Variable declarations: Martin suggests that declarations should be as close to the usage of the variables as possible. Others feel they should all be at the top, or at least together somewhere. What's your opinion?
When it comes to variables within a program - it's important to remember 'scope'. We need to make it clear what scope the variables are in.
If we are declaring variables at the top of the program (not in a main but the very top) - this is what I would consider global variables. These will be used throughout the entire program and therefore be rather important e.g. A Window object. This variable will be altered through the program.
We've talked about global variables - let's talk about local ones.
I would say there are two types of local variables. One's within a methods usages and ones within loops.
Method usage local are the ones you would declare at the start of a function. These could be used to calculate a sum and then be returned. There is no need to put the variable else where. Now I believe you should use the variable right after declaring it in this situation.
Sometimes it's helpful to declare all the variables at the start - this helps give context to what the function will be doing and possibly what it is returning without the need of comments. I find this helpful for functions that handle complex maths equations.
The variables used within loops should come before the loops start; this gives me as a programmer better control on the scope and also context. An example could be line number, character position and file name before a for loop. I know that these three variables will be used in this for loop for keeping track of where we are in the loop.
Do you have any of your own formatting rules you follow (even if only sometimes?)
I try and reduce the amount of nesting I do - this falls under refactoring. I also like to still use {} around one line if functions too.
The Visitor Pattern is used to generate the necessary low level instructions.
The algorithm we are using follows the visitor pattern. We can already see this in the visualisation of the AST.
We specifically write visitNode methods to view the tree.
The Visitor pattern walks the AST calling emit() methods to generate machine code instructions as it goes. Lookups into the AST are used to decide things like the value of literals or specific operators to use.
Backpatching is used when we need to make forward jumps when we need them (look at visitIf command).
let
const b ~ 10;
var i : Integer
in
i := i * b
b is bound to 10 and i is bound to an address large enough to hold an integer.
When b is called in the program - it should be translated to a 10 by the compiler. Each time i is used, it should be translated to a memory address.
In this example, the address for i is 4. The machine code could look like this:
LOAD 4[SB]
LOADL 10
CALL mult
STORE 4[SB]
In any declaration, identifiers can be bound to values or addresses, and these might be known or unknown at compile time. So at any declaration, there is one of four possible known values:
Known Value: This is a constant declaration, e.g. b in the above program. It was bound to a literal value of 10.
Unknown Value: This is a constant expression where the right-side needs to be evaluated at runtime e.g. const b ~ a + 5 or a parameter received by a routine.
Known Address: This is a known address that is allocated and bound to a variable declaration e.g. i in the example above.
Unknown Address: This is an address bound to a variable parameter in a routine.
The specific machine codes to read and write variables and constants are wrapped up in objects that are attached to the relevant declaration nodes in the AST
These codes include either the literal value (in the case of constants bound to literals) or the necessary steps to work out the right address
Each time we have a declaration, we need to generate the instructions to increase the size of the current frame; when that declaration drops out of scope, the frame is decreased in size again by “popping” elements off the stack
...
public KnownValue(int size, int value){ ... }
...
public void encodeFetch(Emitter ...){
emitter.emit(OpCode.LOADL, 0, value)
}
...
A Known value (KnownValue.java) is simple. Size is the amount of memory taken up by this value and Value is the value itself. encodeFetch loads the literal value onto the stack (0).
...
public UnknownValue(int size, int level, int displacement){ ... }
...
public void encodeFetch(Emitter ...){
if (vname.indexed){
emitter.emit(OpCode.LOADA ...
emitter.emit(OpCode.CALL ...
emitter.emit(OpCode.LOADI ...
} else {
emitter.emit(OpCode.LOAD ...
}
}
...
An Unknown Value is made up of two parts - level and displacement. Level is how nested the routine that contained this declaration. Displacement is where the entity is located relative to the base of the frame (i.e. how many words is from the start if the frame it is).
We can use the frame as the displacement; that's the top of the stack.
How do we handle procedures and functions? These both translate to low level routines. A routine is a series of instructions and the template might look something like this:
elaborate [proc I () ~ C] =
JUMP g
e: execute c
RETURN (0) 0
h:
Compiler optimisation can happen in a few places:
Having a step before the code generation whereby the AST is manipulated or manipulating intermediate code if that is being generated.
At the point where code is being generated - we can use processor specific instructions can be used/exploited.
Common Sub-expression Elimination: attempts to reduce calculations that are repeated. (This only works when the code is simple).
Constant Propagation: At some point in a program - a variable might always have the same value. An algorithm to trace the flow of constant variables through the programme can be run to determine where this is guaranteed to be the case. The reference to the variable can be replaced by a literal value - reducing the need for fetches from memory.
Hoisting: If part of a computation in a loop is independent of the values that change inside the loop, it can be moved outside the loop - meaning that it is only calculated once e.g:
while(j < k) {
a[j] := b + c;
j++;
}
In this example: b and c can be moved outside the loop so they are only computed once. We could turn the above code into this:
if (j < k){
tmp = b + c;
while (j < k){
a[j] := tmp;
j++;
}
}
Optional reading: More examples are mentioned in the book Introduction to Compiler Design (Mogensen) cited in the module home page; also here https://compileroptimizations.com
Python is an interpreted language, meaning that the instructions called are done in real time - this creates more slow-down when running the program compared to a compiled language like C.
Java is in the middle. Javac generates bytecode which is targeted at a virtual machine. This is then interpreted, so there is still some overhead involved when running with Java.
Our toy language compiles into a Tam file. This is that intermediate language similar to bytecode in Java.
JIT (Just-in-Time) compilation was introduced to help speed up Java's slow interpreted bytecode. Java's compiler doesn't do much optimisation itself, the idea of JIT is to offload certain expensive CPU executions into native machine code. Only parts of the program are complied in this way rather than the whole thing (most of it wont be ran often enough to make it worthwhile.).
The process of targeting this extra compilation step is where the HotSpot (Oracles implementation of Java gets it name) - the targeted code is where the program runs hot.
Simply put, there's a count of the number of times each method, loop and numerous other structures are executed. If that number would reach a particular threshold, that relevant block of code is compiled natively on the machine!
JIT will also monitor branches of code and do the heavy work of lifting the never executed blocks of code from the compilation process off.
We must not prematurely optimise! We should write straight forward clean code. We don't want to write hard-to-read code; slows down development, introduce bugs that are hard to track and make maintenance so much harder.
log.log(Level.FINE, "..." + calcX() ... + calcY() ... );
This simple line has to be compiled and the method calls have to be computed i.e moving values on and off the stack and tracing ourselves around the program calls. If there is an error, log only reports levels higher than FINE so all that computation is wasted! We can rewrite this to:
if(log.isLoaggable(Level.FINE)){_
log.log(Level.FINE, "..." + calcX() ... + calcY() ... );
}
This is much more efficient.
Sometimes we want to swap values around. We could do it like this:
temp = a
a = b
b = temp
This can be done better though. Maybe we can remove the temp variable. Let's have a look at the XOR binary function.
XOR works like this:
| ----- | - | | 0 + 0 | 0 | | 0 + 1 | 1 | | 1 + 0 | 1 | | 1 + 0 | 0 |
Turns out, if we use XOR in a variable three times - it swaps!
Unfortunately, it turns out that in practise, at best this will perform the same as using a temporary variable. On many modern CPU's, copies between registers are extremely fast. Some provide low-level instructions to swap variables anyway.
In summary: