C-Generating Back End - Incorrect CFront ABI Layout for Base Class Subobject in Microsoft Mode #479

Issue Summary

When using the CFront-ABI, the generated C does not respect the alignas specifier semantics of Microsoft-mode.

When using the IA64-ABI, the generated C code is correct.

Reproducing Source Code

struct B { alignas(16) char c; };
struct D : B { char d; };

extern "C" int printf(const char *, ...);

int main() {
  D x;
  long fe_offset = __builtin_offsetof(D, d);  // Front end's layout.
  long be_offset = (char *)&x.d - (char *)&x;  // Lowered C's layout.
  printf("%ld %ld\n", fe_offset, be_offset);
  return fe_offset != be_offset;
}

Command-line Options

cpfe --ms_c++20 --target linux_aarch64 test.cpp

Type of Issue

[Back End] There is a problem with the generated C code.

Additional Details

The reduction in size is caused by the following logic in layout.c:

#if MICROSOFT_EXTENSIONS_ALLOWED
    if (microsoft_mode) {
      a_targ_alignment  max_alignment_for_base_class;
      /* MSVC ignores explicitly-specified alignment requirements that are more
         strict than platform-specific defaults when calculating the size of a
         a base class. */
      max_alignment_for_base_class = current_max_alignment_for_class_members();
      if (max_alignment_for_base_class == 0) {
        if (target_is_arm_based() || !target_is_64_bits()) {
          max_alignment_for_base_class = 8;
        } else {
          max_alignment_for_base_class = 16;
        }  /* if */
      }  /* if */
      if (max_alignment_for_base_class < (target_is_64_bits() ? 16u : 8u) &&
          max_alignment_for_base_class < class_type->alignment) {
        ...
        if (do_alignment(&end_of_fields_byte_offset, &bit_offset,
                         max_alignment_for_base_class)) {
          ctsp->size_without_virtual_base_classes = end_of_fields_byte_offset;

This was observed due to variance in processing of the test edg/cmeerw/gnats-27181-run.sft.cpp between IA64 and CFront ABIs for #460.

Reproducing (Standard) Configuration(s)

No response

Reproducing (Non-standard) Configuration

Any config with IA64_ABI=0 and Microsoft mode support on non-x86_64 targets.

I acknowledge that:

• I have checked open issues and existing discussions before filing this report.
• The reproducing source code that I have provided does not depend on headers that I have not included with my submission.
• I have reduced the reproducing source code to the best of my ability.
• I have selected at least one standard configuration or uploaded a non-standard configuration dump.
• I understand code shared via this form is NOT confidential.
• I have permission to share the reproducing source code and grant permission for the project to use the source code (or derivatives of the source code) to create tests.
• I agree that any tests created using the reproducing source code (or deriatives of it) will be licensed under project license unless I included a LICENSE.txt file in my .zip or .tar.gz upload. If I included a LICENSE.txt file, any tests created using the reproducing source code will be governed by the terms of the license I provided instead.