zondag 31 januari 2016

SPO600 Lab 4

In our fourth lab we looked at some of the different compiler options and the differences it made in the assembly output. In order to look at the differences we wrote a simple hello world program that was compiled with the different options.

I will go by the options we went over one by one below.

0. Original.
The original command we used was gcc -g -O0 -fno-builtin.
This version is used to compile a file with debugging information, no optimization and without use of builtin functionality. 
1. Static.
Adding -static means the compiler pulls in the entire library the function you are using is in. In this case it pulled in the library that contains the printf function we used to print our message. This results in about a 10 times increase in file size in this case. The advantage of using static is that no matter what system you go to, you can guarantee that the functions you are using will be present, since you include the entire library. The disadvantage is the strain on the file size and if you use static a lot, a lot of your programs will have the same libraries, but with different versions. It will be hard to keep track of security flaws and versioning.

2. Not using fno-builtin.
 fno-builtin means you are asking the compiler not to use the builtin functionality. In this case the compiler recognizes it has a more efficient way to do the printf function because it is only using 1 argument. The compiler realizes that it can use the builtin function puts instead for more efficiency.

3. Removing -g .
 By removing -g from the command we are compiling the file in without debug information, this means the file becomes smaller, but also harder to read. the --source command no longer shows you where the compiler put what part of your code.

4. Adding additional arguments to the printf.
On aarch64 after assigning 7 arguments to different registers, the rest of the arguments get put on the stack through r1. On x86_64 there were only 5 arguments assigned to the registers before putting the rest on the stack. This makes sense because x86_64 has less registers to work with. Interestingly enough, on the aarch64 side the assembly made use of store instead of put to add the arguments to the stack.

5. Moving printf to a separate function called output and calling that from the main.
With this one I thought the compiler would recognise that it is just calling printf and the function did not really have any use, but I was wrong. The assembly called output from the main and in the output was the same section that was previously in the main. If you use arguments to pass the string it also stores some overhead on the stack.

6. Replace O0 with O3.
This instruction asks the compiler to use a lot of optimization(severe), it will actually move some of your code around to do so, so if you compile something with O3 you always have to check whether the functionality of your program remains the same. One of the differences in the output is that instead of returning to the main function after calling the printf function the compiler realized that the printf was the last statement and with an unconditional branch told the program not to bother coming back to the main. Severe optimization also deleted the set up for the stack in this case, because this program makes no use of it.

So this week we saw a small part of what the compiler can do for us and what we can ask it to do. I found it very interesting to finally see what makes it so that any code we write can actually run. I think understanding how the compiler turns our code into assembler can really help us decide what options to use when we want to go for maximum efficiency in our code.

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