.bp
.NH 1
Stack pollution
.NH 2
Introduction
.PP
The "Stack Pollution" optimization technique (SP) decreases the costs
(time as well as space) of procedure calls.
In the EM calling sequence, the actual parameters are popped from
the stack by the \fIcalling\fR procedure.
The ASP (Adjust Stack Pointer) instruction is used for this purpose.
A call in EM is shown in Fig. 8.1
.DS
.TS
l l.
Pascal:	EM:

f(a,2)	LOC 2
	LOE A
	CAL F
	ASP 4    -- pop 4 bytes
.TE

Fig. 8.1 An example procedure call in Pascal and EM
.DE
As procedure calls occur often in most programs,
the ASP is one of the most frequently used EM instructions.
.PP
The main intention of removing the actual parameters after a procedure call
is to avoid the stack size to increase rapidly.
Yet, in some cases, it is possible to \fIdelay\fR or even \fIavoid\fR the
removal of the parameters without letting the stack grow
significantly.
In this way, considerable savings in code size and execution time may
be achieved, at the cost of a slightly increased stack size.
.PP
A stack adjustment may be delayed if there is some other stack adjustment
later on in the same basic block.
The two ASPs can be combined into one.
.DS
.TS
l l l.
Pascal:	EM:	optimized EM:

f(a,2)	LOC 2	LOC 2
g(3,b,c)	LOE A	LOE A
	CAL F	CAL F
	ASP 4	LOE C
	LOE C	LOE B
	LOE B	LOC 3
	LOC 3	CAL G
	CAL G	ASP 10
	ASP 6
.TE

Fig. 8.2 An example of local Stack Pollution
.DE
The stacksize will be increased only temporarily.
If the basic block contains another ASP, the ASP 10 may subsequently be
combined with that next ASP, and so on.
.PP
For some back ends, a stack adjustment also takes place
at the point of a procedure return.
There is no need to specify the number of bytes to be popped at a
return.
This provides an opportunity to remove ASPs more globally.
If all ASPs outside any loop are removed, the increase of the
stack size will still only be small, as no such ASP is executed more
than once without an intervening return from the procedure it is part of.
.PP
This second approach is not generally applicable to all target machines,
as some back ends require the stack to be cleaned up at the point of
a procedure return.
.NH 2
Implementation
.PP
There is one main problem the implementation has to solve.
In EM, the stack is not only used for passing parameters,
but also for evaluating expressions.
Hence, ASP instructions can only be combined or removed
if certain conditions are satisfied.
.PP
Two consecutive ASPs of one basic block can only be combined
(as described above) if:
.IP 1.
On no point of text in between the two ASPs, any item is popped from
the stack that was pushed onto it before the first ASP.
.IP 2.
The number of bytes popped from the stack by the second ASP must equal
the number of bytes pushed since the first ASP.
.LP
Condition 1. is not satisfied in Fig. 8.3.
.DS
.TS
l l.
Pascal:	EM:

5 + f(10) + g(30)	LOC 5
	LOC 10
	CAL F
	ASP 2    -- cannot be removed
	LFR 2    -- push function result
	ADI 2
	LOC 30
	CAL G
	ASP 2
	LFR 2
	ADI 2
.TE

Fig. 8.3 An illegal transformation
.DE
If the first ASP were removed (delayed), the first ADI would add
10 and f(10), instead of 5 and f(10).
.sp
Condition 2. is not satisfied in Fig. 8.4.
.DS
.TS
l l.
Pascal:	EM:

f(10) + 5 * g(30)	LOC 10
	CAL F
	ASP 2
	LFR 2
	LOC 5
	LOC 30
	CAL G
	ASP 2
	LFR 2
	MLI 2   --  5 * g(30)
	ADI 2
.TE

Fig. 8.4 A second illegal transformation
.DE
If the two ASPs were combined into one 'ASP 4', the constant 5 would
have been popped, rather than the parameter 10 (so '10 + f(10)*g(30)'
would have been computed).
.PP
The second approach to deleting ASPs (i.e. let the procedure return
do the stack clean-up)
is only applied to the last ASP of every basic block.
Any preceding ASPs are dealt with by the first approach.
The last ASP of a basic block B will only be removed if:
.IP -
on no path in the control flow graph from B to any block containing a
RET (return) there is a basic block that, at some point of its text, pops
items from the stack that it has not itself pushed earlier.
.LP
Clearly, if this condition is satisfied, no harm can be done; no
other basic block will ever access items that were pushed
on the stack before the ASP.
.PP
The number of bytes pushed onto or popped from the stack can be
easily encoded in a so called "pop-push table".
The numbers in general depend on the target machine word- and pointer
size and on the argument given to the instruction.
For example, an ADS instruction is described by:
.DS
   -a-p+p
.DE
which means: an 'ADS n' first pops an n-byte value (n being the argument),
next pops a pointer-size value and finally pushes a pointer-size value.
For some infrequently used EM instructions the pop-push numbers
cannot be computed statically.
.PP
The stack pollution algorithm first performs a depth first search over
the control flow graph and marks all blocks that do not satisfy
the global condition.
Next it visits all basic blocks in turn.
For every pair of adjacent ASPs, it checks conditions 1. and 2. and
combines the ASPs if they are satisfied.
The new ASP may be used as first ASP in the next pair.
If a condition fails, it simply continues with the next ASP.
Finally, the last ASP is removed if:
.IP -
nothing has been popped from the stack after the last ASP that was
pushed before it
.IP -
the block was not marked by the depth first search
.IP -
the block is not in a loop
.LP
