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c++: Fix ABI issue with alignas on armv7hl [PR94050]

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The static_assert in the following test was failing on armv7hl because
we were disregarding the alignas specifier on Cell.  BaseShape's data
takes up 20B on 32-bit architectures, but we failed to round up its
TYPE_SIZE.  This happens since the
<https://gcc.gnu.org/ml/gcc-patches/2019-06/msg01189.html>
patch: here, in layout_class_type for TenuredCell, we see that the size
of TenuredCell and its CLASSTYPE_AS_BASE match, so we set

  CLASSTYPE_AS_BASE (t) = t;

While TYPE_USER_ALIGN of TenuredCell was 0, because finalize_type_size
called from finish_record_layout reset it, TYPE_USER_ALIGN of its
CLASSTYPE_AS_BASE still remained 1.  After we replace it, it's no longer
1.  Then we perform layout_empty_base_or_field for TenuredCell and since
TYPE_USER_ALIGN of its CLASSTYPE_AS_BASE is now 0, we don't do this
adjustment:

  if (CLASSTYPE_USER_ALIGN (type))
    {
      rli->record_align = MAX (rli->record_align, CLASSTYPE_ALIGN (type));
      if (warn_packed)
        rli->unpacked_align = MAX (rli->unpacked_align, CLASSTYPE_ALIGN (type));
      TYPE_USER_ALIGN (rli->t) = 1;
    }

where rli->t is BaseShape.  Then finalize_record_size won't use the
correct rli->record_align and therefore
  /* Round the size up to be a multiple of the required alignment.  */
  TYPE_SIZE (rli->t) = round_up (unpadded_size, TYPE_ALIGN (rli->t));
after this we end up with the wrong size.

Since the original fix was to avoid creating extra copies for LTO
purposes, I think the following fix should be acceptable.

	PR c++/94050 - ABI issue with alignas on armv7hl.
	* class.c (layout_class_type): Don't replace a class's
	CLASSTYPE_AS_BASE if their TYPE_USER_ALIGN don't match.

	* g++.dg/abi/align3.C: New test.
This commit is contained in:
Marek Polacek 2020-03-05 14:07:25 -05:00
parent 157e23d880
commit 8475f2902a
4 changed files with 27 additions and 0 deletions
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6
gcc/cp/ChangeLog
View File

@ -1,3 +1,9 @@
2020-03-09 Marek Polacek <polacek@redhat.com>
PR c++/94050 - ABI issue with alignas on armv7hl.
* class.c (layout_class_type): Don't replace a class's
CLASSTYPE_AS_BASE if their TYPE_USER_ALIGN don't match.
2020-03-09 Bin Cheng <bin.cheng@linux.alibaba.com>
* coroutines.cc (build_actor_fn): Factor out code inserting the

4
gcc/cp/class.c
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@ -6705,6 +6705,10 @@ layout_class_type (tree t, tree *virtuals_p)
/* If we didn't end up needing an as-base type, don't use it. */
if (CLASSTYPE_AS_BASE (t) != t
/* If T's CLASSTYPE_AS_BASE is TYPE_USER_ALIGN, but T is not,
replacing the as-base type would change CLASSTYPE_USER_ALIGN,
causing us to lose the user-specified alignment as in PR94050. */
&& TYPE_USER_ALIGN (t) == TYPE_USER_ALIGN (CLASSTYPE_AS_BASE (t))
&& tree_int_cst_equal (TYPE_SIZE (t),
TYPE_SIZE (CLASSTYPE_AS_BASE (t))))
CLASSTYPE_AS_BASE (t) = t;

5
gcc/testsuite/ChangeLog
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@ -1,3 +1,8 @@
2020-03-09 Marek Polacek <polacek@redhat.com>
PR c++/94050 - ABI issue with alignas on armv7hl.
* g++.dg/abi/align3.C: New test.
2020-03-09 Christophe Lyon <christophe.lyon@linaro.org>
* gcc.target/arm/fuse-caller-save.c: Fix DejaGnu typo.

12
gcc/testsuite/g++.dg/abi/align3.C Normal file
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@ -0,0 +1,12 @@
// PR c++/94050 - ABI issue with alignas on armv7hl.
// { dg-do compile { target c++11 } }
struct alignas(8) Cell {};
struct TenuredCell : public Cell {};
struct BaseShape : public TenuredCell {
void *p;
unsigned q, r;
void *s;
__UINTPTR_TYPE__ t;
};
static_assert (sizeof (BaseShape) % 8 == 0, "");