Guide: Online C Compiling

An interesting tool to observe how C code translates into assembly language is Compiler Explorer.

  1. Go to Compiler Explorer.
  2. Load the “sum over array” program from the examples (accessible using the load button, shaped like a floppy disk).
  3. Make sure x86-64 gcc 4.8.2 is selected under Compiler:.
  4. Use the option -m32 (in Compiler options) to display code in 32-bit assembly language (as opposed to 64-bit by default).
  5. If you see the message <Compilation failed>, add the option -std=c99.
  6. Initially, the code might be quite cumbersome. To make it more human-readable, add the option -O2 to the compilation options (Compiler options).
  7. You may notice the presence of symbols like .L3: and .L4:. These represent fixed points in the program, labels, quite similar to what is found in C.
  8. Go through the compilers corresponding to the following architectures one by one: ARM, ARM64, AVR, PowerPC. Note: for ARM, ARM64, and AVR, you will need to remove the previously set -m32 flag. You can observe how the generated code differs from one architecture to another.
  9. Also, try the following compilers: clang and icc. As you can see, even though it’s the same C code and the same architecture, the generated code differs. This happens because each compiler can have a different optimization and code generation strategy.

NOTE: clang is an open-source C/C++ compiler. It is often used in IDEs due to its very suggestive compilation error messages.

NOTE: icc is the C/C++ compiler from Intel.

Write the following code sequence in the Code editor area:

int simple_fn(void)
{
    int a = 1;
    a++;
    return a;
}

Observe the assembly code when the compilation options (Compiler options) are -m32, and when the compilation options are -m32 -O2. Notice the effect of optimization options on the generated assembly code.