cxx-ada-char8-type

C++ Ada char8_t mapping

The C++ Ada spec dumper emits char8_t references even though GNAT's Interfaces.C has no such Ada type.

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.

C++20 gives char8_t a distinct fundamental type with the size, alignment, and unsigned representation of unsigned char. The Ada spec dumper prints the C++ spelling directly, but GNAT's Interfaces.C has no char8_t declaration.

The offending C++ is a function with a char8_t parameter and result:

char8_t
char8_value (char8_t value)
{
  return value;
}

The unpatched mapper produces an Ada type name that does not exist:

function char8_value (value : char8_t) return char8_t
with Import => True,
     Convention => CPP,
     External_Name => "_Z11char8_valueDu";

The correct generated binding uses the ABI-equivalent type already provided by Interfaces.C while retaining the C++ mangled name that identifies char8_t:

function char8_value (value : unsigned_char) return unsigned_char
with Import => True,
     Convention => CPP,
     External_Name => "_Z11char8_valueDu";

The executable regression compiles the generated Ada and round-trips a value through the C++ function at -O0 and -O2. The existing wchar_t, char16_t, and char32_t mappings remain unchanged.

Run it against an unpatched or patched compiler root:

./bundles/cxx-ada-char8-type/run-test.sh \
  TOOLCHAIN_ROOT GCC_VERSION unpatched
./bundles/cxx-ada-char8-type/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.

+19 −0 2 files

SHA-256
7167ebbb4862a00b75e69f3d2496892a71e768659740fbb1ae0cd2038daeae4f
Patch
bundles/cxx-ada-char8-type/patches/gcc-13-14.patch

Download the patch

gcc/c-family/c-ada-spec.cc +7−0modified
@@ -2189 +2189 @@dump_ada_node (pretty_printer *buffer, tree node, tree type, int spc,
21892189 case FIXED_POINT_TYPE:
21902190 case BOOLEAN_TYPE:
21912191 if (TYPE_NAME (node)
2192 && ((TREE_CODE (TYPE_NAME (node)) == IDENTIFIER_NODE
2193 && id_equal (TYPE_NAME (node), "char8_t"))
2194 || (TREE_CODE (TYPE_NAME (node)) == TYPE_DECL
2195 && DECL_NAME (TYPE_NAME (node))
2196 && id_equal (DECL_NAME (TYPE_NAME (node)), "char8_t"))))
2197 pp_string (buffer, "unsigned_char");
2198 else if (TYPE_NAME (node)
21922199 && !(TREE_CODE (TYPE_NAME (node)) == TYPE_DECL
21932200 && !strcmp (IDENTIFIER_POINTER (DECL_NAME (TYPE_NAME (node))),
21942201 "__int128")))
gcc/testsuite/g++.dg/ada-spec/char8-type.C +12−0new file
@@ -0 +1 @@
1/* { dg-do compile { target c++20 } } */
2/* { dg-options "-fdump-ada-spec-slim" } */
3/* { dg-final { scan-file char8_type_c.ads "value : unsigned_char" } } */
4/* { dg-final { scan-file char8_type_c.ads "return unsigned_char" } } */
5
6char8_t
7char8_value (char8_t value)
8{
9 return value;
10}
11
12/* { dg-final { cleanup-ada-spec } } */
13

Variant gcc-15-16

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

+19 −0 2 files

SHA-256
398e5d2774547c0f45d17c7253839ebe13e7c8b9b2868ecd06edff1c31196aeb
Patch
bundles/cxx-ada-char8-type/patches/gcc-15-16.patch

