666.
What actually happens in GHC is that this source code is parsed into a
control-flow graph in which execution loops forever, repeating the
call.  The relevant fragment of the prettyprinted CFG looks like this:

    {offset
      ca: // global
          _c1::I32 = %MO_XX_Conv_W64_W32(R1);
          //tick src<programs/panic.cmm:(1,21)-(9,1)>
          switch [0 .. 4] _c1::I32 {
              case 0, 1 : goto c5;
              case 2 : goto c7;
              case 4 : goto c9;
              default: {goto c3;}
          }
      ...
      c5: // global
          //tick src<programs/panic.cmm:3:16-35>
          _c4::I64 = A;
          call "ccall" arg hints:  []  result hints:  [] (_c4::I64)();
          goto c5;
      ...
    }

Surprising, at least to me.

Is this behavior a bug or a feature?  And if it is a feature, can
anyone explain it to me?


Norman
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