[Dwarf-Discuss] Does gcc optimization impacts retriving Dwarf information?

M. Mohan Kumar mohan@in.ibm.com
Thu May 28 10:35:23 GMT 2009

Hi Eager,

Thank you very much for the response.

> The optimizations invoked by -O2 are removing variables like
> 'cpu' from the generated code.  There is no missing DWARF info
> which should describe where to find their values, since they no
> longer exist.
> To confirm this, look at the generated code and see if the local
> variables like 'cpu' are allocated either on the stack or in a
> register.  If this is the case, it's possible that your compiler
> is generating incorrect DWARF.  But the more likely situation is
> simply that the local variables have been either evaluated or
> incorporated into expressions and no longer exist.

Let me check the code and verify it.

> > 2. Does it require handling it differently in our implementation?
> Well, yes and no.  You can compile without optimizations, so the
> local variables are not removed.  That's generally not desirable.
> You can make changes to the source so that the local variables are
> not eliminated, such as making them static or volatile.  Again,
> not desirable.

Obviously we can't ask for compiling the kernel without optimization. Also
we can't make changes to the function just to retain the local variables
info for later debugging :)

> DIEs for variables which do not have DW_AT_location attributes
> do not have physical locations.  Your crash dump utility should
> not attempt to find the locations or print the values for these
> variables.

Makes sense.

So we assume that our implementation is correct and works fine.

M. Mohan Kumar.

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