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

[Jason Merrill <jason@redhat.com>]

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>;