[Bug c/111884] New: unsigned char no longer aliases anything under -std=c2x

[Bug c/111884] New: unsigned char no longer aliases anything under -std=c2x

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111884

            Bug ID: 111884
           Summary: unsigned char no longer aliases anything under
                    -std=c2x
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amonakov at gcc dot gnu.org
  Target Milestone: ---

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];
}


gcc -O2 -std=c2x compiles 'f' as though inspecting representation via an
'unsigned char *' is not valid (with a confusing warning under -Wall).

[Bug c/111884] unsigned char no longer aliases anything under -std=c2x

[sjames at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111884

Sam James <sjames at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jsm28 at gcc dot gnu.org,
                   |                            |sjames at gcc dot gnu.org

--- Comment #1 from Sam James <sjames at gcc dot gnu.org> ---
Originally reported at
https://inbox.sourceware.org/gcc-help/e18818e9-396c-41fe-6c4a-9ac86c564525@ispras.ru/T/#m147d6322a26e2bc094dd5e0935b7f2223bc910ac.
Curious indeed..

[Bug c/111884] unsigned char no longer aliases anything under -std=c2x

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111884

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
I'm going to guess this was introduced by the char8_t changes ("C: 
Implement C2X N2653 char8_t and UTF-8 string literal changes", commit 
703837b2cc8ac03c53ac7cc0fb1327055acaebd2).

  /* Unlike char, char8_t doesn't alias. */
  if (flag_char8_t && t == char8_type_node)
    return -1;

is not correct for C, where char8_t is not a distinct type.

[Bug c/111884] unsigned char no longer aliases anything under -std=c2x

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111884

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Thanks, I can test

--- a/gcc/c-family/c-common.cc
+++ b/gcc/c-family/c-common.cc
@@ -3828,8 +3828,9 @@ 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

[Bug c/111884] [13/14 Regression] unsigned char no longer aliases anything under -std=c2x

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111884

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|unsigned char no longer     |[13/14 Regression] unsigned
                   |aliases anything under      |char no longer aliases
                   |-std=c2x                    |anything under -std=c2x
           Keywords|                            |diagnostic
   Target Milestone|---                         |13.3

[Bug c/111884] [13/14 Regression] unsigned char no longer aliases anything under -std=c2x

[tom at honermann dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111884

--- Comment #4 from Tom Honermann <tom at honermann dot net> ---
(In reply to Marek Polacek from comment #3)
> Thanks, I can test

Thank you. That change looks right. My apologies for introducing the
regression.

[Bug c/111884] [13/14 Regression] unsigned char no longer aliases anything under -std=c2x

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111884

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug c/111884] [13/14 Regression] unsigned char no longer aliases anything under -std=c2x

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111884

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:281699fbff6262766674ab13087d37db751cd40a

commit r14-4802-g281699fbff6262766674ab13087d37db751cd40a
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Oct 19 16:32:10 2023 -0400

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

    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.

[Bug c/111884] [13/14 Regression] unsigned char no longer aliases anything under -std=c2x

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111884

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:4f93c6027fb09c94ca8dc7fe13f92a3b9205b68a

commit r13-7967-g4f93c6027fb09c94ca8dc7fe13f92a3b9205b68a
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Oct 19 16:32:10 2023 -0400

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

    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.

    (cherry picked from commit 281699fbff6262766674ab13087d37db751cd40a)

[Bug c/111884] [13/14 Regression] unsigned char no longer aliases anything under -std=c2x

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111884

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
         Resolution|---                         |FIXED

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.