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));
