c++/modules: local class merging [PR99426]

  Bootstrapped and regtested on x86_64-pc-linux-gnu, does this approach
look reasonable?

-- >8 --

One known missing piece in the modules implementation is merging of a
streamed-in local class with the corresponding in-TU version of the
local class.  This missing piece turns out to cause a hard-to-reduce
use-after-free GC issue due to the entity_ary not being marked as a GC
root (deliberately), and manifests as a serialization error on stream-in
as in PR99426 (see comment #6 for a reduction).  It's also reproducible
on trunk when running the xtreme-header tests without -fno-module-lazy.

This patch makes us merge such local classes according to their position
within the containing function's definition, similar to how we merge
FIELD_DECLs of a class according to their index in the TYPE_FIELDS
list.

	PR c++/99426

gcc/cp/ChangeLog:

	* module.cc (merge_kind::MK_local_class): New enumerator.
	(merge_kind_name): Update.
	(trees_out::chained_decls): Move BLOCK-specific handling
	of DECL_LOCAL_DECL_P decls to ...
	(trees_out::core_vals) <case BLOCK>: ... here.  Stream
	BLOCK_VARS manually.
	(trees_in::core_vals) <case BLOCK>: Stream BLOCK_VARS
	manually.  Handle deduplicated local classes.
	(trees_out::key_local_class): Define.
	(trees_in::key_local_class): Define.
	(trees_out::get_merge_kind) <case FUNCTION_DECL>: Return
	MK_local_class for a local class.
	(trees_out::key_mergeable) <case FUNCTION_DECL>: Use
	key_local_class.
	(trees_in::key_mergeable) <case FUNCTION_DECL>: Likewise.
	(trees_in::is_matching_decl): Be flexible with type mismatches
	for local entities.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/xtreme-header-7_a.H: New test.
	* g++.dg/modules/xtreme-header-7_b.C: New test.
---
 gcc/cp/module.cc                              | 167 +++++++++++++++---
 .../g++.dg/modules/xtreme-header-7_a.H        |   4 +
 .../g++.dg/modules/xtreme-header-7_b.C        |   6 +
 3 files changed, 149 insertions(+), 28 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H
 create mode 100644 gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C
  

On Mon, 26 Feb 2024, Patrick Palka wrote:

> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this approach
> look reasonable?
> 
> -- >8 --
> 
> One known missing piece in the modules implementation is merging of a
> streamed-in local class with the corresponding in-TU version of the
> local class.  This missing piece turns out to cause a hard-to-reduce
> use-after-free GC issue due to the entity_ary not being marked as a GC
> root (deliberately), and manifests as a serialization error on stream-in
> as in PR99426 (see comment #6 for a reduction).  It's also reproducible
> on trunk when running the xtreme-header tests without -fno-module-lazy.
> 
> This patch makes us merge such local classes according to their position
> within the containing function's definition, similar to how we merge
> FIELD_DECLs of a class according to their index in the TYPE_FIELDS
> list.
> 
> 	PR c++/99426
> 
> gcc/cp/ChangeLog:
> 
> 	* module.cc (merge_kind::MK_local_class): New enumerator.
> 	(merge_kind_name): Update.
> 	(trees_out::chained_decls): Move BLOCK-specific handling
> 	of DECL_LOCAL_DECL_P decls to ...
> 	(trees_out::core_vals) <case BLOCK>: ... here.  Stream
> 	BLOCK_VARS manually.
> 	(trees_in::core_vals) <case BLOCK>: Stream BLOCK_VARS
> 	manually.  Handle deduplicated local classes.
> 	(trees_out::key_local_class): Define.
> 	(trees_in::key_local_class): Define.
> 	(trees_out::get_merge_kind) <case FUNCTION_DECL>: Return
> 	MK_local_class for a local class.
> 	(trees_out::key_mergeable) <case FUNCTION_DECL>: Use
> 	key_local_class.
> 	(trees_in::key_mergeable) <case FUNCTION_DECL>: Likewise.
> 	(trees_in::is_matching_decl): Be flexible with type mismatches
> 	for local entities.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/modules/xtreme-header-7_a.H: New test.
> 	* g++.dg/modules/xtreme-header-7_b.C: New test.

