Re: x86_64

[John Fine <johnsfine at verizon dot net>]

I forget where it is documented.  I have the following two register 
lists taped on the bottom bezel of one of my CRT's, because I debug a 
lot of C++ code in assembler view and this key part of the calling 
convention is important whenever you debug C++ in assembler view:

1) Parameters passed in registers, left to right:  rdi, rsi, rdx, rcx, 
r8, r9
2) registers preserved by call:  rbp, rbx, r12-r15

so in your example, assuming foo was NOT optimized to be inline, "a" 
would be passed in rdi, "b" would be passed in rsi and "c" would be 
passed in rdx.

Obviously it gets more complicated when you mix in data types other than 
pointers and integers.  Also, I hope obvious, in member functions "this" 
is passed in as if it were the leftmost parameter.

If you understand the 32-bit and you can read the AMD manuals, I don't 
see what else might be confusing you (actually even the above register 
lists were in one of the AMD manuals).

What is confusing about the use of local labels by the compiler?


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; }