{"id":2221534,"url":"http://patchwork.ozlabs.org/api/1.1/patches/2221534/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/adfVRQUvuvgBZXuF@tucnak/","project":{"id":17,"url":"http://patchwork.ozlabs.org/api/1.1/projects/17/?format=json","name":"GNU Compiler Collection","link_name":"gcc","list_id":"gcc-patches.gcc.gnu.org","list_email":"gcc-patches@gcc.gnu.org","web_url":null,"scm_url":null,"webscm_url":null},"msgid":"","date":"2026-04-09T16:35:17","name":"c++: Unshare expr in get_template_parm_object [PR124792]","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"47e53f61fb28e44e9a603685aacd907b93d4f994","submitter":{"id":671,"url":"http://patchwork.ozlabs.org/api/1.1/people/671/?format=json","name":"Jakub Jelinek","email":"jakub@redhat.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/adfVRQUvuvgBZXuF@tucnak/mbox/","series":[{"id":499328,"url":"http://patchwork.ozlabs.org/api/1.1/series/499328/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=499328","date":"2026-04-09T16:35:17","name":"c++: Unshare expr in get_template_parm_object [PR124792]","version":1,"mbox":"http://patchwork.ozlabs.org/series/499328/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2221534/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2221534/checks/","tags":{},"headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=NG9OqtPz;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; 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charset=us-ascii","Content-Disposition":"inline","X-BeenThere":"gcc-patches@gcc.gnu.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Gcc-patches mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Reply-To":"Jakub Jelinek ","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"},"content":"On Thu, Apr 09, 2026 at 12:07:42PM -0400, Jason Merrill wrote:\n> > With reflection, we can create the NTTPOs during constant evaluation rather than\n> > after cxx_eval_outermost_constant_expr has unshared those, so later\n> > processing within the same cxx_eval_outermost_constant_expr call can modify\n> > those not unshared trees.\n> > Now, from reflect.cc get_template_parm_object is called directly from\n> > eval_reflect_constant_string (I think not a problem, the argument should be\n> > always STRING_CST in that case), eval_reflect_constant_array (also not\n> > a problem IMHO, as the outer CONSTRUCTOR is freshly created by\n> > get_range_elts and what is inside of it is unshared by\n> > if (VAR_P (expr))\n> > expr = unshare_expr (DECL_INITIAL (expr));\n> > ) and then indirectly (convert_reflect_constant_arg ->\n> > convert_nontype_argument -> create_template_parm_object) from\n> > within get_range_elts, eval_reflect_constant, eval_reflect_object,\n> > eval_reflect_function. So, if we don't do unsharing in\n> > get_template_parm_object because it is philosophically attractive but\n> > wastes compile time memory too much when things are usually unshared,\n> \n> Actually, I'm not sure it does waste too much memory; a quick check with\n> -fmem-report shows no difference in the totals on compiling stdc++.h as\n> C++26. And since we'd only unshare in the case where we're actually\n> creating a new variable, it might actually mean less unsharing in the case\n> of repetitive patterns.\n> \n> > (and yet another question is unshare_constructor vs. unshare_expr),\n> \n> Or even unshare_expr_without_location (like PR91165) if we unshare in\n> get_TPO since all occurrences of the same value get the same variable.\n> \n> It seems that many places use unshare_constructor to protect against things\n> like split_nonconstant_init or cxx_eval_store_expression but not a modifying\n> walk_tree like cp_genericize.\n\nSo, do you want something like the patch below then?\nI think in the modules.cc (aka !check_init) case the CONSTRUCTOR is already\nnot shared because it has been read from the module, but I could be wrong.\n\nAnd another question is about that\n /* If EXPR contains any PTRMEM_CST, they will get clobbered by\n lower_var_init before we're done mangling. So store the original\n value elsewhere. We only need to unshare EXPR if it's not yet\n been processed. */\n tree copy = check_init ? unshare_constructor (expr) : expr;\n hash_map_safe_put (tparm_obj_values, decl, copy);\nI guess lower_var_init is done late these days, but still, if we need\ntparm_obj_values at all, couldn't we do it solely for the cases where\nexpr contains any PTRMEM_CSTs inside of it (so search all the nested\nCONSTRUCTORs if we find PTRMEM_CSTs, similarly how cplus_expand_constant\ndoes that, just don't expand anything, just return bool that it contains\nthem and unshare/register in that case), and otherwise just use\nDECL_INITIAL?\n\n2026-04-09 Jakub Jelinek \n\n\tPR c++/124792\n\t* pt.cc (get_template_parm_object): Call unshare_expr_without_location\n\ton expr if check_init.\n\n\t* g++.dg/reflect/pr124792.C: New test.\n\n\n\n\tJakub","diff":"--- gcc/cp/pt.cc.jj\t2026-04-09 18:18:26.461536563 +0200\n+++ gcc/cp/pt.cc\t2026-04-09 18:23:33.840288646 +0200\n@@ -7551,6 +7551,8 @@ get_template_parm_object (tree expr, tre\n DECL_NAME (decl) = name;\n SET_DECL_ASSEMBLER_NAME (decl, name);\n comdat_linkage (decl);\n+ if (check_init)\n+ expr = unshare_expr_without_location (expr);\n \n if (!zero_init_p (type))\n {\n--- gcc/testsuite/g++.dg/reflect/pr124792.C.jj\t2026-04-09 18:18:08.189674154 +0200\n+++ gcc/testsuite/g++.dg/reflect/pr124792.C\t2026-04-09 18:18:08.189674154 +0200\n@@ -0,0 +1,11 @@\n+// PR c++/124792\n+// { dg-do compile { target c++26 } }\n+// { dg-additional-options \"-freflection\" }\n+\n+#include \n+\n+struct S { const char *name = nullptr; };\n+\n+constexpr auto a = std::define_static_array (std::vector { S { std::define_static_string (\"foo\") } });\n+constexpr auto b = std::define_static_array (std::vector { S { std::define_static_string (\"foo\") },\n+\t\t\t\t\t\t\t S { std::define_static_string (\"bar\") } });\n","prefixes":[]}