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[130.44.146.16]) by smtp.gmail.com with ESMTPSA id v26-20020ac8729a000000b004039a43ddbasm3385398qto.53.2023.07.13.13.44.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 13 Jul 2023 13:44:53 -0700 (PDT) Message-ID: Date: Thu, 13 Jul 2023 16:44:52 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH] c++: redundant targ coercion for var/alias tmpls To: Patrick Palka Cc: gcc-patches@gcc.gnu.org References: <20230621171920.1283054-1-ppalka@redhat.com> <94b29e42-dfa9-5c05-4f1d-3c5beb998fdf@redhat.com> <26258d18-5718-9387-087b-be58a25048c2@redhat.com> <4388a939-94bd-048c-f234-e54bab92d937@idea> From: Jason Merrill In-Reply-To: <4388a939-94bd-048c-f234-e54bab92d937@idea> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-12.7 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,KAM_SHORT,NICE_REPLY_A,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H4,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_NONE,TXREP,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On 7/13/23 11:48, Patrick Palka wrote: > On Wed, 28 Jun 2023, Patrick Palka wrote: > >> On Wed, Jun 28, 2023 at 11:50 AM Jason Merrill wrote: >>> >>> On 6/23/23 12:23, Patrick Palka wrote: >>>> On Fri, 23 Jun 2023, Jason Merrill wrote: >>>> >>>>> On 6/21/23 13:19, Patrick Palka wrote: >>>>>> When stepping through the variable/alias template specialization code >>>>>> paths, I noticed we perform template argument coercion twice: first from >>>>>> instantiate_alias_template / finish_template_variable and again from >>>>>> tsubst_decl (during instantiate_template). It should suffice to perform >>>>>> coercion once. >>>>>> >>>>>> To that end patch elides this second coercion from tsubst_decl when >>>>>> possible. We can't get rid of it completely because we don't always >>>>>> specialize a variable template from finish_template_variable: we could >>>>>> also be doing so directly from instantiate_template during variable >>>>>> template partial specialization selection, in which case the coercion >>>>>> from tsubst_decl would be the first and only coercion. >>>>> >>>>> Perhaps we should be coercing in lookup_template_variable rather than >>>>> finish_template_variable? >>>> >>>> Ah yes, there's a patch for that at >>>> https://gcc.gnu.org/pipermail/gcc-patches/2023-May/617377.html :) >>> >>> So after that patch, can we get rid of the second coercion completely? >> >> On second thought it should be possible to get rid of it, if we >> rearrange things to always pass the primary arguments to tsubst_decl, >> and perform partial specialization selection from there instead of >> instantiate_template. Let me try... > > Like so? Bootstrapped and regtested on x86_64-pc-linux-gnu. > > -- >8 -- > > When stepping through the variable/alias template specialization code > paths, I noticed we perform template argument coercion twice: first from > instantiate_alias_template / finish_template_variable and again from > tsubst_decl (during instantiate_template). It'd be good to avoid this > redundant coercion. > > It turns out that this coercion could be safely elided whenever > specializing a primary variable/alias template, because we can rely on > lookup_template_variable and instantiate_alias_template to already have > coerced the arguments. > > The other situation to consider is when fully specializing a partial > variable template specialization (from instantiate_template), in which > case the passed 'args' are the (already coerced) arguments relative to > the partial template and 'argvec', the result of substitution into > DECL_TI_ARGS, are the (uncoerced) arguments relative to the primary > template, so coercion is still necessary. We can still avoid this > coercion however if we always pass the primary variable template to > tsubst_decl from instantiate_template, and instead perform partial > specialization selection directly from tsubst_decl. This patch > implements this approach. The relationship between instantiate_template and tsubst_decl is pretty tangled. We use the former to substitute (often deduced) template arguments into a template, and the latter to substitute template arguments into a use of a template...and also to implement the former. For substitution of uses of a template, we expect to need to coerce the arguments after substitution. But we avoid this issue for variable templates by keeping them as TEMPLATE_ID_EXPR until substitution time, so if we see a VAR_DECL in tsubst_decl it's either a non-template variable or under instantiate_template. So it seems like the current coercion for variable templates is only needed in this case to support the redundant hash table lookup that we just did in instantiate_template. Perhaps instead of doing coercion here or moving the partial spec