Download the patch

gcc/c-family/c-ada-spec.cc +7−0modified
@@ -2254 +2254 @@dump_ada_node (pretty_printer *pp, tree node, tree type, int spc,
22542254 case FIXED_POINT_TYPE:
22552255 case BOOLEAN_TYPE:
22562256 if (TYPE_NAME (node)
2257 && ((TREE_CODE (TYPE_NAME (node)) == IDENTIFIER_NODE
2258 && id_equal (TYPE_NAME (node), "char8_t"))
2259 || (TREE_CODE (TYPE_NAME (node)) == TYPE_DECL
2260 && DECL_NAME (TYPE_NAME (node))
2261 && id_equal (DECL_NAME (TYPE_NAME (node)), "char8_t"))))
2262 pp_string (pp, "unsigned_char");
2263 else if (TYPE_NAME (node)
22572264 && !(TREE_CODE (TYPE_NAME (node)) == TYPE_DECL
22582265 && !strncmp (IDENTIFIER_POINTER (DECL_NAME (TYPE_NAME (node))),
22592266 "__int128", 8)))
gcc/testsuite/g++.dg/ada-spec/char8-type.C +12−0new file
@@ -0 +1 @@
1/* { dg-do compile { target c++20 } } */
2/* { dg-options "-fdump-ada-spec-slim" } */
3/* { dg-final { scan-file char8_type_c.ads "value : unsigned_char" } } */
4/* { dg-final { scan-file char8_type_c.ads "return unsigned_char" } } */
5
6char8_t
7char8_value (char8_t value)
8{
9 return value;
10}
11
12/* { dg-final { cleanup-ada-spec } } */
13

Tests.

Before the patch
GNAT rejects the generated specification because char8_t is undefined.
After the patch
The binding uses Interfaces.C.unsigned_char and round-trips a char8_t value through C++ at -O0 and -O2.
char8-type.C C++ · 12 lines 
/* { dg-do compile { target c++20 } } */
/* { dg-options "-fdump-ada-spec-slim" } */
/* { dg-final { scan-file char8_type_c.ads "value : unsigned_char" } } */
/* { dg-final { scan-file char8_type_c.ads "return unsigned_char" } } */

char8_t
char8_value (char8_t value)
{
  return value;
}

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

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char8_type_consumer.adb Ada · 11 lines 
with Interfaces.C; use Interfaces.C;
with Char8_Type_C;

procedure Char8_Type_Consumer is
   package Bindings renames Char8_Type_C;
   Value : constant unsigned_char := 16#A5#;
begin
   if Bindings.char8_value (Value) /= Value then
      raise Program_Error with "char8_t value did not round-trip";
   end if;
end Char8_Type_Consumer;

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run-test.sh shell · 77 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="$root/bundles/cxx-ada-char8-type/tests/char8-type.C"
ada_fixture="$root/bundles/cxx-ada-char8-type/tests/char8_type_consumer.adb"
work=$(mktemp -d "${TMPDIR:-/tmp}/gnat-cxx-ada-char8-type.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 char8-type.C
  )

  spec="$case_dir/char8_type_c.ads"
  [[ -f "$spec" ]] || {
    echo "error: g++ did not generate $spec" >&2
    exit 1
  }

  set +e
  (
    cd "$case_dir"
    "${REGRESSION_ENV[@]}" "$REGRESSION_GNATMAKE" -q -f \
      "-O$optimization" char8_type_consumer.adb \
      -largs char8-type.o -lstdc++
    "${REGRESSION_ENV[@]}" ./char8_type_consumer
  ) >"$case_dir/run.log" 2>&1
  run_status=$?
  set -e

  if [[ "$state" == unpatched ]]; then
    [[ $run_status -ne 0 ]] || {
      echo "error: unpatched char8_t regression unexpectedly passed" >&2
      exit 1
    }
    grep -Eiq 'char8_t.*undefined' "$case_dir/run.log" || {
      cat "$case_dir/run.log"
      exit 1
    }
    echo "cxx-ada-char8-type -O$optimization: expected undefined type (GCC $version)"
    continue
  fi

  [[ $run_status -eq 0 ]] || {
    cat "$case_dir/run.log"
    exit 1
  }
  grep -Eq 'value : unsigned_char' "$spec"
  grep -Eq 'return unsigned_char' "$spec"
  echo "cxx-ada-char8-type -O$optimization: patched (GCC $version)"
done

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Commands.

Apply the patch
patch --fuzz=0 -p1 -i bundles/cxx-ada-char8-type/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/char8-type.C
Regression tests
  • gcc/testsuite/g++.dg/ada-spec/char8-type.C
Before the patch
GNAT rejects the generated specification because char8_t is undefined.
After the patch
The binding uses Interfaces.C.unsigned_char and round-trips a char8_t value through 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; 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-char8-type/tests/char8-type.C
Fixture SHA-256
e7e136cb00f5b735a27c5b2d8f201063345c5022dff689045cbfc8bb0382afa8
Test runner
bundles/cxx-ada-char8-type/run-test.sh