Getting GCC to Help

One of the nice things about GCC is its ability to spit out assembly language code. To convert a C language file to assembly, you can simply do:

 gcc -S file.c 

The output will be in file.s. It's not the most readable output - most of the variable names have been removed and replaced either with numeric stack locations or references to automatically-generated labels. To start with, you probably want to turn off optimizations with -O0 so that the assembly language output will follow your source code better.

Something else you might notice is that GCC reserves more stack space for local variables than we do, and then AND's %esp ^[1] This is to increase memory and cache efficiency by double-word aligning variables.

Finally, at the end of functions, we usually do the following instructions to clean up the stack before issuing a ret instruction:

  movl %ebp, %esp  popl %ebp 

However, GCC output will usually just include the instruction leave. This instruction is simply the combination of the above two instructions. We do not use leave in this text because we want to be clear about exactly what is happening at the processor level.

I encourage you to take a C program you have written and compile it to assembly language and trace the logic. Then, add in optimizations and try again. See how the compiler chose to rearrange your program to be more optimized, and try to figure out why it chose the arrangement and instructions it did.

^[1]Note that different versions of GCC do this differently.