lookup, we could skip the hash table lookup for the case of a variable template? > gcc/cp/ChangeLog: > > * pt.cc (tsubst_decl) : Don't call > coerce_template_parms. Call most_specialized_partial_spec > when fully specializing a variable template here ... > (instantiate_template): ... instead of here. Always pass > the primary variable template pattern to tsubst_decl. > --- > gcc/cp/pt.cc | 62 +++++++++++++++++++++++----------------------------- > 1 file changed, 27 insertions(+), 35 deletions(-) > > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc > index fa15b75b9c5..53968b823d5 100644 > --- a/gcc/cp/pt.cc > +++ b/gcc/cp/pt.cc > @@ -15194,6 +15194,7 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain) > /* Check to see if we already have the specialization we > need. */ > tree spec = NULL_TREE; > + tree partial_ti = NULL_TREE; > bool local_p = false; > tree ctx = DECL_CONTEXT (t); > if (!(VAR_P (t) && DECL_LOCAL_DECL_P (t)) > @@ -15230,17 +15231,29 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain) > tmpl = DECL_TI_TEMPLATE (t); > gen_tmpl = most_general_template (tmpl); > argvec = tsubst (DECL_TI_ARGS (t), args, complain, in_decl); > - if (argvec != error_mark_node > - && PRIMARY_TEMPLATE_P (gen_tmpl) > - && TMPL_ARGS_DEPTH (args) >= TMPL_ARGS_DEPTH (argvec)) > - /* We're fully specializing a template declaration, so > - we need to coerce the innermost arguments corresponding to > - the template. */ > - argvec = (coerce_template_parms > - (DECL_TEMPLATE_PARMS (gen_tmpl), > - argvec, tmpl, complain)); > if (argvec == error_mark_node) > RETURN (error_mark_node); > + if (variable_template_p (gen_tmpl) > + && TMPL_ARGS_DEPTH (args) >= TMPL_ARGS_DEPTH (argvec)) > + { > + /* We need to determine if we're using a partial > + specialization now, because the type of the > + variable could be different. */ > + iloc_sentinel ils (saved_loc); > + tree tid = build_nt (TEMPLATE_ID_EXPR, gen_tmpl, argvec); > + partial_ti = most_specialized_partial_spec (tid, complain); > + if (partial_ti == error_mark_node) > + RETURN (error_mark_node); > + else if (partial_ti) > + { > + tree partial_tmpl = TI_TEMPLATE (partial_ti); > + tree partial_args = TI_ARGS (partial_ti); > + tree partial_pat = DECL_TEMPLATE_RESULT (partial_tmpl); > + t = partial_pat; > + args = partial_args; > + in_decl = partial_pat; > + } > + } > hash = spec_hasher::hash (gen_tmpl, argvec); > spec = retrieve_specialization (gen_tmpl, argvec, hash); If you disagree with my suggestion above, the hash lookup should come before the partial specialization selection. > } > @@ -15403,6 +15416,10 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain) > DECL_NOT_REALLY_EXTERN (r) = 1; > > DECL_TEMPLATE_INFO (r) = build_template_info (tmpl, argvec); > + if (variable_template_p (tmpl)) > + /* Now that we we've formed this variable template specialization, > + remember the result of most_specialized_partial_spec for it. */ > + TI_PARTIAL_INFO (DECL_TEMPLATE_INFO (r)) = partial_ti; > SET_DECL_IMPLICIT_INSTANTIATION (r); > if (!error_operand_p (r) || (complain & tf_error)) > register_specialization (r, gen_tmpl, argvec, false, hash); > @@ -22092,29 +22109,8 @@ instantiate_template (tree tmpl, tree orig_args, tsubst_flags_t complain) > > tree pattern = DECL_TEMPLATE_RESULT (gen_tmpl); > > - tree partial_ti = NULL_TREE; > - fndecl = NULL_TREE; > - if (VAR_P (pattern)) > - { > - /* We need to determine if we're using a partial or explicit > - specialization now, because the type of the variable could be > - different. */ > - tree tid = build2 (TEMPLATE_ID_EXPR, NULL_TREE, tmpl, targ_ptr); > - partial_ti = most_specialized_partial_spec (tid, complain); > - if (partial_ti == error_mark_node) > - pattern = error_mark_node; > - else if (partial_ti) > - { > - tree partial_tmpl = TI_TEMPLATE (partial_ti); > - tree partial_args = TI_ARGS (partial_ti); > - tree partial_pat = DECL_TEMPLATE_RESULT (partial_tmpl); > - fndecl = tsubst (partial_pat, partial_args, complain, gen_tmpl); > - } > - } > - > /* Substitute template parameters to obtain the specialization. */ > - if (fndecl == NULL_TREE) > - fndecl = tsubst (pattern, targ_ptr, complain, gen_tmpl); > + fndecl = tsubst (pattern, targ_ptr, complain, gen_tmpl); > if (DECL_CLASS_SCOPE_P (gen_tmpl)) > pop_nested_class (); > pop_from_top_level (); > @@ -22129,10 +22125,6 @@ instantiate_template (tree tmpl, tree orig_args, tsubst_flags_t complain) > template, not the most general template. */ > DECL_TI_TEMPLATE (fndecl) = tmpl; > DECL_TI_ARGS (fndecl) = targ_ptr; > - if (VAR_P (pattern)) > - /* Now that we we've formed this variable template specialization, > - remember the result of most_specialized_partial_spec for it. */ > - TI_PARTIAL_INFO (DECL_TEMPLATE_INFO (fndecl)) = partial_ti; > > set_instantiating_module (fndecl); >