cxx-ada-explicit-alignment

C++ Ada explicit alignment

The C++ Ada spec dumper omits user-specified record alignment unless the layout is also packed or contains bit fields.

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 mapper preserves packed/bit-field alignment because those paths already emit Ada representation aspects, but it drops an otherwise ordinary C++ type's explicit alignas requirement.

For example:

struct alignas(32) Aligned
{
  int value;
};

The unpatched output keeps only the convention, so GNAT gives the record its natural alignment:

type Aligned is record
   value : aliased int;
end record
with Convention => C_Pass_By_Copy;

The corrected output carries the C++ alignment into the Ada view:

type Aligned is record
   value : aliased int;
end record
with Convention => C_Pass_By_Copy,
     Alignment => 32;

The patch keeps Pack limited to packed or bit-field layouts and emits Alignment independently when GCC records a user-specified type alignment. The executable regression links C++ and Ada at -O0 and -O2 and compares sizeof, alignof, Ada Object_Size, and Ada Alignment.

Patch.

Variant gcc-13-14

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

+25 −2 2 files

SHA-256
aa50ec3ed9c2ca4f5f03f406add2d9480e9052908e0316d388a1e713574d14bb
Patch
bundles/cxx-ada-explicit-alignment/patches/gcc-13-14.patch

Download the patch

gcc/c-family/c-ada-spec.cc +12−2modified
@@ -3440 +3440 @@dump_ada_structure (pretty_printer *buffer, tree node, tree type, bool nested,
34403440 pp_string (buffer, "Unchecked_Union => True");
34413441 }
34423442
3443 if (bitfield_used || packed_layout)
3443 const bool needs_pack = bitfield_used || packed_layout;
3444
3445 if (needs_pack)
34443446 {
3445 char buf[32];
34463447 pp_comma (buffer);
34473448 newline_and_indent (buffer, spc + 5);
34483449 pp_string (buffer, "Pack => True");
3450 }
3451
3452 if (needs_pack || TYPE_USER_ALIGN (node))
3453 {
3454 char buf[32];
34493455 pp_comma (buffer);
34503456 newline_and_indent (buffer, spc + 5);
34513457 sprintf (buf, "Alignment => %d", TYPE_ALIGN (node) / BITS_PER_UNIT);
34523458 pp_string (buffer, buf);
3459 }
3460
3461 if (needs_pack)
3462 {
34533463 bitfield_used = false;
34543464 packed_layout = false;
34553465 }
gcc/testsuite/g++.dg/ada-spec/explicit-alignment.C +13−0new file
@@ -0 +1 @@
1/* { dg-do compile } */
2/* { dg-options "-fdump-ada-spec-slim" } */
3/* { dg-final { scan-file explicit_alignment_c.ads "Alignment => 32" } } */
4
5struct alignas(32) Aligned
6{
7 int value;
8};
9
10extern "C" unsigned long cpp_size () { return sizeof (Aligned); }
11extern "C" unsigned long cpp_alignment () { return alignof (Aligned); }
12
13/* { dg-final { cleanup-ada-spec } } */
14

Variant gcc-15-16

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

+25 −2 2 files

SHA-256
ce99dbd96103e0cf6b4303cc5a4ab694135b28faca7d5e631bc9aca5a15206ad
Patch
bundles/cxx-ada-explicit-alignment/patches/gcc-15-16.patch

Download the patch

gcc/c-family/c-ada-spec.cc +12−2modified
@@ -3507 +3507 @@dump_ada_structure (pretty_printer *pp, tree node, tree type, bool nested,
35073507 pp_string (pp, "Unchecked_Union => True");
35083508 }
35093509
3510 if (bitfield_used || packed_layout)
3510 const bool needs_pack = bitfield_used || packed_layout;
3511
3512 if (needs_pack)
35113513 {
3512 char buf[32];
35133514 pp_comma (pp);
35143515 newline_and_indent (pp, spc + 5);
35153516 pp_string (pp, "Pack => True");
3517 }
3518
3519 if (needs_pack || TYPE_USER_ALIGN (node))
3520 {
3521 char buf[32];
35163522 pp_comma (pp);
35173523 newline_and_indent (pp, spc + 5);
35183524 sprintf (buf, "Alignment => %d", TYPE_ALIGN (node) / BITS_PER_UNIT);
35193525 pp_string (pp, buf);
3526 }
3527
3528 if (needs_pack)
3529 {
35203530 bitfield_used = false;
35213531 packed_layout = false;
35223532 }
gcc/testsuite/g++.dg/ada-spec/explicit-alignment.C +13−0new file
@@ -0 +1 @@
1/* { dg-do compile } */
2/* { dg-options "-fdump-ada-spec-slim" } */
3/* { dg-final { scan-file explicit_alignment_c.ads "Alignment => 32" } } */
4
5struct alignas(32) Aligned
6{
7 int value;
8};
9
10extern "C" unsigned long cpp_size () { return sizeof (Aligned); }
11extern "C" unsigned long cpp_alignment () { return alignof (Aligned); }
12
13/* { dg-final { cleanup-ada-spec } } */
14

Tests.