> ---
>  gcc/cp/module.cc                              | 167 +++++++++++++++---
>  .../g++.dg/modules/xtreme-header-7_a.H        |   4 +
>  .../g++.dg/modules/xtreme-header-7_b.C        |   6 +
>  3 files changed, 149 insertions(+), 28 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H
>  create mode 100644 gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C
> 
> diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> index fa91c6ff9cb..f77f73a59ed 100644
> --- a/gcc/cp/module.cc
> +++ b/gcc/cp/module.cc
> @@ -2771,6 +2771,7 @@ enum merge_kind
>  
>    MK_enum,	/* Found by CTX, & 1stMemberNAME.  */
>    MK_keyed,     /* Found by key & index.  */
> +  MK_local_class, /* Found by CTX, index.  */
>  
>    MK_friend_spec,  /* Like named, but has a tmpl & args too.  */
>    MK_local_friend, /* Found by CTX, index.  */
> @@ -2799,7 +2800,7 @@ static char const *const merge_kind_name[MK_hwm] =
>      "unique", "named", "field", "vtable",	/* 0...3  */
>      "asbase", "partial", "enum", "attached",	/* 4...7  */
>  
> -    "friend spec", "local friend", NULL, NULL,  /* 8...11 */
> +    "local class", "friend spec", "local friend", NULL,  /* 8...11 */
>      NULL, NULL, NULL, NULL,
>  
>      "type spec", "type tmpl spec",	/* 16,17 type (template).  */
> @@ -2928,6 +2929,7 @@ public:
>    unsigned binfo_mergeable (tree *);
>  
>  private:
> +  tree key_local_class (const merge_key&, tree);
>    uintptr_t *find_duplicate (tree existing);
>    void register_duplicate (tree decl, tree existing);
>    /* Mark as an already diagnosed bad duplicate.  */
> @@ -3086,6 +3088,7 @@ public:
>    void binfo_mergeable (tree binfo);
>  
>  private:
> +  void key_local_class (merge_key&, tree, tree);
>    bool decl_node (tree, walk_kind ref);
>    void type_node (tree);
>    void tree_value (tree);
> @@ -4952,18 +4955,7 @@ void
>  trees_out::chained_decls (tree decls)
>  {
>    for (; decls; decls = DECL_CHAIN (decls))
> -    {
> -      if (VAR_OR_FUNCTION_DECL_P (decls)
> -	  && DECL_LOCAL_DECL_P (decls))
> -	{
> -	  /* Make sure this is the first encounter, and mark for
> -	     walk-by-value.  */
> -	  gcc_checking_assert (!TREE_VISITED (decls)
> -			       && !DECL_TEMPLATE_INFO (decls));
> -	  mark_by_value (decls);
> -	}
> -      tree_node (decls);
> -    }
> +    tree_node (decls);
>    tree_node (NULL_TREE);
>  }
>  
> @@ -6204,7 +6196,21 @@ trees_out::core_vals (tree t)
>  
>        /* DECL_LOCAL_DECL_P decls are first encountered here and
>           streamed by value.  */
> -      chained_decls (t->block.vars);
> +      for (tree decls = t->block.vars; decls; decls = DECL_CHAIN (decls))
> +	{
> +	  if (VAR_OR_FUNCTION_DECL_P (decls)
> +	      && DECL_LOCAL_DECL_P (decls))
> +	    {
> +	      /* Make sure this is the first encounter, and mark for
> +		 walk-by-value.  */
> +	      gcc_checking_assert (!TREE_VISITED (decls)
> +				   && !DECL_TEMPLATE_INFO (decls));
> +	      mark_by_value (decls);
> +	    }
> +	  tree_node (decls);
> +	}
> +      tree_node (NULL_TREE);
> +
>        /* nonlocalized_vars is a middle-end thing.  */
>        WT (t->block.subblocks);
>        WT (t->block.supercontext);
> @@ -6717,7 +6723,34 @@ trees_in::core_vals (tree t)
>      case BLOCK:
>        t->block.locus = state->read_location (*this);
>        t->block.end_locus = state->read_location (*this);
> -      t->block.vars = chained_decls ();
> +
> +      for (tree *chain = &t->block.vars;;)
> +	if (tree decl = tree_node ())
> +	  {
> +	    /* For a deduplicated local class, chain the to-be-discarded
> +	       decl not the in-TU decl (which is already chained to in-TU
> +	       entities).  */
> +	    if (is_duplicate (decl))
> +	      decl = maybe_duplicate (decl);
> +	    else if (DECL_IMPLICIT_TYPEDEF_P (decl)
> +		     && TYPE_TEMPLATE_INFO (TREE_TYPE (decl)))
> +	      {
> +		tree tmpl = TYPE_TI_TEMPLATE (TREE_TYPE (decl));
> +		if (DECL_TEMPLATE_RESULT (tmpl) == decl && is_duplicate (tmpl))
> +		  decl = DECL_TEMPLATE_RESULT (maybe_duplicate (tmpl));
> +	      }
> +
> +	    if (!DECL_P (decl) || DECL_CHAIN (decl))
> +	      {
> +		set_overrun ();
> +		break;
> +	      }
> +	    *chain = decl;
> +	    chain = &DECL_CHAIN (decl);
> +	  }
> +	else
> +	  break;
> +
>        /* nonlocalized_vars is middle-end.  */
>        RT (t->block.subblocks);
>        RT (t->block.supercontext);
> @@ -10335,6 +10368,83 @@ trees_in::fn_parms_fini (int tag, tree fn, tree existing, bool is_defn)
>      }
>  }
>  
> +/* Encode into KEY the position of the local class declaration DECL
> +   within FN.  The position is encoded as the index of the innermost
> +   BLOCK (numbered in BFS order) along with the index within its
> +   BLOCK_VARS list.  */
> +
> +void
> +trees_out::key_local_class (merge_key& key, tree decl, tree fn)
> +{
> +  auto_vec<tree, 4> blocks;
> +  blocks.quick_push (DECL_INITIAL (fn));
> +  unsigned block_ix = 0;
> +  while (block_ix != blocks.length ())
> +    {
> +      tree block = blocks[block_ix];
