Re: [PATCH 2/2] gdb: Add support for tracking the DWARF line table is-stmt field

[Luis Machado <luis.machado@linaro.org>]

Hi,

I still need to check the patch itself, but i had a question about one 
particular paragraph...

On 2/5/20 8:37 AM, Andrew Burgess wrote:
> This commit brings support for the DWARF line table is_stmt field to
> GDB.  The is_stmt field is used by the compiler when a single source
> line is split into multiple assembler instructions, especially if the
> assembler instructions are interleaved with instruction from other
> source lines.
> 
> The compiler will set the is_stmt flag false from some instructions
> from the source lines, these instructions are not a good place to
> insert a breakpoint in order to stop at the source line.
> Instructions which are marked with the is_stmt flag true are a good
> place to insert a breakpoint for that source line.
> 
> Currently GDB ignores all instructions for which is_stmt is false.
> This is fine in a lot of cases, however, there are some cases where
> this means the debug experience is not as good as it could be.
> 
> Consider stopping at a random instruction, currently this instruction
> will be attributed to the last line table entry before this point for
> which is_stmt was true - as these are the only line table entries that
> GDB tracks.  This can easily be incorrect in code with even a low
> level of optimisation.
> 
> With is_stmt tracking in place, when stopping at a random instruction
> we now attribute the instruction back to the real source line, even
> when is_stmt is false for that instruction in the line table.
> 
> When inserting breakpoints we still select line table entries for
> which is_stmt is true, so the breakpoint placing behaviour should not
> change.
> 
> When stepping though code (at the line level, not the instruction
> level) we will still stop at instruction where is_stmt is true, I
> think this is more likely to be the desired behaviour.

Considering a block of continuous instructions that map to the same 
source line, will line-stepping stop at each one of these instructions 
if is_stmt is true? As opposed to stepping over the the whole block 
until we see a line change?

I'm wondering if this would help this bug 
(https://sourceware.org/bugzilla/show_bug.cgi?id=21221) in any way.