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: { }
it's treated as if you wrote
case n: jmp codeBlock;
...
codeBlock:
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
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
_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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