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17
section_1/funcs/README.md
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@ -1,10 +1,11 @@
# Section 1 / Calling and Returning From Functions # Section 1 / Calling and Returning From Functions
Calling functions, passing parameters to them and receiving back return Calling functions, passing parameters to them and receiving back return
values is basic to using `C` and and `C++`. Calling methods (which are values is basic to using `C` and and `C++`.
functions connected to objects) is similar but with enough differences
to warrant its own discussion to be provided later in the chapter on Calling *methods* (which are functions connected to objects) is similar
[structs](../structs/using.md). but with enough differences to warrant its own discussion to be provided
later in the chapter on [structs](../structs/using.md).
Be sure to read [this](./README2.md) for information about Be sure to read [this](./README2.md) for information about
passing parameters to functions. passing parameters to functions.
@ -169,7 +170,7 @@ First, let's take a trip back in time to the early days of C.
[Stephen Bourne](https://en.wikipedia.org/wiki/Stephen_R._Bourne) was [Stephen Bourne](https://en.wikipedia.org/wiki/Stephen_R._Bourne) was
writing `sh`, the first shell for Unix. He noticed that every function writing `sh`, the first shell for Unix. He noticed that every function
had to return a value - even functions that had no reason to return had to return a value - even functions that had no reason to return
a value. a value.
In these early days, `void` functions did not yet exist. In these early days, `void` functions did not yet exist.
@ -232,6 +233,12 @@ ReturnsADouble: // 13
Note, the use of the floating point move instruction as well as the Note, the use of the floating point move instruction as well as the
single precision and double precision registers. single precision and double precision registers.
## Inline functions
Functions that are declared as *inline* don't actually make function
calls. Instead, the code from the function is type checked and inserted
directly where the "call" is made after adjusting for parameter names.
## Repeating the TL;DR ## Repeating the TL;DR
If your functions call *any* other functions, `x30` must be backed If your functions call *any* other functions, `x30` must be backed

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section_1/funcs/README2.md
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# Section 1 / Passing Parameters To Functions # Section 1 / Passing Parameters To Functions
Up to 8 parameters can be passed directly via registers. Each parameter can be up to the size of an address, long or double (8 bytes). If you need to pass more than 8 parameters or you need to pass parameters which are larger than 8 bytes or are `structs`, you would use a different technique described later. How parameters are passed to functions can be different from OS to OS.
This chapter is written to the standard implemented for Linux. It
differs from the **calling convention** used on, for example, the Mac in
that parameters are principally passed via the scratch registers.
For the purposes of the present discussion, we assume all parameters are `long int` and are therefore stored in `x` registers. Up to 8 parameters can be passed directly via registers. Each parameter
can be up to the size of an address, long or double (8 bytes). If you
need to pass more than 8 parameters or you need to pass parameters which
are larger than 8 bytes or are `structs`, you would use a different
technique described later.
Up to 8 parameters are passed in the scratch registers (of which there are 8). These are `x0` through `x7`. *Scratch* means the value of the register can be changed at will without any need to backup or restore their values. Remember that even large data structures that are passed by reference
are, in fact, passed via their base address (as a pointer).
**This means that you cannot count on the contents of the scratch registers maintaining their value if your function makes any function calls.** For the purposes of the present discussion, we assume all parameters are
`long int` and are therefore stored in `x` registers.
Up to 8 parameters are passed in the scratch registers (of which there
are a matching 8). These are `x0` through `x7`. *Scratch* means the
value of the register can be changed at will without any need to backup
or restore their values.
**This means that you cannot count on the contents of the scratch
registers maintaining their value if your function makes any function
calls.**
For example: For example:
@ -24,83 +42,34 @@ func: add x0, x0, x1 // 1
ret // 2 ret // 2
``` ```
The first parameter (`p1`) goes in the first scratch register (`x0`). It's an `x` because the parameter's type is `long int`. It is the `0` register, because that is the first scratch register. The value of the first parameter (`p1`) is copied into the first scratch
register (`x0`). It's an `x` because the parameter's type is `long int`.
