[PATCH] c-family: char8_t and aliasing in C vs C++ [PR111884]

[Marek Polacek]

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