Re: Parameter Passing Via Registers (Michael K. Gschwind)
30 Apr 91 14:22:06 GMT

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Newsgroups: comp.compilers
From: (Michael K. Gschwind)
Keywords: Pascal, registers, optimize, Modula
Organization: Compilers Central
References: <>
Date: 30 Apr 91 14:22:06 GMT

In article <> (Chuck Lins) writes:
>Does anyone know how nested procedures affect the ability to pass parameters
>via registers? ...

Well, there is an easy solution: just generate a list of all variables
used by nested procedures - then you can store these (AND ONLY THESE, they
are bound to be very few) in well-known stack locations. All other
variables can be kept in registers and pronounced dead.

In this set-up, a variable is dead, if

*) it is dead in the procedure
*) and
+) either no nested procedures are called
+) or the varaible is dead upon entry in this

There are two possibilities to act, if you know a parameter is going to be
referenced in nested procedures. Either push it on the stack upon function
entry or only if you're calling a nested procedure which uses it (You will
need the transitive closure, I guess) - the latter mey be preferable in
some circumstances, but the fromer is easier to implement: Just search the
nested functions and check if the parameter is ever used. If so, reserve a
stack slot and push immediately.

PROCEDURE x( a, b, c ,d : integer) ;
PROCEDURE y ( e,f,g : real);

write (a+f);
if (d = 0)
write (a);

You either do:

/* a into well-know location, no need to save b,c,d */

st rA, fp+_a
test rD
jne @L1
<call y>
@L1: <write>



test rD
jne @L1
/* a into well-know loc */
st rA, fp+_a
<call y>
@L1: <write>

      [Can one reference a named parameter in an enclosing procedure? If so, I
      can imagine that would force them onto the stack. -John]

I dunno, but I guess so. Shouldn't be any complication in any case. Maybe
somebody could check out the relevant facts?

Register allocation across all nested functions is *) impossible in Pascal
(since nested procedures may be transmitted via PROCEDURE parameters to
non-related functions) and *) (I guess) not worthwhile in Modula-2 (where
only top-level functions may be assigned to procedure-type variables -
thus you can have a fall-back strategy for all top-level functions and
thus restrict register allocation to a top-level function and all its
nested functions) except if used with a global register allocation

BTW, does anybody know about compilers trying to find 'optimal' register
assignment for top-level Modula-2 functions and all nested functions?
Register allocation could probably done quite easily for such a block of
related functions, as the only entry point from without this block is the
top-level function. Thus the compiler knows all the places where the
nested functions are called.

Michael K. Gschwind, Dept. of VLSI-Design, Vienna University of Technology || mike@vlsivie.uucp
Voice: (++43).1.58801 8144 || Fax: (++43).1.569697

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