Before the patch
The generated Ada type uses natural alignment and disagrees with C++ sizeof and alignof.
After the patch
The generated Alignment aspect makes Ada Object_Size and Alignment agree with C++ at -O0 and -O2.
explicit-alignment.C C++ · 13 lines 
/* { dg-do compile } */
/* { dg-options "-fdump-ada-spec-slim" } */
/* { dg-final { scan-file explicit_alignment_c.ads "Alignment => 32" } } */

struct alignas(32) Aligned
{
  int value;
};

extern "C" unsigned long cpp_size () { return sizeof (Aligned); }
extern "C" unsigned long cpp_alignment () { return alignof (Aligned); }

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

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explicit_alignment_consumer.adb Ada · 20 lines 
with Ada.Text_IO;
with Explicit_Alignment_C;
with Interfaces.C; use Interfaces.C;
with System;

procedure Explicit_Alignment_Consumer is
   package Bindings renames Explicit_Alignment_C;
   Ada_Size : constant unsigned_long :=
     unsigned_long (Bindings.Aligned'Object_Size / System.Storage_Unit);
   Ada_Alignment : constant unsigned_long :=
     unsigned_long (Bindings.Aligned'Alignment);
begin
   if Ada_Size = Bindings.cpp_size
     and then Ada_Alignment = Bindings.cpp_alignment
   then
      Ada.Text_IO.Put_Line ("MATCH C++ Ada explicit alignment");
   else
      Ada.Text_IO.Put_Line ("MISMATCH C++ Ada explicit alignment");
   end if;
end Explicit_Alignment_Consumer;

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run-test.sh shell · 61 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="$root/bundles/cxx-ada-explicit-alignment/tests/explicit-alignment.C"
ada_fixture="$root/bundles/cxx-ada-explicit-alignment/tests/explicit_alignment_consumer.adb"
work=$(mktemp -d "${TMPDIR:-/tmp}/gnat-cxx-ada-alignment-test.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[@]}" "$gxx" -c "-O$optimization" \
      -fdump-ada-spec-slim explicit-alignment.C
    "${REGRESSION_ENV[@]}" "$REGRESSION_GNATMAKE" -q -f \
      "-O$optimization" explicit_alignment_consumer.adb \
      -largs explicit-alignment.o -lstdc++
    "${REGRESSION_ENV[@]}" ./explicit_alignment_consumer
  ) >"$case_dir/output.log" 2>&1 || {
    cat "$case_dir/output.log"
    exit 1
  }

  spec="$case_dir/explicit_alignment_c.ads"
  if [[ "$state" == unpatched ]]; then
    grep -F "MISMATCH C++ Ada explicit alignment" "$case_dir/output.log"
    if grep -Fq "Alignment => 32" "$spec"; then
      echo "error: unpatched mapper unexpectedly preserved explicit alignment" >&2
      exit 1
    fi
    echo "cxx-ada-explicit-alignment -O$optimization: expected mismatch (GCC $version)"
    continue
  fi

  grep -F "Alignment => 32" "$spec"
  grep -F "MATCH C++ Ada explicit alignment" "$case_dir/output.log"
  echo "cxx-ada-explicit-alignment -O$optimization: patched (GCC $version)"
done

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Commands.

Apply the patch
patch --fuzz=0 -p1 -i bundles/cxx-ada-explicit-alignment/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/explicit-alignment.C
Regression tests
  • gcc/testsuite/g++.dg/ada-spec/explicit-alignment.C
Before the patch
The generated Ada type uses natural alignment and disagrees with C++ sizeof and alignof.
After the patch
The generated Alignment aspect makes Ada Object_Size and Alignment agree with 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-explicit-alignment/tests/explicit-alignment.C
Fixture SHA-256
d9882744731168b6b8d1bd6f68f092c35b8a738fb865e8861c452b77b727442e
Test runner
bundles/cxx-ada-explicit-alignment/run-test.sh