From: Jakub Jelinek <jakub@redhat.com>
Date: Wed, 25 Aug 2021 20:35:21 +0000 (+0200)
Subject: c++: Fix up value initialization of structs with zero width bitfields [PR102019]
X-Git-Tag: basepoints/gcc-13~5170
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=1ab84eda5548119908c4e24c6ad953dd7c00a5b7;p=thirdparty%2Fgcc.git

c++: Fix up value initialization of structs with zero width bitfields [PR102019]

The removal of remove_zero_width_bit_fields, in addition to triggering
some ABI issues that need solving anyway (ABI incompatibility between
C and C++) also resulted in UB inside of gcc, we now call build_zero_init
which calls build_int_cst on an integral type with TYPE_PRECISION of 0.

Fixed by ignoring the zero width bitfields.  I understand
build_value_init_noctor wants to initialize to 0 even unnamed bitfields
(of non-zero width), at least until we have some CONSTRUCTOR flag that says
that even all the padding bits should be cleared.

2021-08-25  Jakub Jelinek  <jakub@redhat.com>

	PR c++/102019
	* init.c (build_value_init_noctor): Ignore unnamed zero-width
	bitfields.
---

diff --git a/gcc/cp/init.c b/gcc/cp/init.c
index 229c84e1d74d..1426f9a54345 100644
--- a/gcc/cp/init.c
+++ b/gcc/cp/init.c
@@ -427,6 +427,11 @@ build_value_init_noctor (tree type, tsubst_flags_t complain)
 		      == NULL_TREE))
 		continue;
 
+	      /* Ignore unnamed zero-width bitfields.  */
+	      if (DECL_UNNAMED_BIT_FIELD (field)
+		  && integer_zerop (DECL_SIZE (field)))
+		continue;
+
 	      /* We could skip vfields and fields of types with
 		 user-defined constructors, but I think that won't improve
 		 performance at all; it should be simpler in general just