It is the `0` register, because that is the first scratch register.
The second parameter (`p2`) goes in the second scratch register (the `1` register) because it is the second argument, and so on. The value of the second parameter (`p2`) is copied into the second
scratch register (the `1` register) because it is the second argument,
and so on.
`Line 1` of the assembly language provides the label `func` to which a `bl` can be made. `Line 1` of the assembly language provides the label `func` to which a
`bl` can be made.
`Line 1` also provides the full body of the function - the third argument to `add` is added to the second and the result is put in the first. Thus it is: `x0 = x0 + x1`. `Line 1` also provides the full body of the function - the third
argument to `add` is added to the second and the result is put in the
first. Thus it is: `x0 = x0 + x1`.
Just as scratch registers are used for passing (up to 8) parameters, the `0` register is used for function returns. Just as scratch registers are used for passing (up to 8) parameters, the
In the case of the current code, the result of the addition is already sitting in `x0` so all we do is `ret` on `Line 2`. `0` register is used for function returns. In the case of the current
code, the result of the addition is already sitting in `x0` so all we do
is `ret` on `Line 2`.
## Passing Pointers **This is an advanced topic:** If you are the author of both the caller
and the callee and both are in assembly language, you can play loosey
goosey with how you return values. Specifically, you can return more
than one value. **But** if you do so, you give up the possibility of
calling these functions from C or C++. Maybe you should forget you read
this paragraph?
A pointer is an address of something. The word *pointer* is scary. The words *address of* are not as scary. They mean **exactly** the same thing. ## `const`
Here is a function which *also* adds two parameters together but this time using pointers to `long int` rather than the values themselves.
```c
void func(long * p1, long * p2) // 1
{ // 2
*p1 = *p1 + *p2; // 3
} // 4
```
`Line 1` passes the *address of* `p1` and `p2` as parameters. That is, the addresses of `p1` and `p2` are passed in registers `x0` and `x1` rather than their contents. The contents of the underlying
longs still reside in memory. That is:
* The address of `p1` arrives in `x0`.
* The value of `p1` is found in memory at the address specified by the parameter.
`Line 3` *dereferences* the addresses to fetch their underlying values.
The values are added together and the result overwrites the value pointed to by `p1`.
Here it is in assembly language:
```asm
func: ldr x2, [x0] // 1
ldr x3, [x1] // 2
add x2, x2, x3 // 3
str x2, [x0] // 4
ret // 5
```
The `add` instruction cannot operate on values in memory. With little exception, all the *action* takes place in registers, not memory. Therefore, the underlying values pointed to by the parameters must be fetched from memory.
`Line 1` provides the label to which a use of `bl` can branch with link register.
Remember that up to the first 8 parameters are passed in the 8 scratch registers. Thus, the address of `p1` and the address of `p2` are stored in `x0` and `x1` respectively. `0` and `1` because these are the first two parameters. The
`x` form of the `0` and `1` registers are used because the parameters' type are addresses.
* Addresses (pointers) to any type are 64 bits wide and therefore must use `x` registers.
* `long` and `unsigned long` integers are 64 bits wide and ...
* `double` floats are 64 bits wide
`Line 1` also dereferences the address held in `x0` going out to memory and loading (`ldr`) the value found there into `x2`, another scratch register. It's scratch so it doesn't need backing up and restoring.
`Line 2` does the same for `p2`, putting its value in `x3`.
Why didn't we reuse `x0` and `x1` as in:
```asm
ldr x0, [x0]
ldr x1, [x1]
```
Doing so would be legal but would end in tears. Doing so would blow away the address of `p1` (and `p2` too
but this doesn't matter). Destroying the address of `p1` would prevent us from copying the result of the
addition back into memory since the address to which we would want to store the result of the addition
would be gone. Can't have that!
So, as the smart *human*, we decided to use `x2` and `x3` because, well, they're scratch.
`Line 3` performs the addition.
