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 <ja...@redhat.com> 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) <case VAR/TYPE_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);
