[Bug c/53037] warn_if_not_aligned(X)

[aldyh at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53037

--- Comment #41 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
Author: aldyh
Date: Wed Sep 13 17:08:36 2017
New Revision: 252478

URL: https://gcc.gnu.org/viewcvs?rev=252478&root=gcc&view=rev
Log:
Add warn_if_not_aligned attribute

Add warn_if_not_aligned attribute as well as  command line options:
-Wif-not-aligned and -Wpacked-not-aligned.

__attribute__((warn_if_not_aligned(N))) causes compiler to issue a
warning if the field in a struct or union is not aligned to N:

typedef unsigned long long __u64
  __attribute__((aligned(4),warn_if_not_aligned(8)));

struct foo
{
  int i1;
  int i2;
  __u64 x;
};

__u64 is aligned to 4 bytes.  But inside struct foo, __u64 should be
aligned at 8 bytes.  It is used to define struct foo in such a way that
struct foo has the same layout and x has the same alignment when __u64
is aligned at either 4 or 8 bytes.

Since struct foo is normally aligned to 4 bytes, a warning will be issued:

warning: alignment 4 of 'struct foo' is less than 8

Align struct foo to 8 bytes:

struct foo
{
  int i1;
  int i2;
  __u64 x;
} __attribute__((aligned(8)));

silences the warning.  It also warns the field with misaligned offset:

struct foo
{
  int i1;
  int i2;
  int i3;
  __u64 x;
} __attribute__((aligned(8)));

warning: 'x' offset 12 in 'struct foo' isn't aligned to 8

This warning is controlled by -Wif-not-aligned and is enabled by default.

When -Wpacked-not-aligned is used, the same warning is also issued for
the field with explicitly specified alignment in a packed struct or union:

struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
  struct S8 s8;
};

warning: alignment 1 of 'struct S' is less than 8

This warning is disabled by default and enabled by -Wall.

gcc/

        PR c/53037
        * print-tree.c (print_node): Support DECL_WARN_IF_NOT_ALIGN
        and TYPE_WARN_IF_NOT_ALIGN.
        * stor-layout.c (do_type_align): Merge DECL_WARN_IF_NOT_ALIGN.
        (handle_warn_if_not_align): New.
        (place_union_field): Call handle_warn_if_not_align.
        (place_field): Call handle_warn_if_not_align.  Copy
        TYPE_WARN_IF_NOT_ALIGN.
        (finish_builtin_struct): Copy TYPE_WARN_IF_NOT_ALIGN.
        (layout_type): Likewise.
        * tree-core.h (tree_type_common): Add warn_if_not_align.  Set
        spare to 18.
        (tree_decl_common): Add warn_if_not_align.
        * tree.c (build_range_type_1): Copy TYPE_WARN_IF_NOT_ALIGN.
        * tree.h (TYPE_WARN_IF_NOT_ALIGN): New.
        (SET_TYPE_WARN_IF_NOT_ALIGN): Likewise.
        (DECL_WARN_IF_NOT_ALIGN): Likewise.
        (SET_DECL_WARN_IF_NOT_ALIGN): Likewise.
        * doc/extend.texi: Document warn_if_not_aligned attribute.
        * doc/invoke.texi: Document -Wif-not-aligned and
        -Wpacked-not-aligned.

gcc/c-family/

        PR c/53037
        * c-attribs.c (handle_warn_if_not_aligned_attribute): New.
        (c_common_attribute_table): Add warn_if_not_aligned.
        (handle_aligned_attribute): Renamed to ...
        (common_handle_aligned_attribute): Remove argument, name, and add
        argument, warn_if_not_aligned.  Handle warn_if_not_aligned.
        (handle_aligned_attribute): New.
        * c.opt: Add -Wif-not-aligned and -Wpacked-not-aligned.

gcc/c/

        PR c/53037
        * c-decl.c (merge_decls): Also merge DECL_WARN_IF_NOT_ALIGN.
        (check_bitfield_type_and_width): Don't allow bit-field with
        warn_if_not_aligned type.

gcc/cp/

        PR c/53037
        * decl.c (duplicate_decls): Also merge DECL_WARN_IF_NOT_ALIGN.
        * decl2.c (grokbitfield): Don't allow bit-field with
        warn_if_not_aligned type.

gcc/testsuite/

        PR c/53037
        * c-c++-common/pr53037-5.c: New test.
        * g++.dg/pr53037-1.C: Likewise.
        * g++.dg/pr53037-2.C: Likewise.
        * g++.dg/pr53037-3.C: Likewise.
        * g++.dg/pr53037-4.C: Likewise.
        * gcc.dg/pr53037-1.c: Likewise.
        * gcc.dg/pr53037-2.c: Likewise.
        * gcc.dg/pr53037-3.c: Likewise.
        * gcc.dg/pr53037-4.c: Likewise.

Added:
    branches/range-gen2/gcc/testsuite/c-c++-common/pr53037-5.c
    branches/range-gen2/gcc/testsuite/g++.dg/pr53037-1.C
    branches/range-gen2/gcc/testsuite/g++.dg/pr53037-2.C
    branches/range-gen2/gcc/testsuite/g++.dg/pr53037-3.C
    branches/range-gen2/gcc/testsuite/g++.dg/pr53037-4.C
    branches/range-gen2/gcc/testsuite/gcc.dg/pr53037-1.c
    branches/range-gen2/gcc/testsuite/gcc.dg/pr53037-2.c
    branches/range-gen2/gcc/testsuite/gcc.dg/pr53037-3.c
    branches/range-gen2/gcc/testsuite/gcc.dg/pr53037-4.c
Modified:
    branches/range-gen2/gcc/ChangeLog
    branches/range-gen2/gcc/c-family/ChangeLog
    branches/range-gen2/gcc/c-family/c-attribs.c
    branches/range-gen2/gcc/c-family/c.opt
    branches/range-gen2/gcc/c/ChangeLog
    branches/range-gen2/gcc/c/c-decl.c
    branches/range-gen2/gcc/cp/ChangeLog
    branches/range-gen2/gcc/cp/decl.c
    branches/range-gen2/gcc/cp/decl2.c
    branches/range-gen2/gcc/doc/extend.texi
    branches/range-gen2/gcc/doc/invoke.texi
    branches/range-gen2/gcc/print-tree.c
    branches/range-gen2/gcc/stor-layout.c
    branches/range-gen2/gcc/testsuite/ChangeLog
    branches/range-gen2/gcc/tree-core.h
    branches/range-gen2/gcc/tree.c
    branches/range-gen2/gcc/tree.h