[Bug tree-optimization/66512] PRE fails to optimize calls to pure functions in C++, ok in C

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66512

--- Comment #2 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
In that case I'd like to contribute a documentation patch to make that clear in
the pure/const attribute information, but I need more explanation.  I see that

int p(void) __attribute__((const));
void f()
{
  p();
  p();
}

is optimized to an empty function, even though p may throw.  Is that not a bug?

Also, could you please expand your explanation in the first paragraph, i.e.
this:

  What we miss here is the fact that it should only matter
  if p throws internally for IL consistency.  Of course it
  still matters for observing other side-effects if p throws
  and after the transform now does so before side-effects
  that should be observed otherwise.

I'm probably missing a lot of contextual knowledge to understand that.  TIA.