how to examine data with compiler optimization option set?

Robert Dewar
Tue Sep 2 21:36:00 GMT 2008

J R wrote:
> Extract of GDB user manual:
> "Another possible effect of compiler optimizations is to optimize unused
> variables out of existence, or assign variables to registers (as opposed to
> memory addresses). Depending on the support for such cases offered by the debug
> info format used by the compiler, GDB might not be able to display values for
> such local variables.
> To solve such problems, either recompile without optimizations, or use a
> different debug info format, if the compiler supports several such formats. For
> example, GCC, the GNU C/C++ compiler usually supports the `-gstabs' option.
> `-gstabs' produces debug info in a format that is superior to formats such as
> COFF. You may be able to use DWARF2 (`-gdwarf-2'), which is also an effective
> form for debug info. See section `Options for Debugging Your Program or GNU CC'
> in Using GNU CC, for more information."
> I wanted to keep the optimization option, so I tried to recompile with GCC,
> using the -gstabs and -gdwarf-2 options, and even -g3, with this very simple
> program:
> int main(void){
> 	int a = 1;
> 	int b = 2;
> 	int c = a+b;
> 	printf("Value c = %d \n", c);
> }
> But still couldn't display the variables a, b and c!

That's because if you ask for the compiler to optimize, it
will change this program to

      int main(void){
         printf ("Value c = %d \n", 3");

if it was a bit cleverer, it might even change it to

      int main(void){
         printf ("Value c = 3 \n");

but in either case a,b, and c are gone!

It might be theoretically possible to retain the values of
a,b,c in the debugging information, but this is a huge amount
of work, and not something likely to be done in the near future.

> Is there a particular compiling option configuration to set?

Yes,  -O0, if you want junk code kept around for debugging
purposes, you have to ask for it!
> Is there a particular compiling option configuration to set?
> Many thanks in advance.
> Regards.

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