> +      unsigned decl_ix = 0;
> +      for (tree var = BLOCK_VARS (block); var; var = DECL_CHAIN (var))
> +	{
> +	  if (TREE_CODE (var) != TYPE_DECL)
> +	    continue;
> +	  if (var == decl)
> +	    {
> +	      key.index = (block_ix << 10) | decl_ix;
> +	      return;
> +	    }
> +	  ++decl_ix;
> +	}
> +      for (tree sub = BLOCK_SUBBLOCKS (block); sub; sub = BLOCK_CHAIN (sub))
> +	blocks.safe_push (sub);
> +      ++block_ix;
> +    }
> +
> +  /* Not-found value.  */
> +  key.index = 1023;
> +}
> +
> +/* Look up the local class corresponding at the position encoded by
> +   KEY within FN.  */
> +
> +tree
> +trees_in::key_local_class (const merge_key& key, tree fn)
> +{
> +  if (!DECL_INITIAL (fn))
> +    return NULL_TREE;
> +
> +  const unsigned block_pos = key.index >> 10;
> +  const unsigned decl_pos = key.index & 1023;
> +
> +  if (decl_pos == 1023)
> +    return NULL_TREE;
> +
> +  auto_vec<tree, 4> blocks;
> +  blocks.quick_push (DECL_INITIAL (fn));
> +  unsigned block_ix = 0;
> +  while (block_ix != blocks.length ())
> +    {
> +      tree block = blocks[block_ix];
> +      if (block_ix == block_pos)
> +	{
> +	  unsigned decl_ix = 0;
> +	  for (tree var = BLOCK_VARS (block); var; var = DECL_CHAIN (var))
> +	    {
> +	      if (TREE_CODE (var) != TYPE_DECL)
> +		continue;
> +	      if (decl_ix == decl_pos)
> +		return var;
> +	      ++decl_ix;
> +	    }
> +	  return NULL_TREE;
> +	}
> +      for (tree sub = BLOCK_SUBBLOCKS (block); sub; sub = BLOCK_CHAIN (sub))
> +	blocks.safe_push (sub);
> +      ++block_ix;
> +    }
> +
> +  return NULL_TREE;
> +}
> +
>  /* DEP is the depset of some decl we're streaming by value.  Determine
>     the merging behaviour.  */
>  
> @@ -10454,17 +10564,10 @@ trees_out::get_merge_kind (tree decl, depset *dep)
>  	    gcc_unreachable ();
>  
>  	  case FUNCTION_DECL:
> -	    // FIXME: This can occur for (a) voldemorty TYPE_DECLS
> -	    // (which are returned from a function), or (b)
> -	    // block-scope class definitions in template functions.
> -	    // These are as unique as the containing function.  While
> -	    // on read-back we can discover if the CTX was a
> -	    // duplicate, we don't have a mechanism to get from the
> -	    // existing CTX to the existing version of this decl.
>  	    gcc_checking_assert
>  	      (DECL_IMPLICIT_TYPEDEF_P (STRIP_TEMPLATE (decl)));
>  
> -	    mk = MK_unique;
> +	    mk = MK_local_class;
>  	    break;
>  
>  	  case RECORD_TYPE:
> @@ -10768,6 +10871,10 @@ trees_out::key_mergeable (int tag, merge_kind mk, tree decl, tree inner,
>  	  }
>  	  break;
>  
> +	case MK_local_class:
> +	  key_local_class (key, STRIP_TEMPLATE (decl), container);
> +	  break;
> +
>  	case MK_enum:
>  	  {
>  	    /* Anonymous enums are located by their first identifier,
> @@ -11117,11 +11224,10 @@ trees_in::key_mergeable (int tag, merge_kind mk, tree decl, tree inner,
>  	    break;
>  
>  	  case FUNCTION_DECL:
> -	    // FIXME: What about a voldemort? how do we find what it
> -	    // duplicates? Do we have to number vmorts relative to
> -	    // their containing function?  But how would that work
> -	    // when matching an in-TU declaration?
> -	    kind = "unique";
> +	    gcc_checking_assert (mk == MK_local_class);
> +	    existing = key_local_class (key, container);
> +	    if (existing && inner != decl)
> +	      existing = TYPE_TI_TEMPLATE (TREE_TYPE (existing));
>  	    break;
>  
>  	  case TYPE_DECL:
> @@ -11374,6 +11480,11 @@ trees_in::is_matching_decl (tree existing, tree decl, bool is_typedef)
>  	/* Just like duplicate_decls, presum the user knows what
>  	   they're doing in overriding a builtin.   */
>  	TREE_TYPE (existing) = TREE_TYPE (decl);
> +      else if (decl_function_context (decl))
> +	/* The type of a mergeable local entity (such as a function scope
> +	   capturing lambda's closure type fields) can depend on an
> +	   unmergeable local entity (such as a local variable), so type
> +	   equality isn't feasible in general for local entities.  */;
>        else
>  	{
>  	  // FIXME:QOI Might be template specialization from a module,
> diff --git a/gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H b/gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H
> new file mode 100644
> index 00000000000..bf7859fba99
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/xtreme-header-7_a.H
> @@ -0,0 +1,4 @@
> +// { dg-additional-options -fmodule-header }
> +
> +// { dg-module-cmi {} }
> +#include "xtreme-header.h"
> diff --git a/gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C b/gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C
> new file mode 100644
> index 00000000000..03f3dc1bae6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/xtreme-header-7_b.C
> @@ -0,0 +1,6 @@
> +// A version of xtreme-header_{a.H,b.C} that doesn't pass
> +// -fno-module-lazy.
> +// { dg-additional-options -fmodules-ts }
> +
> +#include "xtreme-header.h"
> +import "xtreme-header-7_a.H";
> -- 