`Line 4` stored the value in `x2` at the address in memory still sitting in `x0`.
### `const`
Suppose we had: Suppose we had:
@ -115,7 +84,105 @@ how would the assembly language change?
Answer: no change at all! Answer: no change at all!
`const` is an instruction to the compiler ordering it to prohibit changing the values of `p1` and `p2`. We're smart humans and realize that our assembly language makes no attempt to change `p1` and `p2` so no changes are warranted. `const` is an instruction to the compiler ordering it to prohibit
changing the values of `p1` and `p2`. We're smart humans and realize
that our assembly language makes no attempt to change `p1` and `p2` so
no changes are warranted.
## Passing Pointers
A pointer is an address of something. The word *pointer* is scary. The
words *address of* are not as scary. They mean **exactly** the same
thing.
Here is a function which *also* adds two parameters together but this
time using pointers to `long int` rather than the values themselves.
```c
void func(long * p1, long * p2) // 1
{ // 2
*p1 = *p1 + *p2; // 3
} // 4
```
`Line 1` passes the *address of* `p1` and `p2` as parameters. That is,
the addresses of `p1` and `p2` are passed in registers `x0` and `x1`
rather than their contents. The contents of the underlying longs still
reside in memory. That is:
* The address of `p1` arrives in `x0`. The value of `p1` still
resides in memory.
* The value of `p1` is found in memory at the address specified by the
parameter.
`Line 3` *dereferences* the addresses to fetch their underlying values.
The values are added together and the result overwrites the value
pointed to by `p1`.
Here it is in assembly language:
```asm
func: ldr x2, [x0] // 1
ldr x3, [x1] // 2
add x2, x2, x3 // 3
str x2, [x0] // 4
ret // 5
```
The `add` instruction cannot operate on values in memory.
With little exception, all the *action* takes place in registers, not
memory. Therefore, the underlying values pointed to by the parameters
must be fetched from memory.
`Line 1` provides the label to which a use of `bl` can branch with link
register.
Remember that up to the first 8 parameters are passed in the 8 scratch
registers. Thus, the address of `p1` and the address of `p2` are stored
in `x0` and `x1` respectively. `0` and `1` because these are the first
two parameters. The `x` form of the `0` and `1` registers are used
because the parameters' type are addresses.
* Addresses (pointers) to any type are 64 bits wide and therefore must
use `x` registers.
* `long` and `unsigned long` integers are 64 bits wide and ...
* `double` floats are 64 bits wide
`Line 1` also dereferences the address held in `x0` going out to memory
and loading (`ldr`) the value found there into `x2`, another scratch
register. It's scratch so it doesn't need backing up and restoring.
`Line 2` does the same for `p2`, putting its value in `x3`.
To say this again but differently, the syntax `[` then an `x` register
followed by `]` means use the `x` register as an address in RAM. Go
to that address and fetch its value. This is a *dereference*.
Why didn't we reuse `x0` and `x1` as in:
```asm
ldr x0, [x0]
ldr x1, [x1]
```
Doing so would be legal but would end in tears.
Doing so would blow away the address of `p1` (and `p2` too). Destroying
the address of `p1` would prevent us from copying the result of the
addition back into memory since the address to which we would want to
store the result of the addition would be gone. Can't have that!
So, as the smart *human*, we decided to use `x2` and `x3` because, well,
they're scratch.
`Line 3` performs the addition.
`Line 4` stored the value in `x2` at the address in memory still sitting
in `x0`.
### Passing by Reference ### Passing by Reference
@ -130,22 +197,56 @@ long func(long & p1, long & p2) // 1
how would the assembly language change? how would the assembly language change?
Answer: no change at all! Answer: just a little:
Passing by reference is also an instruction to the compiler to treat pointers a little differently - the differences don't show up here so there is no change needed to the assembly language we wrote to handle passing pointers. ```asm
func: ldr x2, [x0] // 1
ldr x3, [x1] // 2
add x2, x2, x3 // 3
mov x0, x2 // 4
ret // 5
```
Passing by reference is also an instruction to the compiler to treat
pointers a little differently - the differences don't show up here so
there the only change to our pointer passing version is how we return
the answer.
