cxx-ada-member-pointers

C++ Ada pointers to members

C++ data-member and member-function pointer representations contain blank underlying Ada types and make the generated specification invalid.

AcceptedApplies standaloneSince 1.2.0

Bundle JSON Repository directory

Where it applies.

How each patchset treats this bundle on each GCC major
PatchsetGCC 13GCC 14GCC 15GCC 16
1.2.0 (latest)Patchedgcc-13-1413.2.0Patchedgcc-13-1414.2.0Patchedgcc-15-1615.3.0Patchedgcc-15-1616.2.0
1.1.0Not applicable13.2.0Not applicable14.2.0Not applicable15.3.0Not applicable16.2.0
1.0.1Not applicable13.2.0Not applicable14.2.0Not applicable15.3.0Not applicable16.1.0
1.0.0Not applicable13.2.0Not applicable14.2.0Not applicable15.3.0Not applicable16.1.0

Explanation.

The C++ front end uses two related internal representations for pointers to members. A data-member pointer is an OFFSET_TYPE; a member-function pointer is an ABI record containing a pointer-to-METHOD_TYPE and a this adjustment. The Ada dumper handles neither internal type, so both forms produce malformed declarations.

The offending C++ forms are:

struct Data_Object { int first; int field; };
using Data_Member = int Data_Object::*;

struct Method_Object {
  virtual int virtual_method (int) const;
  int nonvirtual_method (int) const;
};
using Method_Member = int (Method_Object::*) (int) const;

The unpatched mapper leaves both underlying types blank:

subtype Data_Member is ;

type Method_Member is record
   uu_pfn : access ;
   uu_delta : aliased long;
end record
with Convention => C_Pass_By_Copy;

On the Itanium C++ ABI used by every supported repository host, a data-member pointer is a ptrdiff_t byte offset with -1 reserved for null. A member-function pointer is two words: the first is either a function address or an encoded virtual-table offset, and the second adjusts this. The corrected Ada preserves those ABI values without claiming that Ada can dereference them:

subtype Data_Member is ptrdiff_t;

type Method_Member is record
   uu_pfn : System.Address;
   uu_delta : aliased long;
end record
with Convention => C_Pass_By_Copy;

System.Address is deliberately opaque: a virtual member-function encoding is not an Ada access-to-subprogram value. Invocation still belongs in C++, but the generated Ada can safely store and pass the complete value. The executable regression obtains data, nonvirtual-function, virtual-function, and null member pointers from C++, round-trips them through Ada, and asks C++ to apply or classify them at -O0 and -O2.

Run it against an unpatched or patched compiler root:

./bundles/cxx-ada-member-pointers/run-test.sh \
  TOOLCHAIN_ROOT GCC_VERSION unpatched
./bundles/cxx-ada-member-pointers/run-test.sh \
  TOOLCHAIN_ROOT GCC_VERSION patched

Patch.

Variant gcc-13-14

Applies to 13.2.0, 14.2.0. Source flavors: linux, darwin_arm64.

+60 −0 2 files

SHA-256
0f9516b6baedf127ba1dae85228ec5df9b8701c7fe36d0b89ef43b31238e4d7a
Patch
bundles/cxx-ada-member-pointers/patches/gcc-13-14.patch

