Improving GCC's line table information to help GDB

[Luis Machado]

On 10/16/19 5:59 AM, Richard Biener wrote:
> I think that adding an extra jump is unwanted.  Instead - if you disregard
> the single-source-line case - there's always the jump and the label we jump
> to which might/should get different source locations.  Like in one of the above
> cases:
> 
> main ()
> {
>    int D.1803;
> 
>    [t.c:2:1] {
>      int var;
> 
>      [t.c:3:5] var = 0;
>      <D.1801>:
>      [t.c:7:8] var = var + 1;
>      [t.c:7:8] goto <D.1801>;
>      [t.c:10:8] D.1803 = 0;
>      [t.c:10:8] return D.1803;
> 
> seen at GIMPLE.  Of course we lose the label once we build the CFG,
> but we retain a goto-locus which we could then put back on the
> jump statement.  For this case we at the moment get
> 
> .L2:
>          .loc 1 7 0 discriminator 1
>          addl    $1, -4(%rbp)
>          jmp     .L2
> 
> and we could do
> 
> .L2:
>          .loc 1 7 0 discriminator 1
>          addl    $1, -4(%rbp)
>          .loc 1 5 0
>          jmp     .L2
> 
> thus assign the "destination" location to the jump instruction?

On a first look, i considered reusing the jump instruction that did not 
get a location assigned to it, but that didn't work right for all cases, 
such as the one you showed below with the incorrect line ordering.

> 
> The first question is of course what happens with the edges
> goto_locus at the moment and why we get the code we get.
> 
> The above solution might also be a bit odd since for the loop
> entry we'd first see line 7 and only after that line 5.  But fixing
> that would mean we have to output an extra instruction
> (where I'd chose a nop instead of some random extra jump).

Right. I wanted to preserve the correct order of execution, at least 
from a O0 perspective. A nop would work just as well. I'll give this a try.

I don't think it makes sense to output additional instructions in O1+ 
cases just because we want to have more debug info, but we do need a new 
instruction address in some cases, in order to use it in the line table 
for the line transition.

Would it make sense to have it restricted to O0?