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.

From-SVN: r251180

1 files changed, 28 insertions, 1 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index ec29f1d..40d0c8d 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -284,6 +284,7 @@ Objective-C and Objective-C++ Dialects}.
 -Wformat-security  -Wformat-signedness  -Wformat-truncation=@var{n} @gol
 -Wformat-y2k  -Wframe-address @gol
 -Wframe-larger-than=@var{len}  -Wno-free-nonheap-object  -Wjump-misses-init @gol
+-Wif-not-aligned @gol
 -Wignored-qualifiers  -Wignored-attributes  -Wincompatible-pointer-types @gol
 -Wimplicit  -Wimplicit-fallthrough  -Wimplicit-fallthrough=@var{n} @gol
 -Wimplicit-function-declaration  -Wimplicit-int @gol
@@ -298,7 +299,7 @@ Objective-C and Objective-C++ Dialects}.
 -Wnormalized=@r{[}none@r{|}id@r{|}nfc@r{|}nfkc@r{]} @gol
 -Wnull-dereference  -Wodr  -Wno-overflow  -Wopenmp-simd  @gol
 -Woverride-init-side-effects  -Woverlength-strings @gol
--Wpacked  -Wpacked-bitfield-compat  -Wpadded @gol
+-Wpacked  -Wpacked-bitfield-compat -Wpacked-not-aligned -Wpadded @gol
 -Wparentheses  -Wno-pedantic-ms-format @gol
 -Wplacement-new  -Wplacement-new=@var{n} @gol
 -Wpointer-arith  -Wpointer-compare  -Wno-pointer-to-int-cast @gol
@@ -4427,6 +4428,13 @@ switch (cond)
 
 The @option{-Wimplicit-fallthrough=3} warning is enabled by @option{-Wextra}.
 
+@item -Wif-not-aligned @r{(C, C++, Objective-C and Objective-C++ only)}
+@opindex Wif-not-aligned
+@opindex Wno-if-not-aligned
+Control if warning triggered by the @code{warn_if_not_aligned} attribute
+should be issued.  This is is enabled by default.
+Use @option{-Wno-if-not-aligned} to disable it.
+
 @item -Wignored-qualifiers @r{(C and C++ only)}
 @opindex Wignored-qualifiers
 @opindex Wno-ignored-qualifiers
@@ -6520,6 +6528,25 @@ struct foo
 This warning is enabled by default.  Use
 @option{-Wno-packed-bitfield-compat} to disable this warning.
 
+@item -Wpacked-not-aligned @r{(C, C++, Objective-C and Objective-C++ only)}
+@opindex Wpacked-not-aligned
+@opindex Wno-packed-not-aligned
+Warn if a structure field with explicitly specified alignment in a
+packed struct or union is misaligned.  For example, a warning will
+be issued on @code{struct S}, like, @code{warning: alignment 1 of
+'struct S' is less than 8}, in this code:
+
+@smallexample
+@group
+struct __attribute__ ((aligned (8))) S8 @{ char a[8]; @};
+struct __attribute__ ((packed)) S @{
+  struct S8 s8;
+@};
+@end group
+@end smallexample
+
+This warning is enabled by @option{-Wall}.
+
 @item -Wpadded
 @opindex Wpadded
 @opindex Wno-padded