Download the patch

gcc/c-family/c-ada-spec.cc +15−0modified
@@ -2129 +2129 @@dump_ada_node (pretty_printer *buffer, tree node, tree type, int spc,
21292129 pp_string (buffer, "address");
21302130 break;
21312131
2132 case OFFSET_TYPE:
2133 /* The Itanium C++ ABI represents a data-member pointer as a ptrdiff_t
2134 offset, with -1 reserved for null. */
2135 pp_string (buffer, "ptrdiff_t");
2136 break;
2137
21322138 case VECTOR_TYPE:
21332139 pp_string (buffer, "<vector>");
21342140 break;
@@ -2227 +2233 @@dump_ada_node (pretty_printer *buffer, tree node, tree type, int spc,
22272233 dump_ada_node (buffer, TYPE_NAME (node), node, spc, limited_access,
22282234 true);
22292235
2236 else if (TREE_CODE (TREE_TYPE (node)) == METHOD_TYPE)
2237 {
2238 /* A member-function pointer's first word is either a function pointer
2239 or an encoded virtual-table offset, so it is not an Ada access-to-
2240 subprogram value. Preserve it as an opaque address-sized word. */
2241 append_withs ("System", false);
2242 pp_string (buffer, "System.Address");
2243 }
2244
22302245 else if (TREE_CODE (TREE_TYPE (node)) == FUNCTION_TYPE)
22312246 {
22322247 if (VOID_TYPE_P (TREE_TYPE (TREE_TYPE (node))))
gcc/testsuite/g++.dg/ada-spec/member-pointers.C +45−0new file
@@ -0 +1 @@
1/* { dg-do compile } */
2/* { dg-options "-std=gnu++20 -fdump-ada-spec-slim" } */
3/* { dg-final { scan-file member_pointers_c.ads "subtype Data_Member is ptrdiff_t;" } } */
4/* { dg-final { scan-file member_pointers_c.ads "uu_pfn : System.Address;" } } */
5/* { dg-final { scan-file member_pointers_c.ads "uu_delta : aliased long;" } } */
6
7struct Data_Object
8{
9 int first;
10 int field;
11};
12
13using Data_Member = int Data_Object::*;
14
15struct Method_Object
16{
17 virtual int virtual_method (int value) const { return value + 20; }
18 int nonvirtual_method (int value) const { return value + 10; }
19};
20
21using Method_Member = int (Method_Object::*) (int) const;
22
23Data_Member get_data_member () { return &Data_Object::field; }
24Data_Member get_null_data_member () { return nullptr; }
25int apply_data_member (Data_Member member)
26{
27 Data_Object object { 1, 42 };
28 return object.*member;
29}
30bool is_null_data_member (Data_Member member) { return member == nullptr; }
31
32Method_Member get_nonvirtual_method ()
33{
34 return &Method_Object::nonvirtual_method;
35}
36Method_Member get_virtual_method () { return &Method_Object::virtual_method; }
37Method_Member get_null_method () { return nullptr; }
38int apply_method (Method_Member member, int value)
39{
40 Method_Object object;
41 return (object.*member) (value);
42}
43bool is_null_method (Method_Member member) { return member == nullptr; }
44
45/* { dg-final { cleanup-ada-spec } } */
46

Variant gcc-15-16

Applies to 15.3.0, 16.1.0, 16.2.0. Source flavors: linux, darwin_arm64.

+60 −0 2 files

SHA-256
d814882b5f602aa065f08a2b31ba6a1105b524e51d5d9a5616a885e7da19ba8d
Patch
bundles/cxx-ada-member-pointers/patches/gcc-15-16.patch

Download the patch

gcc/c-family/c-ada-spec.cc +15−0modified
@@ -2189 +2189 @@dump_ada_node (pretty_printer *pp, tree node, tree type, int spc,
21892189 pp_string (pp, "address");
21902190 break;
21912191
2192 case OFFSET_TYPE:
2193 /* The Itanium C++ ABI represents a data-member pointer as a ptrdiff_t
2194 offset, with -1 reserved for null. */
2195 pp_string (pp, "ptrdiff_t");
2196 break;
2197
21922198 case VECTOR_TYPE:
21932199 pp_string (pp, "<vector>");
21942200 break;
@@ -2292 +2298 @@dump_ada_node (pretty_printer *pp, tree node, tree type, int spc,
22922298 dump_ada_node (pp, TYPE_NAME (node), node, spc, limited_access,
22932299 true);
22942300
2301 else if (TREE_CODE (TREE_TYPE (node)) == METHOD_TYPE)
2302 {
2303 /* A member-function pointer's first word is either a function pointer
2304 or an encoded virtual-table offset, so it is not an Ada access-to-
2305 subprogram value. Preserve it as an opaque address-sized word. */
2306 append_withs ("System", false);
2307 pp_string (pp, "System.Address");
2308 }
2309
22952310 else if (TREE_CODE (TREE_TYPE (node)) == FUNCTION_TYPE)
22962311 {
22972312 if (VOID_TYPE_P (TREE_TYPE (TREE_TYPE (node))))
gcc/testsuite/g++.dg/ada-spec/member-pointers.C +45−0new file
@@ -0 +1 @@
1/* { dg-do compile } */
2/* { dg-options "-std=gnu++20 -fdump-ada-spec-slim" } */
3/* { dg-final { scan-file member_pointers_c.ads "subtype Data_Member is ptrdiff_t;" } } */
4/* { dg-final { scan-file member_pointers_c.ads "uu_pfn : System.Address;" } } */
5/* { dg-final { scan-file member_pointers_c.ads "uu_delta : aliased long;" } } */
6
7struct Data_Object
8{
9 int first;
10 int field;
11};
12
13using Data_Member = int Data_Object::*;
14
15struct Method_Object
16{
17 virtual int virtual_method (int value) const { return value + 20; }
18 int nonvirtual_method (int value) const { return value + 10; }
19};
20
21using Method_Member = int (Method_Object::*) (int) const;
22
23Data_Member get_data_member () { return &Data_Object::field; }
24Data_Member get_null_data_member () { return nullptr; }
25int apply_data_member (Data_Member member)
26{
27 Data_Object object { 1, 42 };
28 return object.*member;
29}
30bool is_null_data_member (Data_Member member) { return member == nullptr; }
31
32Method_Member get_nonvirtual_method ()
33{
34 return &Method_Object::nonvirtual_method;
35}
36Method_Member get_virtual_method () { return &Method_Object::virtual_method; }
37Method_Member get_null_method () { return nullptr; }
38int apply_method (Method_Member member, int value)
39{
40 Method_Object object;
41 return (object.*member) (value);
42}
43bool is_null_method (Method_Member member) { return member == nullptr; }
44
45/* { dg-final { cleanup-ada-spec } } */
46

