On 3/5/20 2:40 PM, Marek Polacek wrote:
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;
But while TYPE_USER_ALIGN of TenuredCell was 0, TYPE_USER_ALIGN of its
CLASSTYPE_AS_BASE was 1.
Surely the bug is that TYPE_USER_ALIGN isn't set for TenuredCell?
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.
Bootstrapped/regtested on x86_64-linux, ok for trunk? I verified the
fix on the attached testcase using a --target=armv7hl-linux-gnueabi cross,
but haven't actually run the testsuite.
PR c++/94050 - ABI issue with alignas on armv7hl.
* class.c (layout_class_type): Don't replace a class's
CLASSTYPE_AS_BASE if it is CLASSTYPE_USER_ALIGN.
* g++.dg/abi/align3.C: New test.
---
gcc/cp/class.c | 4 ++++
gcc/testsuite/g++.dg/abi/align3.C | 12 ++++++++++++
2 files changed, 16 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/abi/align3.C
diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index b3787f75d7b..4a17751d70b 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -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. */
+ && !CLASSTYPE_USER_ALIGN (t)
&& tree_int_cst_equal (TYPE_SIZE (t),
TYPE_SIZE (CLASSTYPE_AS_BASE (t))))
CLASSTYPE_AS_BASE (t) = t;
diff --git a/gcc/testsuite/g++.dg/abi/align3.C
b/gcc/testsuite/g++.dg/abi/align3.C
new file mode 100644
index 00000000000..a56693a34b8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/abi/align3.C
@@ -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, "");
base-commit: 2d22ab64c4774d7d30c7e014652b28a13d744aec
