On 6/29/22 13:42, Patrick Palka wrote:
In r13-1045-gcb7fd1ea85feea I assumed that substitution into generic
DECL_TI_ARGS corresponds to an identity mapping of the given arguments,
and hence its safe to always elide such substitution. But this PR
demonstrates that such a substitution isn't always the identity mapping,
in particular when there's an ARGUMENT_PACK_SELECT argument, which gets
handled specially during substitution:
* when substituting an APS into a template parameter, we strip the
APS to its underlying argument;
* and when substituting an APS into a pack expansion, we strip the
APS to its underlying argument pack.
Ah, right. For instance, in variadic96.C we have
10 template < typename... T >
11 struct derived
12 : public base< T, derived< T... > >...
so when substituting into the base-specifier, we're approaching it from
the outside in, so when we get to the inner T... we need some way to
find the T pack again. It might be possible to remove the need for APS
by substituting inner pack expansions before outer ones, which could
improve worst-case complexity, but I don't know how relevant that is in
real code; I imagine most inner pack expansions are as simple as this one.
In this testcase, when expanding the pack expansion pattern (idx + Ns)...
with Ns={0,1}, we specialize idx twice, first with Ns=APS<0,{0,1}> and
then Ns=APS<1,{0,1}>. The DECL_TI_ARGS of idx are the generic template
arguments of the enclosing class template impl, so before r13-1045,
we'd substitute into its DECL_TI_ARGS which gave Ns={0,1} as desired.
But after r13-1045, we elide this substitution and end up attempting to
hash the original Ns argument, an APS, which ICEs.
So this patch partially reverts this part of r13-1045. I considered
using preserve_args in this case instead, but that'd break the
static_assert in the testcase because preserve_args always strips APS to
its underlying argument, but here we want to strip it to its underlying
argument pack, so we'd incorrectly end up forming the specializations
impl<0>::idx and impl<1>::idx instead of impl<0,1>::idx.
Although we can't elide the substitution into DECL_TI_ARGS in light of
ARGUMENT_PACK_SELECT, it should still be safe to elide template argument
coercion in the case of a non-template decl, which this patch preserves.
It's unfortunate that we need to remove this optimization just because
it doesn't hold for one special tree code. So this patch implements a
heuristic in tsubst_template_args to avoid allocating a new TREE_VEC if
the substituted elements are identical to those of a level from ARGS.
It turns out that about 30% of all calls to tsubst_template_args benefit
from this optimization, and it reduces memory usage by about 1.5% for
e.g. stdc++.h (relative to r13-1045). (This is the maybe_reuse stuff,
the rest of the changes to tsubst_template_args are just drive-by
cleanups.)
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk? Patch generated with -w to ignore noisy whitespace changes.
PR c++/105956
gcc/cp/ChangeLog:
* pt.cc (tsubst_template_args): Move variable declarations
closer to their first use. Replace 'orig_t' with 'r'. Rename
'need_new' to 'const_subst_p'. Heuristically detect if the
substituted elements are identical to that of a level from
'args' and avoid allocating a new TREE_VEC if so.
(tsubst_decl) <case TYPE_DECL, case VAR_DECL>: Revert
r13-1045-gcb7fd1ea85feea change for avoiding substitution into
DECL_TI_ARGS, but still avoid coercion in this case.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/variadic183.C: New test.
