Question on Dwarf2 unwind info and optimized code

[Christophe LYON]

Hi all,

I have been look at the Dwarf2 frame info generated by GCC, and how it 
works.
 From what I can see, only the register saves are recorded, and not the 
restores. Why?

I guess it may loose GDB if one sets a breakpoint inside a function 
epilogue, right?



I am currently working on the debug_frame info emission in our C/C++ 
compiler (based on Open64) and I have recently come across optimized 
code which I don't know how to handle.

Consider the following function:

foo() {
     PROLOGUE

     if (COND1) {
         SAVE_REG r1 to stack (r1 is callee save)
         compute...
         bar1()
         RESTORE r1
     }
     else if (COND2) {
         bar2()
     }

     EPILOGUE
}

Suppose that the generated code looks like:
foo:
     PROLOGUE
     COND1
     if true goto LABEL1

     COND2
     if true goto LABEL2

epilogue_label:
     EPILOGUE

LABEL1:
     SAVE register r1 to stack
     compute....
     goto CALL_BAR1

LABEL2:
     CALL bar2
     goto epilogue_label

CALL_BAR1:
     CALL bar1
     RESTORE r1
     goto epilogue_label

My question is:
what debug_frame info should I generate around epilogue, LABEL1, LABEL2, 
and CALL_BAR1 ?
Should I repeat all the info of the prologue before LABEL1 and LABEL2?
for CALL_BAR1, should I also add the save of register r1 ?

It's also unclear to me if I can/should use the 
remember_state/restore_state dwarf operators.
Indeed, what is the rule?

Use remember_state after the prologue, and use restore state at LABEL1 
and LABEL2?
But as states are defined as a stack, it means I would restore twice the 
same thing...

Any help/suggestion highly appreciated!

Thanks,

Christophe.