[Dwarf-discuss] Purpose of basic_block flag in line number table?

[Michael Eager]

Cary Coutant wrote:
> I've got a question about the basic_block flag in the DWARF line  
> number table.
> 
> It seems to me that the point of this flag is to enable efficient  
> single stepping -- i.e., instead of stepping one instruction at a  
> time until you get to a different line number, the debugger could  
> look down the line table until it got to either a new source line or  
> the end of a basic block. If it found a new line before the end of  
> the basic block, it could set a breakpoint and free-run, fairly  
> confident that it'll hit that breakpoint. Otherwise, it could set a  
> breakpoint at the end of the basic block, then single step to  
> establish where it's going next (which will probably be at a new  
> source line).
> 
> Is this in fact what the flag is for, or is there some other purpose  
> that I'm overlooking?

I believe that the basic block flag was added for profiling
source code, not for optimizing stepping.  I don't know if any
tools are actually using the flag -- I'd be interested in hearing.

I not sure how a debugger might use the basic block flag to
optimize single stepping, assuming that this means executing a
single line.  If you are stepping over calls, then finding
the next line should be independent of the basic block flag.

Also, DWARF indicates the start of a basic block, not the end.
I don't know that you can assume that the line before a basic
block is the end of a preceding basic block.

> What about step-into as opposed to step-over? The DWARF spec extends  
> the concept of basic block to include calls, so there'd be no way for  
> the debugger to step into the call and stop without single stepping  
> or disassembling instructions.

-- 
Michael Eager	 eager at eagercon.com
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