On Mon, 2009-01-19 at 05:16 -0500, Yang Zhang wrote:
> Are there any good resources for learning about gcc's x86_64 assembly
> output? When I run `gcc -S a.c` for a simple test C program like the
> one below, I can completely understand the resulting go.s on a 32-bit
> x86, but as soon as I run that on a 64-bit machine, I'm lost. I know
> what the instructions are (i.e. I know how to use the AMD manuals), but
> I'd like to learn about gcc's assembly output: the exact new calling
> conventions (registers seem to be more liberally used over the stack now
> even without opt), all these
> .LFB/.LCFI/.LFE/.Lframe/.LSCIE/.LSFDE/.LASFDE/... labels, and so on.
> Thanks in advance for any guidance.
>
> void foo(int a, int b, int c) { char xs[5]; char ys[10]; }
> int main() { foo(1,2,3); return 0; }
>
Many of the local labels you see are coming from the DWARF2 unwind
table. I don't know a lot about it, but I believe it's used for
exception processing, for example, using a debugger.
You can eliminate this table with the following options:
$ gcc -O0 -g -Wa,-adhls -fno-asynchronous-unwind-tables \
> myProg.c > myProg.lst
These options will also provide some comments in the assembly listing
that refer back to the C source code. This should help you "cut to the
chase" in trying to figure this out.
Bob Plantz
