C main 32/64 bit prologues differ

C main 32/64 bit prologues differ

[Bob Plantz]

The prologues for the main function in C differ (at the assembly
language level) depending on whether it is compiled for 32-bit or 64-bit
in the x86 architecture.

Also, adding a dwarf2 unwind table is the default in 64-bit mode, but
not in 32-bit.

Can anyone point me to discussions about why it's done this way?

Re: C main 32/64 bit prologues differ

[Andrew Haley]

Bob Plantz wrote:
> The prologues for the main function in C differ (at the assembly
> language level) depending on whether it is compiled for 32-bit or 64-bit
> in the x86 architecture.
> 
> Also, adding a dwarf2 unwind table is the default in 64-bit mode, but
> not in 32-bit.
> 
> Can anyone point me to discussions about why it's done this way?

The DWARF question I can answer: it's required by the processor-
specific ABI.

Andrew.

Re: C main 32/64 bit prologues differ

[Bob Plantz]

On Sat, 2008-02-23 at 15:40 +0000, Andrew Haley wrote:
> The DWARF question I can answer: it's required by the processor-
> specific ABI.
> 
> Andrew.
> 
Thank you.

I should explain my application. I'm working on an introductory machine
organization textbook. It's based on x86-64 running a GNU/Linux system.
My approach is to start with C, look at gcc's assembly language, then
show how a programmer might write the assembly language. Since it's
introductory, I'm less concerned about industrial quality error
handling.

I use gdb to single-step through code and look at registers and memory.

I don't need to explain all the details in my book, but I would like to
be able to say something beyond "the stork brought it."

My take at this point is that the DWARF is used for 64 bit because
parameters are passed in registers, making it more difficult to unwind
the stack. Since 32 bit passes parameters only in the stack frame, it is
easier to unwind the stack. (Pointing out my misconceptions is much
appreciated.)

Bob