But wait...
There is a small optimization we can make here:
```asm
func: ldr x0, [x0] // 1
ldr x1, [x1] // 2
add x0, x0, x1 // 3
ret // 4
```
This time we're not storing anything back to `p1` or `p2` so we can
reuse `x0` and `x1` since the addresses they contained aren't needed
again. Smart human!
## What If We Need More Than Eight Parameters? ## What If We Need More Than Eight Parameters?
First, do you **really** need to pass more than 8 parameters? **REALLY?** First, do you **really** need to pass more than 8 parameters?
**REALLY?**
If for some reason you do, you can pass the first 8 in registers are described above. Beginning with the ninth parameters, these would be passed on the stack. If for some reason you do, you can pass the first 8 in registers are
described above. Beginning with the ninth parameter, these would be
passed on the stack.
**REMEMBER THAT ANY ADJUSTMENT TO THE STACK MUST BE DONE IN MULTIPLES OF 16!** **REMEMBER THAT ANY ADJUSTMENT TO THE STACK MUST BE DONE IN MULTIPLES OF
16!**
* If you need just 1 byte of stack, the stack pointer must be changed by 16. * If you need just 1 byte of stack, the stack pointer must be changed by
* If you need 17 bytes of stack, the stack pointer must be changed by 32 and so on. 16.
Here is a sample function that requires 9 parameters (for who knows what reason): * If you need 17 bytes of stack, the stack pointer must be changed by 32
and so on.
Here is a sample function that requires 9 parameters (for who knows what
reason):
```c++ ```c++
#include <stdio.h> #include <stdio.h>
@ -204,7 +305,10 @@ fmt: .asciz "This example hurts: %ld %ld\n" // 30
``` ```
Notice how `main()` puts the first 8 parameters into the scratch registers `x0` through `x7` using `Lines 17` to `24`. But first, it put the ninth parameter on the stack. It did the stack parameter first so that the stack pointer could be manipulated in a scratch register. Notice how `main()` puts the first 8 parameters into the scratch
registers `x0` through `x7` using `Lines 17` to `24`. But first, it put
the ninth parameter onto the stack. It did the stack parameter first so
that the stack pointer could be manipulated in a scratch register.
After executing `Line 14`, the stack will have: After executing `Line 14`, the stack will have:
@ -233,4 +337,34 @@ sp + 32 return address for main
sp + 40 zero sp + 40 zero
``` ```
This means that `Line 8` fetches `p9` from memory and puts its value into x2 (where it becomes the third argument to `printf()`). This means that `Line 8` fetches `p9` from memory and puts its value
into x2 (where it becomes the third argument to `printf()`).
## A bit of history
The early Unix kernels would abuse the calling convention to
miraculously pass return values back to calling functions. Early
versions of C made extensive use of a now obsolete keyword `register`.
It was an instruction to the compiler to store a certain variable in
a register and not in memory in the code the compiler produced.
Particularly abusive functions would call other functions without
passing any actual variables but the parameters would indeed be passed!
The coders assumed the compiler would store specific variables in
specific registers, avoiding the overhead of using the actual calling
convention they themselves defined. Code that did this had to be
rewritten once Unix began to be ported to machines beyond the original
DEC hardware.
This had the author scratching his head until he figured it out, way
way back in the day.
Those were the days when the entire Unix kernel would be printed out to
form a stack of paper less than an inch high. The author knows this
because [Jishnu
Mukerji](<https://www.topionetworks.com/people/jishnu-mukerji-5288f3c41dedae1c01000582>)
presented such a stack to the author the third time the author asked
Jishnu a question about the kernel. He gave the author answers to two
questions. On the third question, he handed the author the print out and
said: *"All your answers are in here."* The author deeply appreciates
Jishnu Mukerji's formative impact on a young undergraduate.