Re: offsets of stack variables with optimizations enabled in GCC

[Jim Wilson <wilson at tuliptree dot org>]

Tao Zhang wrote:
> Otherwise, the offsets are not correct or I can't figure out how to
> use the offsets extracted from the GCC RTL. I think although GCC
> can dump debug information with optimizations enabled, the
> GDB debugger can't debug an optimized binary. Maybe the
> debugger doesn't know how to use the offsets either?

It isn't clear what you are doing.

Note that the stabs format (from dbxout) is not sufficiently flexible to 
 fully describe an optimized executable.  So the debug info we emit is 
a compromise.

dwarf2/3 can do better, but we don't have full support for all of its 
features (e.g. location lists) yet, and so internally we don't have as 
much info as we'd like.

Internally to gcc, the stack offsets must obviously be correct, 
otherwise we would not be able to emit correct code.

When optimizing, a variable does not necessarily live in the stack slot 
allocated to it.  It might be sometimes in a register and sometimes in 
the stack slot.  When it is in the stack slot, it doesn't necessarily 
have the value that you expect it to have from reading the source, 
because various optimizations could have been applied to it.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com