Tests.

Before the patch
GNAT rejects blank data-member and member-function pointer component types.
After the patch
Data, nonvirtual-function, virtual-function, and null member pointers round-trip through Ada and are applied or classified correctly by C++ at -O0 and -O2.
member-pointers.C C++ · 45 lines 
/* { dg-do compile } */
/* { dg-options "-std=gnu++20 -fdump-ada-spec-slim" } */
/* { dg-final { scan-file member_pointers_c.ads "subtype Data_Member is ptrdiff_t;" } } */
/* { dg-final { scan-file member_pointers_c.ads "uu_pfn : System.Address;" } } */
/* { dg-final { scan-file member_pointers_c.ads "uu_delta : aliased long;" } } */

struct Data_Object
{
  int first;
  int field;
};

using Data_Member = int Data_Object::*;

struct Method_Object
{
  virtual int virtual_method (int value) const { return value + 20; }
  int nonvirtual_method (int value) const { return value + 10; }
};

using Method_Member = int (Method_Object::*) (int) const;

Data_Member get_data_member () { return &Data_Object::field; }
Data_Member get_null_data_member () { return nullptr; }
int apply_data_member (Data_Member member)
{
  Data_Object object { 1, 42 };
  return object.*member;
}
bool is_null_data_member (Data_Member member) { return member == nullptr; }

Method_Member get_nonvirtual_method ()
{
  return &Method_Object::nonvirtual_method;
}
Method_Member get_virtual_method () { return &Method_Object::virtual_method; }
Method_Member get_null_method () { return nullptr; }
int apply_method (Method_Member member, int value)
{
  Method_Object object;
  return (object.*member) (value);
}
bool is_null_method (Method_Member member) { return member == nullptr; }

/* { dg-final { cleanup-ada-spec } } */

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member_pointers_consumer.adb Ada · 28 lines 
with Interfaces.C; use Interfaces.C;
with Interfaces.C.Extensions;
with Member_Pointers_C;

procedure Member_Pointers_Consumer is
   package Bindings renames Member_Pointers_C;
   use type Interfaces.C.Extensions.bool;

   Data : constant Bindings.Data_Member := Bindings.get_data_member;
   Null_Data : constant Bindings.Data_Member := Bindings.get_null_data_member;
   Nonvirtual : constant Bindings.Method_Member :=
     Bindings.get_nonvirtual_method;
   Virtual : constant Bindings.Method_Member := Bindings.get_virtual_method;
   Null_Method : constant Bindings.Method_Member := Bindings.get_null_method;
begin
   if Bindings.apply_data_member (Data) /= 42
     or else Bindings.is_null_data_member (Data)
       /= Interfaces.C.Extensions.bool'(False)
     or else Bindings.is_null_data_member (Null_Data)
       /= Interfaces.C.Extensions.bool'(True)
     or else Bindings.apply_method (Nonvirtual, 5) /= 15
     or else Bindings.apply_method (Virtual, 5) /= 25
     or else Bindings.is_null_method (Null_Method)
       /= Interfaces.C.Extensions.bool'(True)
   then
      raise Program_Error with "pointer-to-member ABI mismatch";
   end if;
end Member_Pointers_Consumer;

