[Bug debug/105108] incomplete/incorrect DWARF information at -O1 and -Og after inlining a function returning a constant

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105108

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-03-31

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  At -O2 we end up with

int main ()
{
  <bb 2> [local count: 1073741824]:
  [t.c:4:2] # DEBUG BEGIN_STMT
  [t.c:5:2] # DEBUG BEGIN_STMT
  [t.c:5:8] # DEBUG l_165 => 128
  [t.c:6:2] # DEBUG BEGIN_STMT
  [t.c:6:10] # DEBUG l_144 => 0
  [t.c:7:2] # DEBUG BEGIN_STMT
  [t.c:7:4] a = 1; 
  [<built-in>:0:0] return 0;

}

while -O1 has

int main ()
{
  <bb 2> [local count: 1073741824]:
  [t.c:4:2] # DEBUG BEGIN_STMT
  [t.c:5:2] # DEBUG BEGIN_STMT
  [t.c:5:8] # DEBUG l_165 => 128
  [t.c:6:2] # DEBUG BEGIN_STMT
  [t.c:7:2] # DEBUG BEGIN_STMT
  [t.c:7:4] a = 1; 
  [<built-in>:0:0] return 0;

with -O1 we do not inline b() but instead CCP uses value-range info to
determine
that l_144 cannot be 128 and thus the call to b() is DCEd before it is
inlined.  So indeed at -O1 we do not know the value of l_144, we only know
that it cannot be 128.

Not sure what we can do about the missing debug info - IIRC we cannot
encode in DWARF that l_144 has the value of 'b()' (which is discovered
to have no side-effects besides the return value).