[Bug debug/90946] gcc generates wrong debug information at -O2

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90946

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |aoliva at gcc dot gnu.org
            Summary|gcc generates wrong debug   |gcc generates wrong debug
                   |information at -O3          |information at -O2

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
There isn't really much we can do - l_756 is memory for quite a bit of the
compilation and there the dead-store removal "issue" with debug info
pops up and we elide the last *l = 105487; store without leaving debug
footprint.  Then at some point we are able to rewrite l_756 to a register
and only after that we elide even the first "store" leaving the only
debug footprint but with the outdated value.

So there's something that pops up in my mind we could do - mark everything
that was memory at some point and avoid generating debug stmts for it.
That's of course quite a big hammer....

There's missed optimization that could have made the optimized value
<optimized out>, namely a pass ordering issue between DSE and DCE during
early opts where we have after CSE

  <bb 2> :
  # DEBUG BEGIN_STMT
  # DEBUG BEGIN_STMT
  l_756 = 40369;
  # DEBUG i => 0
  # DEBUG BEGIN_STMT
  l = &l_756;
  # DEBUG BEGIN_STMT
  goto <bb 4>; [INV]

...

  <bb 12> :
  # DEBUG BEGIN_STMT
  l.8_17 = (long int) &l;
  o_37 = (int) l.8_17;
  # DEBUG o => o_37
  l_756 ={v} {CLOBBER};
  l ={v} {CLOBBER};
  return;

where you can see that 'l' is dead and thus we could have written l_756
into SSA much earlier (or eliminated all stores to it).  It shows
us not "quickly" eliminating zero-use SSA defs can lead to such delayed
optimizations.  into-SSA already could see it, doing

   <bb 12> :
   # DEBUG BEGIN_STMT
   l.8_17 = (long int) &l;
-  o = (int) l.8_17;
-  l_756 = {CLOBBER};
-  l = {CLOBBER};
+  o_37 = (int) l.8_17;
+  # DEBUG o => o_37
+  l_756 ={v} {CLOBBER};
+  l ={v} {CLOBBER};
   return;

of course it's a bit premature there and tailored to this particular
testcase.