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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 23:59:29 +0000
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EricP wrote:

> MitchAlsup1 wrote:
>> 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.}

> You still could do it by callee loading the arg address to register
> and jumping directly there.

> If it uses callee stack cleanup then you load the reg and pop the
> stack before jumping.

> If it uses caller stack cleanup then since caller controls what jump
> addresses are passed to the callee, caller follows labels 100 and 200
> each with stack cleanup code then continues normally.

> If it doesn't use a stack at all, as I understand IBM Fortran did,
> then it doesn't need cleanup.


IBM FORTRAN used a list of register store areas--thus pretty much like
a stack, just without the ability to be reentrant or recursive.