---
gcc/cp/pt.cc | 113 ++++++++++++++---------
gcc/testsuite/g++.dg/cpp0x/variadic183.C | 14 +++
2 files changed, 85 insertions(+), 42 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/variadic183.C
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 8672da123f4..7898834faa6 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -27,6 +27,7 @@ along with GCC; see the file COPYING3. If not see
Fixed by: C++20 modules. */
#include "config.h"
+#define INCLUDE_ALGORITHM // for std::equal
#include "system.h"
#include "coretypes.h"
#include "cp-tree.h"
@@ -13544,17 +13545,22 @@ tsubst_argument_pack (tree orig_arg, tree args,
tsubst_flags_t complain,
tree
tsubst_template_args (tree t, tree args, tsubst_flags_t complain, tree
in_decl)
{
- tree orig_t = t;
- int len, need_new = 0, i, expanded_len_adjust = 0, out;
- tree *elts;
-
if (t == error_mark_node)
return error_mark_node;
- len = TREE_VEC_LENGTH (t);
- elts = XALLOCAVEC (tree, len);
+ const int len = TREE_VEC_LENGTH (t);
+ tree *elts = XALLOCAVEC (tree, len);
+ int expanded_len_adjust = 0;
- for (i = 0; i < len; i++)
+ /* True iff no element of T was changed by the substitution. */
+ bool const_subst_p = true;
+
+ /* If MAYBE_REUSE is non-NULL, as an optimization we'll try to reuse and
+ return this TREE_VEC instead of allocating a new one if possible. This
+ will either be ARGS or a level from ARGS. */
+ tree maybe_reuse = NULL_TREE;
+
+ for (int i = 0; i < len; i++)
{
tree orig_arg = TREE_VEC_ELT (t, i);
tree new_arg;
@@ -13580,56 +13586,90 @@ tsubst_template_args (tree t, tree args,
tsubst_flags_t complain, tree in_decl)
else if (ARGUMENT_PACK_P (orig_arg))
new_arg = tsubst_argument_pack (orig_arg, args, complain, in_decl);
else
+ {
new_arg = tsubst_template_arg (orig_arg, args, complain, in_decl);
+ /* If T heuristically appears to be a set of generic template
+ arguments, set MAYBE_REUSE to the corresponding level from
+ ARGS. */
+ if (maybe_reuse == NULL_TREE && orig_arg != NULL_TREE)
+ {
+ if (TEMPLATE_PARM_P (orig_arg))
+ {
This doesn't handle the case of variadic template parameters, which are
represented in the generic args with a pack expansion. If this is a
choice, there should be a comment about it.
+ int level, index;
+ template_parm_level_and_index (orig_arg, &level, &index);
+ if (index == i && TMPL_ARGS_DEPTH (args) >= level)
+ maybe_reuse = TMPL_ARGS_LEVEL (args, level);
+ else
+ maybe_reuse = error_mark_node;
+ }
+ else
+ /* T is not a set of generic template arguments; use
+ error_mark_node to denote this. */
+ maybe_reuse = error_mark_node;
+ }
+ }
+
if (new_arg == error_mark_node)
return error_mark_node;
elts[i] = new_arg;
if (new_arg != orig_arg)
- need_new = 1;
+ const_subst_p = false;
}
- if (!need_new)
+ if (const_subst_p)
return t;
+ /* If ARGS and T are both multi-level, then substituted T may be
+ identical to ARGS (if each level was pairwise identical). */
+ if (maybe_reuse == NULL_TREE
+ && TMPL_ARGS_HAVE_MULTIPLE_LEVELS (t)
+ && TMPL_ARGS_HAVE_MULTIPLE_LEVELS (args))
+ maybe_reuse = args;
+
+ /* Return MAYBE_REUSE and avoid allocating a new TREE_VEC if the substituted
+ result is identical to it. */
+ if (NON_ERROR (maybe_reuse) != NULL_TREE
+ && TREE_VEC_LENGTH (maybe_reuse) == len
+ && std::equal (elts, elts+len, TREE_VEC_BEGIN (maybe_reuse)))
+ return maybe_reuse;
+
/* Make space for the expanded arguments coming from template
argument packs. */
- t = make_tree_vec (len + expanded_len_adjust);
- /* ORIG_T can contain TREE_VECs. That happens if ORIG_T contains the
+ tree r = make_tree_vec (len + expanded_len_adjust);
+ /* T can contain TREE_VECs. That happens if T contains the
arguments for a member template.
- In that case each TREE_VEC in ORIG_T represents a level of template
- arguments, and ORIG_T won't carry any non defaulted argument count.
+ In that case each TREE_VEC in T represents a level of template
+ arguments, and T won't carry any non defaulted argument count.
It will rather be the nested TREE_VECs that will carry one.
