ARM处理器的堆栈和函数调用，以及与Sparc的比较

To understand functions in ARM we first need to get familiar with the structural parts of a function, which are:

The purpose of the prologue is to save the previous state of the program (by storing values of LR and R11 onto the Stack) and set up the Stack for the local variables of the function. While the implementation of the prologue may differ depending on a compiler that was used, generally this is done by using PUSH/ADD/SUB instructions. An example of a prologue would look like this:

函数起始：

（1）保存之前的状态（将LR和R11保存到堆栈，下面第1句）

（2）设置堆栈的fp，一般是将fp=sp+4（因为之前push已经移动了2个单位）

（3）设置堆栈的sp，sp现在已经移动了2个单位，再移动剩余所需的空间即可。

The body part of the function is usually responsible for some kind of unique and specific task. This part of the function may contain various instructions, branches (jumps) to other functions, etc. An example of a body section of a function can be as simple as the following few instructions:

The sample code above shows a snippet of a function which sets up local variables and then branches to another function. This piece of code also shows us that the parameters of a function (in this case function max) are passed via registers. In some cases, when there are more than 4 parameters to be passed, we would additionally use the Stack to store the remaining parameters. It is also worth mentioning, that a result of a function is returned via the register R0. So what ever the result of a function (max) turns out to be, we should be able to pick it up from the register R0 right after the return from the function. One more thing to point out is that in certain situations the result might be 64 bits in length (exceeds the size of a 32bit register). In that case we can use R0 combined with R1 to return a 64 bit result.

不超过4个的输入参数可以通过寄存器传递，若超过4个参数，则超过的需要通过堆栈传递。函数返回值也是通过R0传递。

The last part of the function, the epilogue, is used to restore the program’s state to it’s initial one (before the function call) so that it can continue from where it left of. For that we need to readjust the Stack Pointer. This is done by using the Frame Pointer register (R11) as a reference and performing add or sub operation. Once we readjust the Stack Pointer, we restore the previously (in prologue) saved register values by poping them from the Stack into respective registers. Depending on the function type, the POP instruction might be the final instruction of the epilogue. However, it might be that after restoring the register values we use BX instruction for leaving the function. An example of an epilogue looks like this:

函数结束，恢复初始状态：

（1）设置堆栈的sp，一般通过r11=fp来设置，通常应该是sp=r11+4。

（2）恢复之前保存的r11=fp和lr到r11和PC。

So now we know, that:

Another key point to know about the functions is their types: leaf and non-leaf. The leaf function is a kind of a function which does not call/branch to another function from itself. A non-leaf function is a kind of a function which in addition to it’s own logic’s does call/branch to another function. The implementation of these two kind of functions are similar. However, they have some differences. To analyze the differences of these functions we will use the following piece of code:

另一个关于函数的要点是，函数分叶子函数和非叶子函数。叶子函数里不再继续调用其它函数，非叶子函数里会继续调用其它函数