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19 April 2011

Getting confused when exploring Qemu source? gcc comes to rescue!

Quick summary first: use gcc -save-temps!

Ever dig into Qemu (qemu.org) source code? OK, I assume you ever did that at least once... may I ask, what's your first impression?

Here's mine: it's complex C code...and to make it more like a nightmare, it heavily uses c (gcc, to be precise) tricks almost everywhere. ifdef, "##", define....almost endless. IMHO, since Fabrice Bellard, its author, is somekind of C compiler wizard, he somehow pull out all of those tricks so easy from his mind. I know it should make the code kinda more readable, but for me, is not.

Take one for example: INDEX_op symbols. AFAIK, it has something to do with code generation, to be precise it's an index toward instruction op which will later be translated to target. Previously, I thought it was defined somewhere in header files, but turns out (after long hours of grep and cscope sessions) they were created by preprocessor (token concatenation, to be precise -- explanation here).

So, what is the recipe? I think I found it (thanks to this URL http://stackoverflow.com/questions/3812670/what-are-the-internal-processes-involved-for-a-c-compilation/3814007#3814007) , although not really ideal. During configuration session, use extra cflags like this:
./configure --extra-cflags="-save-temps"
Put additional parameters as needed. Then do "make". Now, if you do this in main Qemu source tree:
find -iname "*.[is]"
you'll find several files. Each of them are result of  preprocessing (.i) and assembling (.s). Yeap, "-save-temps" comes to rescue, folks! So there you go... open them one by one and hopefully you get better picture on how to code works.

regards,

Mulyadi Santosa

1 comment:

Bhaskar said...

use navigation features of a good IDE, like Netbeans. It even has Call Graph feature.