Re: [PATCH] c++: generic targs and identity substitution [PR105956]

[Patrick Palka <ppalka@redhat.com>]

On Tue, 5 Jul 2022, Jason Merrill wrote:

> On 7/5/22 10:06, Patrick Palka wrote:
> > On Fri, 1 Jul 2022, Jason Merrill wrote:
> > 
> > > 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.
> > 
> > Aha, that makes sense.
> > 
> > > 
> > > > 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.
> > 
> > AFAICT substituting such a pack expansion will never be an identity
> > transform -- the relevant targ during tsubst_pack_expansion will be an
> > ARGUMENT_PACK, and we'll return the TREE_VEC from that ARGUMENT_PACK
> > instead of the ARGUMENT_PACK itself, so there's no way we can reuse (a
> > level from) 'args' in this case.
> 
> Hmm, when replacing a level of T... we start with a TREE_VEC containing an
> ARGUMENT_PACK around a TREE_VEC containing a PACK_EXPANSION, and substitution
> should replace that with the argument level, a TREE_VEC containing an
> ARGUMENT_PACK around a TREE_VEC containing an actual set of args, so reuse
> ought to be possible?

Ah, I think see what you mean.  For that, I think it suffices to just
handle in tsubst_template_args the case where we're substituting into a
single-elt TREE_VEC such as {T...}.  That'll allow us to reuse the inner
TREE_VEC containing the actual set of args at least.  And AFAICT that's
the best we can do, since we'll always be creating a new ARGUMENT_PACK
which doesn't appear in any other TREE_VEC we have at our disposal.

So like this?  I also added an assert that 'out' never exceeds 'len',
and simplified the test

  orig_arg
  && (PACK_EXPANSION_P (orig_arg) || ARGUMENT_PACK_P (orig_arg))
  && TREE_CODE (elts[i]) == TREE_VEC

to just

  orig_arg
  && PACK_EXPANSION_P (orig_arg)
  && TREE_CODE (elts[i]) == TREE_VEC

since substitution into an ARGUMENT_PACK will never yield a TREE_VEC.

-- >8 --

Subject: [PATCH] c++: generic targs and identity substitution [PR105956]

	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.  Assert that
	'out' doesn't exceed 'len', and remove useless ARGUMENT_PACK_P
	test.
	(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                             | 128 +++++++++++++++--------
 gcc/testsuite/g++.dg/cpp0x/variadic183.C |  14 +++
 2 files changed, 99 insertions(+), 43 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..c290174a390 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 the substituted result is identical to T.  */
+  bool const_subst_p = true;
+
+  /* If MAYBE_REUSE is a TREE_VEC, 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 +13586,103 @@ 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.
+	     Otherwise set it to error_mark_node.  (Note that this doesn't
+	     handle variadic template parameters, which are represented as a
+	     generic arg by an ARGUMENT_PACK around a one-element TREE_VEC
+	     containing a PACK_EXPANSION.  We partially handle that case
+	     later.)  */
+	  if (maybe_reuse == NULL_TREE && orig_arg != NULL_TREE)
+	    {
+	      if (TEMPLATE_PARM_P (orig_arg))
+		{
+		  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
+		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, the substituted result 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;
+
+  /* If T consists of only a pack expansion for which substitution resulted
+     in a TREE_VEC of the expanded elements, then reuse that TREE_VEC instead
+     of effectively making a copy.  */
+  if (len == 1
+      && PACK_EXPANSION_P (TREE_VEC_ELT (t, 0))
+      && TREE_CODE (elts[0]) == TREE_VEC)
+    return elts[0];
+
   /* 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++)
+
+  int out = 0;
+  for (int i = 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))
+	  && PACK_EXPANSION_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++;
         }
     }
+  gcc_assert (out <= TREE_VEC_LENGTH (r));
 
-  return t;
+  return r;
 }
 
 /* Substitute ARGS into one level PARMS of template parameters.  */
@@ -14965,32 +15018,21 @@ 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..27444ebb594
--- /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...> 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>;
-- 
2.37.0.3.g30cc8d0f14