- In other words, ORIG_T carries a non defaulted argument count only
+ In other words, T carries a non defaulted argument count only
if it doesn't contain any nested TREE_VEC. */
- if (NON_DEFAULT_TEMPLATE_ARGS_COUNT (orig_t))
+ if (NON_DEFAULT_TEMPLATE_ARGS_COUNT (t))
{
- int count = GET_NON_DEFAULT_TEMPLATE_ARGS_COUNT (orig_t);
+ int count = GET_NON_DEFAULT_TEMPLATE_ARGS_COUNT (t);
count += expanded_len_adjust;
- SET_NON_DEFAULT_TEMPLATE_ARGS_COUNT (t, count);
+ SET_NON_DEFAULT_TEMPLATE_ARGS_COUNT (r, count);
}
- for (i = 0, out = 0; i < len; i++)
+ for (int i = 0, out = 0; i < len; i++)
{
- tree orig_arg = TREE_VEC_ELT (orig_t, i);
+ tree orig_arg = TREE_VEC_ELT (t, i);
if (orig_arg
&& (PACK_EXPANSION_P (orig_arg) || ARGUMENT_PACK_P (orig_arg))
&& TREE_CODE (elts[i]) == TREE_VEC)
{
- int idx;
-
/* Now expand the template argument pack "in place". */
- for (idx = 0; idx < TREE_VEC_LENGTH (elts[i]); idx++, out++)
- TREE_VEC_ELT (t, out) = TREE_VEC_ELT (elts[i], idx);
+ for (int idx = 0; idx < TREE_VEC_LENGTH (elts[i]); idx++, out++)
+ TREE_VEC_ELT (r, out) = TREE_VEC_ELT (elts[i], idx);
}
else
{
- TREE_VEC_ELT (t, out) = elts[i];
+ TREE_VEC_ELT (r, out) = elts[i];
out++;
}
}
- return t;
+ return r;
}
/* Substitute ARGS into one level PARMS of template parameters. */
@@ -14965,32 +15005,21 @@ tsubst_decl (tree t, tree args, tsubst_flags_t
complain)
if (!spec)
{
- int args_depth = TMPL_ARGS_DEPTH (args);
- int parms_depth = TMPL_ARGS_DEPTH (DECL_TI_ARGS (t));
tmpl = DECL_TI_TEMPLATE (t);
gen_tmpl = most_general_template (tmpl);
- if (args_depth == parms_depth
- && !PRIMARY_TEMPLATE_P (gen_tmpl))
- /* The DECL_TI_ARGS in this case are the generic template
- arguments for the enclosing class template, so we can
- shortcut substitution (which would just be the identity
- mapping). */
- argvec = args;
- else
- {
argvec = tsubst (DECL_TI_ARGS (t), args, complain, in_decl);
- /* Coerce the innermost arguments again if necessary. If
- there's fewer levels of args than of parms, then the
- substitution could not have changed the innermost args
- (modulo level lowering). */
- if (args_depth >= parms_depth && argvec != error_mark_node)
+ 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_innermost_template_parms
(DECL_TEMPLATE_PARMS (gen_tmpl),
argvec, t, complain,
/*all*/true, /*defarg*/true));
if (argvec == error_mark_node)
RETURN (error_mark_node);
- }
hash = spec_hasher::hash (gen_tmpl, argvec);
spec = retrieve_specialization (gen_tmpl, argvec, hash);
}
diff --git a/gcc/testsuite/g++.dg/cpp0x/variadic183.C
b/gcc/testsuite/g++.dg/cpp0x/variadic183.C
new file mode 100644
index 00000000000..3938e52e0e7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/variadic183.C
@@ -0,0 +1,14 @@
+// PR c++/105956
+// { dg-do compile { target c++11 } }
+
+template<int... Ns> struct list;
+
+template<int... Ns> struct impl {
+ static const int idx = 0;
+ using type = list<(idx + Ns)...>;
+
+ static constexpr const int* a[2] = {(Ns, &idx)...};
+ static_assert(a[0] == &idx && a[1] == &idx, "");
+};
+
+template struct impl<0, 1>;
