[Bug middle-end/106075] Wrong DSE with -fnon-call-exceptions

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106075

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-06-24
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Related:

int a = 1;
int c,d;
void
test()
{
  a=12345;
  c/d;
  a=1;
}

where the possibly throwing division (by zero) does not have virtual operands.
Likewise

void __attribute__((noreturn,const)) foo ()
{
  longjmp (&buf);
}

int a = 1;
int c,d;
void
test()
{
  a=12345;
  foo ();
  a=1;
}

but then we can simply declare 'const' invalid on 'foo'.

For the non-VOP case we'd need to assign a "context" counter to stmts
(in UID for example) and increment it when seeing a possible (implicit)
control flow terminating statement.  When the DSE walk discovers a new
context it has to consider that a use.  The expense is an extra whole-IL
walk over the function [with -fnon-call-exceptions].  Note there's also
the possibility to create a const externally throwing function but its
semantics are disputed (see another PR for that).