It's time for our little compiler to get in touch with the real world!
And by real world,
I mean a PC, such as this one.
How does it work?
The answer, like the answers to all good questions,
lies in the AMD64 Architecture Programmer's Manual.
Page 24 in this manual shows the register architecture –
of this processor architecture.
There are sixteen base registers and some special-purpose registers.
The next opening, pages 26 and 27, explain this more in detail.
You can find even more-in-detail information about this architecture –
in a separate video that I made some time in the future,
but let me quote parts from it:
There are sixteen base registers.
These are like really really fast variables within the CPU itself.
They are named RAX, RBX, RCX, and so on, until R15.
Each register is 64 bits wide,
but the bottom 32 bits can be addressed separately –
with a legacy name: EAX, EBX, ECX, and so on.
Furthermore, the bottom 16 bits can be addressed separately,
and so can the bottom 8 bits as well.
Some of the registers also have a way to address the second set of 8 bits,
because of legacy reasons.
And in my compiler, we might use all of these.
Well, except for the stack pointer because it has a special purpose.
The IR variables will be directly converted into CPU registers,
such that R0 is RDI, R1 is RSI, R2 is RDX, and so on,
until R14 which is RBP.
But what do we do when we run out of registers?
We might add more registers.
These 64-bit registers exist in all x86_64 processors,
but they are usually used for special types of calculation,
not as general-purpose storage for variables.
Nevertheless, it is possible to use them like this.
But we still have a cap.
Only the first 55 variables have been assigned physical storage.
It might not be enough for some programs,
and furthermore, on the SNES, we still only have three registers.
The rest of variables have to be stored somewhere else.
And that somewhere else is memory.
There are two classes of variables:
Ones that are stored in CPU registers,
and ones that are stored in memory.
Not all CPU registers are equal though.
Some registers and instructions have limitations –
on what they may be paired with.
And the rest of variables are stored in memory.
The traditional way to programming on old video game consoles –
is to store all variables in global memory.
However, global variables are disfavored –
by both modern compilers and modern programming guidelines.
The robust way is to put the variables in stack.
IR statement parameters can also be number literals.
So let's study the IR statements themselves.
Here are some sample IR statements.
How do we map these into CPU instructions?
A crude sketch would look like this.
Each statement corresponds to one or more CPU instructions.
This is how simple compilers do the translation –
from IR code into machine code.
However, this little sketch is just that, a starting point.
Let's study this MOV instruction for instance.
How does the MOV instruction actually work?
These are all the valid combinations of operands.
You can move from a base register to another base register.
You can move from memory to a base register,
and vice versa.
You can not move directly from memory to memory.
For number literals,
you can use the XOR instruction to put zero into a base register.
You can also put an arbitrary number literal into a base register,
as long as that number literal is smaller than 32 bits.
The LEA instruction can put the address of a resource into a register,
even if that address is larger than 32 bits.
And you can directly assign a number literal into a memory variable,
as long as that number literal is smaller than 32 bits.
If we add the SIMD registers into the mix,
the number of data movement instructions becomes five times larger.
So it is clear that a simple MOV will not do it.
For the addition, we can abuse the address decoder –
to perform the sum of two registers and store the result in a third.
But this only works if none of the operands is a memory variable.
Now let's have a look at the SNES instructions.
The LDA instruction comes in two varieties:
One reads a memory variable and stores into A,
and the other stores a number literal in A.
Both of these operate on A.
For other combinations of registers and memory,
there is at least 13 different instructions.
All the other statements have similar restrictions,
and we have to figure out ways to deal with them.
The READ instruction is rather straightforward.
This variant shown here –
assumes the source is a memory variable,
that contains the address of the other memory resource we should read.
For that, we use the stack-indexed- indirect LDA instruction.
It works as intended, but as a casualty,
it also clobbers the contents of the Y register.
Some instruction patterns may indeed have side-effects,
that they clobber some registers and put in them an unrelated value.
The compiler must be aware of this.
Now that we have established what we can do,
let's move on to what we should do.
For the x86_64 platform, also called AMD64,
there is an actual official document –
that specifies how everything should be done.
It specifies things like data formats,
function call convention, stack layout,
program models, and even the file formats.
Basically, everything there is to producing well-behaved programs.
I am going to focus on the function call convention.
This is an umbrella term –
that covers how to pass parameters to functions,
where return values are stored,
and where and how to store local variables.
All of these are somewhere in CPU registers or in the stack,
but how and where exactly,
that's what we are going to find out.
Answers are found in chapter 3.2.3, on pages 21 and 22.
In my compiler,
all variables are words that are either integers or pointers.
So, the first six parameters are passed in registers –
RDI, RSI, RDX, RCX, R8 and R9 respectively.
If the function takes more parameters than six,
the rest of the parameters are passed through stack,
in reversed order.
