[PATCH] tree: Fix -fcompare-debug issues due to protected_set_expr_location [PR94323]

[Jakub Jelinek]

On Mon, Mar 30, 2020 at 12:44:40PM +0200, Richard Biener wrote:
> >    type = TREE_TYPE (stmt_expr);
> > +  /* For statement-expressions, force the STATEMENT_LIST
> > +     to be preserved with side-effects, even if it contains
> > +     just one statement or no statements.  See PR93786.  */
> > +  TREE_SIDE_EFFECTS (stmt_expr) = 0;
> >    result = pop_stmt_list (stmt_expr);
> > +  gcc_assert (result == stmt_expr);
> > +  TREE_SIDE_EFFECTS (result) = 1;
> 
> The original code had !TREE_SIDE_EFFECT stmt lists (empty ones?) returned
> directly which you above change to have side-effects - are you sure
> the fallout is not due to that?  Did you look into any of the fallout?

Some of the fallout has been from missed optimizations e.g. in initializers,
where the TREE_SIDE_EFFECTs (sometimes for empty statement exprs like
({ ; ; ; }), at other times for one with a single real statement in there
which didn't have side-effects) is with the patch now set when it wasn't
before, but there were ICEs too.
I guess another option is what Alex wrote, just throw the DEBUG_BEGIN_STMTs
away if there would be just those or those plus a single other statement,
perhaps limited to statement expressions (i.e. do it in the above spot).

	Jakub