[Bug c++/65685] New: Reducing alignment with alignas should be rejected

[Bug c++/65685] New: Reducing alignment with alignas should be rejected

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65685

            Bug ID: 65685
           Summary: Reducing alignment with alignas should be rejected
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
            Blocks: 58601

GCC doesn't give any diagnostic for the example in 7.6.2 [dcl.align] p5:

The combined effect of all alignment-specifiers in a declaration shall not
specify an alignment that is less strict than the alignment that would be
required for the entity being declared if all alignment-specifiers
appertaining to that entity were omitted. [ Example:
  struct alignas(8) S {};
  struct alignas(1) U {
    S s;
  };   // Error: U specifies an alignment that is less strict than
       // if the alignas(1) were omitted.
— end example ]
>From gcc-bugs-return-482857-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Tue Apr 07 11:34:20 2015
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Subject: [Bug target/65676] ICE: in extract_insn, at recog.c:2343 (unrecognizable insn) with -mavx512f -funsigned-char and __builtin_ia32_pmovsxwq512_mask()
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65676

--- Comment #3 from Uroš Bizjak <ubizjak at gmail dot com> ---
Following patch:

--cut here--
Index: config/i386/i386.c
===================================================================
--- config/i386/i386.c  (revision 221888)
+++ config/i386/i386.c  (working copy)
@@ -37509,6 +37509,10 @@ ix86_expand_args_builtin (const struct builtin_des
          if (memory_operand (op, mode))
            num_memory++;

+         /* Fixup modeless constants to fit required mode.  */
+         if (GET_MODE (op) == VOIDmode)
+           op = convert_to_mode (mode, op, 1);
+
          if (GET_MODE (op) == mode || GET_MODE (op) == VOIDmode)
            {
              if (optimize || !match || num_memory > 1)
--cut here--

fixes the testcase failure. This fixup is needed everywhere we have builtin
with QImode operands.
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id)256

--- Comment #48 from Richard Biener <rguenth at gcc dot gnu.org> ---
On ppc64 we generate

.L2:
        lfd 6,8(10)
        lfd 7,16(10)
        lfd 8,24(10)
        lfd 9,32(10)
        lfd 10,40(10)
        lfd 11,48(10)
        lfd 12,56(10)
        lfdu 0,64(10)
        stfd 6,8(9)
        stfd 7,16(9)
        stfd 8,24(9)
        stfd 9,32(9)
        stfd 10,40(9)
        stfd 11,48(9)
        stfd 12,56(9)
        stfdu 0,64(9)
        bdnz .L2

which looks quite optimal to me.  8 copies, indexed loads/stores with an
update variant on the last.

[Bug c++/65685] Reducing alignment with alignas should be rejected

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65685

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-04-07
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.

[Bug c++/65685] Reducing alignment with alignas should be rejected

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65685

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |sinbal2l at gmail dot com

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
*** Bug 94594 has been marked as a duplicate of this bug. ***

[Bug c++/65685] Reducing alignment with alignas should be rejected

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65685

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
*** Bug 94569 has been marked as a duplicate of this bug. ***

[Bug c++/65685] Reducing alignment with alignas should be rejected

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65685

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There are two facets to this. GCC allows alignas with a weaker alignment on a
type, and ignores it. GCC also allows alignas with a weaker alignment on an
object declaration, and reduces the alignment. In both cases it would be
conforming to issue a warning, even if the current behaviour is retained.

struct alignas(8) S {};
struct alignas(2) U {
  S s;
}; // { dg-error "alignment" }

// This assertion passes, meaning the invalid alignas(2) was silently ignored.
static_assert( alignof(U) == alignof(S), "" );

alignas(1) S s;  // { dg-error "align" }
alignas(1) U u;  // { dg-error "align" }

#ifdef __GNUC__
// These assertions pass, so GCC honours the invalid alignas(1) requests.
static_assert( alignof(s) == 1, "GCC reduced alignment of s" ); // WRONG!
static_assert( alignof(u) == 1, "GCC reduced alignment of u" ); // WRONG!
#endif

[Bug c++/65685] Reducing alignment with alignas should be rejected

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65685

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 69571 has been marked as a duplicate of this bug. ***