Very good!
The return value is passed in the RAX register.
On page 23 there is a summary –
that lists the role of every register in the CPU.
Worth noticing here is the column that says "preserved across function calls".
This means whether the register type is "caller-saves" or "callee-saves".
If the register is "caller-saves", such as RAX,
it means that whenever the function calls another function,
if it still needs the value of that register –
after the function call,
it must save it somewhere and reload it after the call.
However, it does not need to save this register –
at the start and restore it at the exit.
If the register is "callee-saves", such as RBX,
it means that if the function uses this register as a temporary variable,
it must save the register –
when entering the function and restore it before exiting,
but when it calls other functions,
it does not need to worry –
about the function clobbering that register,
because the ABI specifies –
that the other function must not clobber the register.
Okay that's everything there is to know about the registers,
but how about the stack?
Pages 17 and 18 explain key facts about the stack.
The order of function parameters, the order of local variables,
the alignment, and also the concept called red zone.
This was a lot to take in,
so let's do a summary of everything that has been learned so far.
All registers and parameters are 64 bits wide.
The first six function parameters –
shall be passed in six particular CPU registers.
The function shall return the value in RAX register.
Six out of the fifteen general-purpose registers are callee-saves;
all the other registers are caller-saves.
The stack must be aligned on sixteen-byte boundary.
And a leaf function may utilize a 128-byte red zone in the stack;
this is an area of the stack –
that is technically not yet allocated for the function,
but the ABI specifies it can be used anyway.
For the SNES there is no official ABI,
but we can make one up.
All registers and pointers are 16 bits wide.
Register parameters shall be passed in X, Y and A.
I chose this order for the simple reason that in my code,
most parameters are pointers,
and increments by one are much simpler on X and Y than on A.
The function return value will be in the A register.
There is a fundamental difference between the x86 and the 65816 –
in how the stack pointer works.
On the x86, the stack pointer always points to the address –
that the next POP would read.
On the 65816 however,
the stack pointer points to where the next push would write.
The red-zone is an area that is ahead of the stack pointer,
where next pushes would write.
On the SNES there is no red-zone.
One tangible reason is –
that the stack-relative addressing mode cannot take negative offsets.
Let's have a case study.
This is an example function –
that takes a plethora of function parameters.
At least three parameters had to be passed through stack.
In red letters you can see –
how to refer to each of these stack items individually.
Now suppose the function also has two local memory variables.
These local variables now occupy the topmost addresses in the stack,
and the incoming parameters are farther out in the stack.
On the x86 you could actually use the red-zone for those locals,
without having to explicitly allocate space for them,
but on the SNES this is not possible.
So let's ignore red-zone for now.
Now suppose this function calls another function –
that also takes a plethora of arguments.
Some of these arguments would have to be stored in the stack.
Notice how the stack allocation grows bigger and bigger –
to reflect these new items in the stack.
Hmm.
But if our function needed temporary variables in the stack,
it probably already used at least some of those callee-save registers.
In this case, RBX and RBP.
So those two have to be saved in the entrance,
and restored at the exit.
Again, the incoming function parameters are further out in the stack.
But wait, we are calling a child function, right?
Let's do a count.
We have added seven items into the stack –
since the function started.
This is an odd number.
The ABI specified –
that the stack must be aligned on 16-byte boundary,
which means an even number of eight-byte words.
We must add padding into the stack in order to not violate the ABI.
Now, we have done everything according to the ABI.
Notice how we allocated space from the stack.
It is a simple matter of mathematics:
Subtract the desired number from the stack pointer.
To release the space,
add that same number back to the stack pointer.
How about on the SNES?
The PHX instruction is used for allocating space.
To allocate two words, issue PHX twice.
More space, more PHX.
To release the space, do just as many PLX instructions.
You can see where this is going.
It goes in the direction of awkward.
In the SNES there is no way to do arithmetic on the stack pointer.
All arithmetics has to be performed on the A register.
If you wish to subtract 30 words from the S register,
it has to be first transferred into the X register,
and from there into the A register,
and then calculation is performed,
and then it's transferred back into X, and then back into S.
There is no simpler way.
Fine and dandy, but it clobbers A and X.
This is very bad,
if the function receives parameters in those registers.
We cannot read the parameters, if we overwrite the registers –
before we even get to the actual function body.
There is a way to save those registers though.
This alternative saves the X register, although it still clobbers A.
And even longer code saves both A and X.
This is the perfect function prologue on the SNES.
Awkward…!
In the function exit, very much similar code has to be done.
This code avoids clobbering the A register,
because it contains the function return value.
It still clobbers X though.
It's possible to preserve A and X in the exit boilerplate as well,
meaning this is the absolutely perfect function exit code,
but we don't need that.
Tune in for more — next time in Making a Compiler!
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