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run-test.sh shell · 79 lines 
#!/usr/bin/env bash
set -euo pipefail

if [[ $# -ne 3 ]]; then
  echo "usage: $0 TOOLCHAIN_ROOT GCC_VERSION unpatched|patched" >&2
  exit 2
fi

root=$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)
source "$root/scripts/regression-common.sh"
resolve_regression_toolchain "$1"
version=$2
state=$3
[[ "$state" == unpatched || "$state" == patched ]] || {
  echo "error: state must be unpatched or patched" >&2
  exit 2
}

gxx="$REGRESSION_TOOLCHAIN/bin/g++"
[[ -x "$gxx" ]] || {
  echo "error: no g++ in $REGRESSION_TOOLCHAIN" >&2
  exit 1
}

cxx_fixture="$root/bundles/cxx-ada-member-pointers/tests/member-pointers.C"
ada_fixture="$root/bundles/cxx-ada-member-pointers/tests/member_pointers_consumer.adb"
work=$(mktemp -d "${TMPDIR:-/tmp}/gnat-cxx-ada-member-pointers.XXXXXX")
trap 'rm -rf "$work"' EXIT

for optimization in 0 2; do
  case_dir="$work/O$optimization"
  mkdir -p "$case_dir"
  cp "$cxx_fixture" "$ada_fixture" "$case_dir/"
  (
    cd "$case_dir"
    "${REGRESSION_ENV[@]}" "$gxx" -std=gnu++20 -c "-O$optimization" \
      -fdump-ada-spec-slim member-pointers.C
  )

  spec="$case_dir/member_pointers_c.ads"
  [[ -f "$spec" ]] || {
    echo "error: g++ did not generate $spec" >&2
    exit 1
  }

  set +e
  (
    cd "$case_dir"
    "${REGRESSION_ENV[@]}" "$REGRESSION_GNATMAKE" -q -f \
      "-O$optimization" member_pointers_consumer.adb \
      -largs member-pointers.o -lstdc++
    "${REGRESSION_ENV[@]}" ./member_pointers_consumer
  ) >"$case_dir/run.log" 2>&1
  run_status=$?
  set -e

  if [[ "$state" == unpatched ]]; then
    [[ $run_status -ne 0 ]] || {
      echo "error: unpatched member-pointer regression unexpectedly passed" >&2
      exit 1
    }
    grep -Eiq 'missing operand|extra.*;|identifier expected' \
      "$case_dir/run.log" || {
      cat "$case_dir/run.log"
      exit 1
    }
    echo "cxx-ada-member-pointers -O$optimization: expected invalid Ada (GCC $version)"
    continue
  fi

  [[ $run_status -eq 0 ]] || {
    cat "$case_dir/run.log"
    exit 1
  }
  grep -Eq 'subtype Data_Member is ptrdiff_t;' "$spec"
  grep -Eq 'uu_pfn : System.Address;' "$spec"
  grep -Eq 'uu_delta : aliased long;' "$spec"
  echo "cxx-ada-member-pointers -O$optimization: patched (GCC $version)"
done

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Commands.

Apply the patch
patch --fuzz=0 -p1 -i bundles/cxx-ada-member-pointers/patches/VARIANT.patch
Build the compiler
PATH=/path/to/bootstrap/bin:$PATH ./scripts/build-gnat.sh SOURCE BUILD INSTALL
Run the regression
./scripts/run-regressions.sh INSTALL 1.2.0 GCC_MAJOR patched

Metadata.

Affected releases
  • 13.2.0
  • 14.2.0
  • 15.3.0
  • 16.1.0
  • 16.2.0
Known-good releases

None.

Files modified
  • gcc/c-family/c-ada-spec.cc
  • gcc/testsuite/g++.dg/ada-spec/member-pointers.C
Regression tests
  • gcc/testsuite/g++.dg/ada-spec/member-pointers.C
Before the patch
GNAT rejects blank data-member and member-function pointer component types.
After the patch
Data, nonvirtual-function, virtual-function, and null member pointers round-trip through Ada and are applied or classified correctly by C++ at -O0 and -O2.
Upstream issue
Pending GCC Bugzilla filing.
Upstream submission
Not yet filed or submitted upstream.
Provenance
Repository-authored GCC patch and executable regression for the Itanium C++ ABI used by all supported hosts; exact application verified locally against the pinned Darwin release identities. Pinned FSF release application is required by Linux CI.
Licensing
GCC-derived compiler hunks remain GPL-3.0-or-later. The new test is proposed for the GCC testsuite under GCC project terms.
Repository fixture
bundles/cxx-ada-member-pointers/tests/member-pointers.C
Fixture SHA-256
506f9ec81a7386556d4ef646c625cbedf9637e871f3ed6f6fcb2deada9e37f3e
Test runner
bundles/cxx-ada-member-pointers/run-test.sh