From: Marek Polacek <polacek@redhat.com>
To: GCC Patches <gcc-patches@gcc.gnu.org>,
Jason Merrill <jason@redhat.com>,
Joseph Myers <joseph@codesourcery.com>
Subject: [PATCH] c-family: char8_t and aliasing in C vs C++ [PR111884]
Date: Fri, 20 Oct 2023 12:31:21 -0400 [thread overview]
Message-ID: <20231020163121.25120-1-polacek@redhat.com> (raw)
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
In the PR, Joseph says that in C char8_t is not a distinct type. So
we should behave as if it can alias anything, like ordinary char.
In C, unsigned_char_type_node == char8_type_node, so with this patch
we return 0 instead of -1. And the following comment says:
/* The C standard guarantees that any object may be accessed via an
lvalue that has narrow character type (except char8_t). */
if (t == char_type_node
|| t == signed_char_type_node
|| t == unsigned_char_type_node)
return 0;
Which appears to be wrong, so I'm adjusting that as well.
PR c/111884
gcc/c-family/ChangeLog:
* c-common.cc (c_common_get_alias_set): Return -1 for char8_t only
in C++.
gcc/testsuite/ChangeLog:
* c-c++-common/alias-1.c: New test.
---
gcc/c-family/c-common.cc | 7 ++++---
gcc/testsuite/c-c++-common/alias-1.c | 23 +++++++++++++++++++++++
2 files changed, 27 insertions(+), 3 deletions(-)
create mode 100644 gcc/testsuite/c-c++-common/alias-1.c
diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
index f044db5b797..0efdc677217 100644
--- a/gcc/c-family/c-common.cc
+++ b/gcc/c-family/c-common.cc
@@ -3828,12 +3828,13 @@ c_common_get_alias_set (tree t)
if (!TYPE_P (t))
return -1;
- /* Unlike char, char8_t doesn't alias. */
- if (flag_char8_t && t == char8_type_node)
+ /* Unlike char, char8_t doesn't alias in C++. (In C, char8_t is not
+ a distinct type.) */
+ if (flag_char8_t && t == char8_type_node && c_dialect_cxx ())
return -1;
/* The C standard guarantees that any object may be accessed via an
- lvalue that has narrow character type (except char8_t). */
+ lvalue that has narrow character type. */
if (t == char_type_node
|| t == signed_char_type_node
|| t == unsigned_char_type_node)
diff --git a/gcc/testsuite/c-c++-common/alias-1.c b/gcc/testsuite/c-c++-common/alias-1.c
new file mode 100644
index 00000000000..d72fec47f76
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/alias-1.c
@@ -0,0 +1,23 @@
+/* PR c/111884 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -Wall" } */
+/* { dg-additional-options "-std=c++20" { target c++ } } */
+/* { dg-additional-options "-std=c2x" { target c } } */
+
+int f(int i)
+{
+ int f = 1;
+ return i[(unsigned char *)&f];
+}
+
+int g(int i)
+{
+ int f = 1;
+ return i[(signed char *)&f];
+}
+
+int h(int i)
+{
+ int f = 1;
+ return i[(char *)&f];
+}
base-commit: eb15fad3190a8b33e3e451b964ff1ecf08bbb113
--
2.41.0
next reply other threads:[~2023-10-20 16:31 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-20 16:31 Marek Polacek [this message]
2023-10-20 19:10 ` Jason Merrill
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