[Bug c++/100409] C++ FE elides pure throwing call

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100409

--- Comment #13 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> -  if (! TREE_SIDE_EFFECTS (expr))
> +  if (! TREE_SIDE_EFFECTS (expr) && expr_noexcept_p (expr, 0))
>      expr = void_node;

The assumption that an expression with TREE_SIDE_EFFECTS unset can be discarded
if its value is not used is made throughout the compiler.  Abandoning that
invariant seems like a bad idea.  And changing from checking a flag to walking
through the whole expression in all of those places sounds like an algorithmic
complexity problem.

If a pure/const function can throw, fine.  But that shouldn't affect how
optimization treats the function call, or we'll end up pessimizing the vast
majority of calls to pure/const functions that don't throw (but are not
explicitly declared noexcept).  In this testcase, if the user doesn't want the
compiler to optimize away a call to foo(), they shouldn't mark it pure.