Consider the following minimal testcase added that verifies we now
properly merge local classes (and local enums):

 gcc/testsuite/g++.dg/modules/merge-17.h   | 28 ++++++++++++++++++++++++++++
 gcc/testsuite/g++.dg/modules/merge-17_a.H |  3 +++
 gcc/testsuite/g++.dg/modules/merge-17_b.C |  3 +++
 3 files changed, 34 insertions(+)

diff --git a/gcc/testsuite/g++.dg/modules/merge-17.h b/gcc/testsuite/g++.dg/modules/merge-17.h
new file mode 100644
index 00000000000..246ccd8011d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/merge-17.h
@@ -0,0 +1,28 @@
+// PR c++/99426
+
+inline auto f() {
+  struct A { int m = 42; };
+  return A{};
+}
+
+template<class T>
+inline auto ft() {
+  decltype(+T()) x;
+  return [&x] { };
+}
+
+inline auto g() {
+  enum E { e };
+  return e;
+}
+
+template<class T>
+inline auto gt() {
+  enum E : T { e };
+  return e;
+}
+
+using ty1 = decltype(f());
+using ty2 = decltype(ft<int>());
+using ty3 = decltype(g());
+using ty4 = decltype(gt<int>());
diff --git a/gcc/testsuite/g++.dg/modules/merge-17_a.H b/gcc/testsuite/g++.dg/modules/merge-17_a.H
new file mode 100644
index 00000000000..0440cd765e9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/merge-17_a.H
@@ -0,0 +1,3 @@
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+#include "merge-17.h"
diff --git a/gcc/testsuite/g++.dg/modules/merge-17_b.C b/gcc/testsuite/g++.dg/modules/merge-17_b.C
new file mode 100644
index 00000000000..4315b99f172
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/merge-17_b.C
@@ -0,0 +1,3 @@
+// { dg-additional-options "-fmodules-ts -fno-module-lazy" }
+#include "merge-17.h"
+import "merge-17_a.H";