Bob Plantz plantz@cds1.net
Mon Jan 19 16:57:00 GMT 2009

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

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