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From: Jason Merrill <jason@redhat.com>
To: Patrick Palka <ppalka@redhat.com>, gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] c++: redundant targ coercion for var/alias tmpls
Date: Fri, 23 Jun 2023 11:11:25 -0400	[thread overview]
Message-ID: <94b29e42-dfa9-5c05-4f1d-3c5beb998fdf@redhat.com> (raw)
In-Reply-To: <20230621171920.1283054-1-ppalka@redhat.com>

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?  It looks like we currently get to 
most_specialized_partial_spec with args that haven't yet been coerced to 
match the primary template.

> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?  This reduces memory usage of range-v3's zip.cpp by ~0.5%.
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (tsubst_decl) <case TYPE_/VAR_DECL>: Call
> 	coercion_template_parms only if DECL_TEMPLATE_SPECIALIZATION
> 	is set.
> ---
>   gcc/cp/pt.cc | 15 +++++++++++----
>   1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index be86051abad..dd10409ce18 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -15232,10 +15232,17 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
>   		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.  */
> +		    && TMPL_ARGS_DEPTH (args) >= TMPL_ARGS_DEPTH (argvec)
> +		    && DECL_TEMPLATE_SPECIALIZATION (t))
> +		  /* We're fully specializing an alias or variable template, so
> +		     coerce the innermost arguments if necessary.  We expect
> +		     instantiate_alias_template and finish_template_variable to
> +		     already have done this relative to the primary template, in
> +		     which case this coercion is unnecessary, but we can also
> +		     get here when substituting a partial variable template
> +		     specialization (directly from instantiate_template), in
> +		     which case DECL_TEMPLATE_SPECIALIZATION is set and coercion
> +		     is necessary.  */
>   		  argvec = (coerce_template_parms
>   			    (DECL_TEMPLATE_PARMS (gen_tmpl),
>   			     argvec, tmpl, complain));


  reply	other threads:[~2023-06-23 15:11 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-06-21 17:19 Patrick Palka
2023-06-23 15:11 ` Jason Merrill [this message]
2023-06-23 16:23   ` Patrick Palka
2023-06-28 15:50     ` Jason Merrill
2023-06-28 20:14       ` Patrick Palka
2023-07-13 15:48         ` Patrick Palka
2023-07-13 20:44           ` Jason Merrill
2023-07-14 18:07             ` Patrick Palka
2023-07-14 21:17               ` Jason Merrill
2023-07-17 21:29                 ` Patrick Palka
2023-07-18 16:52                   ` Jason Merrill

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