[Bug ipa/104377] Unreachable code in create_specialized_node of ipa-prop.c?

["fxue at os dot amperecomputing.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104377

--- Comment #4 from Feng Xue <fxue at os dot amperecomputing.com> ---
(In reply to Martin Jambor from comment #2)
> (In reply to Feng Xue from comment #1)
> > 
> > OK. I does missed something. Here we could not hold assumption that
> > ipcp_decision_stage() only sees raw cgraph node, since sometime in the
> > future some new ipa pass may be added prior to ipa-cp, and this pass
> > introduces clone node.
> 
> Right, initially IPA-SRA was developed as a pass before IPA-CP and it
> may well be that we decide to swap the order again.
> 
> > 
> > However, there is a questionable point about the code snippet
> > 
> > 	  if (!node->can_change_signature
> > 	      || old_adj->op != IPA_PARAM_OP_COPY
> > 	      || (!known_csts[old_adj->base_index]
> > 		  && ipa_is_param_used (info, old_adj->base_index)))
> > 
> > In ipa-cp, known_csts is for the node, has no relation to the node's origin
> > node, but here it is accessed via index of the latter (old_adj->base_index),
> > will this cause out-of-bound error?
> 
> I think the code is correct. Assume IPA-SRA running before IPA-CP, and
> we're compiling a function with two argument, with indices 0 and 1.
> 
> Analysis phases of both passes run before the IPA (WPA) phases of
> either.  This is important to keep in mind.
> 
> IPA SRA removes the first one with index zero as useless, IPA-CP wants
> to remove the second one with index 1, possibly because it is constant
> everywhere.  In oder to that it has to combine the pre-existing
> adjustments with its own changes.
> 
> Before create_specialized_node, the pass checks whether previous
> passes did not kill some parameters and stops caring about them, but
> it does not re-index anything, all lattices, jump functions,
> everything, still keep their positions (and thus indices) they got in
> the analysis phase.
> 
> Then create_specialized_node hits this loop.  For i=0 encounters an
> old_adj element that actually describes the parameter which originally
> had index 1.  The pass looks up in base_index what the original
> (pre-IPA) index of the parameter was (1) and uses those to look up in
> its own structures whether it wants to remove it or not.
> 
> Bounds should be always OK, base_index should never be greater than
> the original pre-IPA number of parameters (-1) and known_csts should
> always have just as many parameters.
> 
> Does that make sense?

Yes. Thanks for your explanation.