https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105956
--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Patrick Palka <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:7b90f07f778caa5e09f3dc20a8c3da1f4cf60c20 commit r13-1565-g7b90f07f778caa5e09f3dc20a8c3da1f4cf60c20 Author: Patrick Palka <ppa...@redhat.com> Date: Thu Jul 7 16:46:29 2022 -0400 c++: generic targs and identity substitution [PR105956] In r13-1045-gcb7fd1ea85feea I assumed that substitution into generic DECL_TI_ARGS corresponds to an identity mapping of the given arguments, and hence its safe to always elide such substitution. But this PR demonstrates that such a substitution isn't always the identity mapping, in particular when there's an ARGUMENT_PACK_SELECT argument, which gets handled specially during substitution: * when substituting an APS into a template parameter, we strip the APS to its underlying argument; * and when substituting an APS into a pack expansion, we strip the APS to its underlying argument pack. In this testcase, when expanding the pack expansion pattern (idx + Ns)... with Ns={0,1}, we specialize idx twice, first with Ns=APS<0,{0,1}> and then Ns=APS<1,{0,1}>. The DECL_TI_ARGS of idx are the generic template arguments of the enclosing class template impl, so before r13-1045, we'd substitute into its DECL_TI_ARGS which gave Ns={0,1} as desired. But after r13-1045, we elide this substitution and end up attempting to hash the original Ns argument, an APS, which ICEs. So this patch reverts that part of r13-1045. I considered using preserve_args in this case instead, but that'd break the static_assert in the testcase because preserve_args always strips APS to its underlying argument, but here we want to strip it to its underlying argument pack, so we'd incorrectly end up forming the specializations impl<0>::idx and impl<1>::idx instead of impl<0,1>::idx. Although we can't elide the substitution into DECL_TI_ARGS in light of ARGUMENT_PACK_SELECT, it should still be safe to elide template argument coercion in the case of a non-template decl, which this patch preserves. It's unfortunate that we need to remove this optimization just because it doesn't hold for one special tree code. So this patch implements a heuristic in tsubst_template_args to avoid allocating a new TREE_VEC if the substituted elements are identical to those of a level from ARGS, as well as a similar heuristic for tsubst_argument_pack. It turns out that about 40% of all calls to tsubst_template_args benefit from this, and it reduces memory usage by about 4% for e.g. range-v3's zip.cpp (relative to r13-1045) which more than makes up for the reversion. PR c++/105956 gcc/cp/ChangeLog: * pt.cc (template_arg_to_parm): Define. (tsubst_argument_pack): Try to reuse the corresponding ARGUMENT_PACK from 'args' when substituting into a generic ARGUMENT_PACK for a variadic template parameter. (tsubst_template_args): Move variable declarations closer to their first use. Replace 'orig_t' with 'r'. Rename 'need_new' to 'const_subst_p'. Heuristically detect if the substituted elements are identical to that of a level from 'args' and avoid allocating a new TREE_VEC if so. Add sanity check for the length of the new TREE_VEC, and remove dead ARGUMENT_PACK_P test. (tsubst_decl) <case TYPE_DECL, case VAR_DECL>: Revert r13-1045-gcb7fd1ea85feea change for avoiding substitution into DECL_TI_ARGS, but still avoid coercion in this case. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/variadic183.C: New test.
