[Bug ipa/93385] [10 Regression] wrong code with u128 modulo at -O2 -fno-dce -fno-ipa-cp -fno-tree-dce

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93385

--- Comment #29 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to rguenther@suse.de from comment #28)
> On Fri, 17 Apr 2020, jakub at gcc dot gnu.org wrote:
> 
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93385
> > 
> > --- Comment #26 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> > For debug stmts, it would be best if we could use those
> >              DEBUG D#Y s=> parm
> >              DEBUG var => D#Y
> > added in if (param_body_adjs && MAY_HAVE_DEBUG_BIND_STMTS).
> > Though, if we remove already during the copy_bb by not actually creating the
> > stmt, I'm afraid that will mean the debug info is lost (debug stmts will be
> > reset), unless we for the lhs of to be dced stmts manually create debug
> > temporaries and debug stmts when we remap_gimple_stmt those stmts (for
> > SSA_NAMEs that are directly (or indirectly!) used in debug stmts).
> > If we do what Martin was proposing instead, i.e. copy the stmts and then DCE
> > them afterwards, it might work properly (perhaps only if we DCE them in the
> > right order).
> 
> It's true that copying everything and then DCEing is easier for debug
> stmt generation.  I didn't consider this.  That also argues for not
> remapping anything to error_mark_node.  Of course this leaves us with
> no automagic verification if we really DCEd everything required
> (esp. his handling of call arguments looks expensive and odd to me).

We could remap those to error_mark_node for -g0 and for -g to the
DEBUG_EXPR_DECLs we'd create and then just during verification make sure we
diagnose DEBUG_EXPR_DECLs in non-debug stmts (and error_mark_node).
Though, I think even right now the debug side of things even without the DCE is
not perfect, we do create a bind to cover the optimized out parameters at the
start of the function, but don't do this remapping of the SSA_NAME to that
DEBUG_EXPR_DECL, which means  that e.g. debug stmts that used to use that
SSA_NAME in some expressions are reset.
So, I think if Martin comes up with something that just doesn't handle the
debug stmts for GCC10, I can try to improve the debug side later.