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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: ALTER, ASSIGN and labels-as-values
Date: Mon, 27 May 2024 18:21:43 +0000
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Anton Ertl wrote:

> EricP <ThatWouldBeTelling@thevillage.com> writes:
>>Don't forget Fortran's alternate returns, indicated by a * in the arg
>> list.
>>A caller subroutine passes *labels in the args list, and callee decides
>>which *label-arg it wants to return to in the RETURN statement constant
>>expression. (This is not ambiguous because in Fortran function return
>>values are done by assignment to the function name, not the return
>>statement expression.)
>>
>>call Foo (*100, *200)
>>...
>>100
>>...
>>200
>>...
>>end
>>
>>subroutine Foo (*, *)
>>...
>>return 1       ! Return to callers first * label arg
>>...
>>return 2       ! Return to callers second * label arg
>>...
>>end

> Interesting.  Rudimentary explicit continuation-passing style at the
> dawn of programming languages.  Of course, these days it would mean
> that you better not use the common call and return instructions,
> because the branch prediction for the return would be wrong if return
> 1 or return 2 would be used.  But using plain jump for the call and
> indirect jumps for the returns should be efficient on modern CPUs.

That is why the subroutine returns a value and the return point 
implements the switch (control transfer to proper label.

{You DO NOT return to the label, you return normally and then 
go to the proper label.}

> - anton