ack/doc/ego/il/il3

.NH 2
Feasibility and desirability analysis
.PP
Feasibility and desirability analysis
of in line substitution differ
somewhat from most other techniques.
Usually, much effort is needed to find
a feasible opportunity for optimization
(e.g. a redundant subexpression).
Desirability analysis then checks
if it is really advantageous to do
the optimization.
For IL, opportunities are easy to find.
To see if an in line expansion is
desirable will not be hard either.
Yet, the main problem is to find the most
desirable ones.
We will deal with this problem later and
we will first attend feasibility and
desirability analysis.
.PP
There are several reasons why a procedure invocation
cannot or should not be expanded in line.
.sp
A call to a procedure P cannot be expanded in line
in any of the following cases:
.IP 1. 3
The body of P is not available as EM text.
Clearly, there is no way to do the substitution.
.IP 2.
P, or any procedure called by P (transitively),
follows the chain of statically enclosing
procedures (via a LXL or LXA instruction)
or follows the chain of dynamically enclosing
procedures (via a DCH).
If the call were expanded in line,
one level would be removed from the chains,
leading to total chaos.
This chaos could be solved by patching up
every LXL, LXA or DCH in all procedures
that could be part of the chains,
but this is hard to implement.
.IP 3.
P, or any procedure called by P (transitively),
calls a procedure whose body is not
available as EM text.
The unknown procedure may use an LXL, LXA or DCH.
However, in several languages a separately
compiled procedure has no access to the
static or dynamic chain.
In this case
this point does not apply.
.IP 4.
P, or any procedure called by P (transitively),
uses the LPB instruction, which converts a
local base to an argument base;
as the locals and parameters are stored
in a non-standard way (differing from the
normal EM calling sequence) this instruction
would yield incorrect results.
.IP 5.
The total number of bytes of the parameters
of P is not known.
P may be a procedure with a variable number
of parameters or may have an array of dynamic size
as value parameter.
.LP
It is undesirable to expand a call to a procedure P in line
in any of the following cases:
.IP 1. 3
P is large, i.e. the number of EM instructions
of P exceeds some threshold.
The expanded code would be large too.
Furthermore, several programs in ACK,
including the global optimizer itself,
may run out of memory if they they have to run
in a small address space and are provided
very large procedures.
The threshold may be set to infinite,
in which case this point does not apply.
.IP 2.
P has many local variables.
All these variables would have to be allocated
in the stack frame of the calling procedure.
.PP
If a call may be expanded in line, we have to
decide how to access its parameters.
In the previous section we stated that we would
use in line parameters whenever possible and desirable.
There are several reasons why a parameter
cannot or should not be expanded in line.
.sp
No parameter of a procedure P can be expanded in line,
in any of the following cases:
.IP 1. 3
P, or any procedure called by P (transitively),
does a store-indirect or a use-indirect (i.e. through
a pointer).
However, if the front-end has generated messages
telling that certain parameters can not be accessed
indirectly, those parameters may be expanded in line.
.IP 2.
P, or any procedure called by P (transitively),
calls a procedure whose body is not available as EM text.
The unknown procedure may do a store-indirect
or a use-indirect.
However, the same remark about front-end messages
as for 1. holds here.
.IP 3.
The address of a parameter location is taken (via a LAL).
In the normal calling sequence, all parameters
are stored sequentially. If the address of one
parameter location is taken, the address of any
other parameter location can be computed from it.
Hence we must put every parameter in a temporary location;
furthermore, all these locations must be in
the same order as for the normal calling sequence.
.IP 4.
P has overlapping parameters; for example, it uses
the parameter at offset 10 both as a 2 byte and as a 4 byte
parameter.
Such code may be produced by the front ends if
the formal parameter is of some record type
with variants.
.PP
Sometimes a specific parameter must not be expanded in line.
.sp 0
An actual parameter expression cannot be expanded in line
in any of the following cases:
.IP 1. 3
P stores into the parameter location.
Even if the actual parameter expression is a simple
variable, it is incorrect to change the 'store into
formal' into a 'store into actual', because of
the parameter mechanism used.
In Pascal, the following expansion is incorrect:
.DS
procedure p (x:integer);
begin
   x := 20;
end;
...
a := 10;                a := 10;
p(a);        --->       a := 20;
write(a);               write(a);
.DE
.IP 2.
P changes any of the operands of the
actual parameter expression.
If the expression is expanded and evaluated
after the operand has been changed,
the wrong value will be used.
.IP 3.
The actual parameter expression has side effects.
It must be evaluated only once,
at the place of the call.
.LP
It is undesirable to expand an actual parameter in line
in the following case:
.IP 1. 3
The parameter is used more than once
(dynamically) and the actual parameter expression
is not just a simple variable or constant.
.LP