on removal of line number notes at the end of BBs

[Alexandre Oliva <aoliva at redhat dot com>]

Hi, Jan,

The patch at http://gcc.gnu.org/ml/gcc-patches/2001-07/msg00399.html
added code to remove line number notes from the end of basic blocks
after say turning a jump at the end of a basic block into a
fallthrough edge.

This unfortunately causes line number info from becoming incorrect for
test programs such as:

int x;
int f() {
  x = 0;
  while(1);
}

We get line number notes for code only up to "x = 0;".

The line number note for the empty infinite loop is dropped when the
back-edge is turned into a fallthrough edge (?!?).  Even if I arrange
for the edge from a block to itself to never be regarded as a
fallthrough edge (it doesn't seem to make sense to me), loop2 still
splits the back-edge such that the loop has separate header and latch
blocks, and at the time of the split, we use the same
try_to_redirect_by_replacing_jump() function and thus lose debug info.

Although I'll be the first to admit that this particular case is not
that useful, it has surprised people on the Frysk project (see
Sourceware bug 3259, for example).

I'm not sufficiently familiar with the code nor do I see a rationale
as to why keeping the notes might change the generated code, even
though I can imagine it might have to do with a loop becoming empty or
some such.

I've considered arranging for such notes to be moved to the
destination of the edge, but in this case the note would still remain
alone in the basic block, and therefore at its end, causing whatever
failure mode it is that got you to add that chunk of code.

I've also considered adding some nop after the source line number
instead of removing, but then I realized that it wouldn't address the
potential problem of generating different code for -g and -g0.


I don't see any such note-removal code in tidy_fallthru_edge(), from
which you claim to have got inspiration, so I wonder if you still
remember what kind of failure modes you envisioned, and whether it
would make sense to come up with some other means to avoid dropping
such line number notes entirely, such as storing
line-number-at-the-end information in basic blocks, or something
similar for loops, and get such notes re-inserted at some convenient
time, such as when re-inserting jumps for fallthru edges that couldn't
be satisfied without a jump, or some such.

Any other ideas?

Thanks in advance,

-- 
Alexandre Oliva         http://www.lsd.ic.unicamp.br/~oliva/
Secretary for FSF Latin America        http://www.fsfla.org/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}