* [PATCH] avoid warning for members declared both aligned and packed (PR 84108)
@ 2018-02-02 18:58 Martin Sebor
2018-02-13 19:04 ` Martin Sebor
2018-02-14 7:14 ` Jeff Law
0 siblings, 2 replies; 3+ messages in thread
From: Martin Sebor @ 2018-02-02 18:58 UTC (permalink / raw)
To: Gcc Patch List
[-- Attachment #1: Type: text/plain, Size: 1494 bytes --]
The design of the attribute exclusion framework includes
support for different exclusions applying to different
kinds of declarations (functions, types, and variables
or fields), but the support is incomplete -- the logic
to consider these differences is missing. This is
because the differences are apparently rare. However,
as the bug below points out, they do exist.
PR middle-end/84108 - incorrect -Wattributes warning for
packed/aligned conflict on struct members, shows that while
declaring a non-member variable aligned is enough to reduce
the its alignment and declaring it both aligned and packed
triggers a -Wattributes warning:
int a __attribute__((packed, aligned (2))); // -Wattributes
a struct member must be declared both aligned and packed in
order to have its alignment reduced. (Declaring a member
just aligned has no effect and doesn't cause a warning).
struct S {
int b __attribute__((packed, aligned (2)));
int c __attribute__((aligned (2))); // no effect
};
As a result of the incomplete logic GCC 8 issues a -Wattributes
for the declaration of b in the struct.
By adding the missing logic the attached patch lets GCC avoid
the spurious warning.
I considered adding support for detecting the ineffective
attribute aligned on the declaration of the member c at
the same time but since that's not a regression I decided
to defer that until GCC 9. I opened bug 84185 to track it.
Tested on x86_64-linux with no regressions.
Martin
[-- Attachment #2: gcc-84108.diff --]
[-- Type: text/x-patch, Size: 2760 bytes --]
PR middle-end/84108 - incorrect -Wattributes warning for packed/aligned conflict on struct members
gcc/ChangeLog:
PR c/84108
* attribs.c (diag_attr_exclusions): Consider the exclusion(s)
that correspond to the kind of a declaration.
gcc/testsuite/ChangeLog:
PR c/84108
* gcc.dg/Wattributes-8.c: New test.
diff --git a/gcc/attribs.c b/gcc/attribs.c
index 2cac9c4..140863b 100644
--- a/gcc/attribs.c
+++ b/gcc/attribs.c
@@ -410,6 +410,22 @@ diag_attr_exclusions (tree last_decl, tree node, tree attrname,
if (!lookup_attribute (excl->name, attrs[i]))
continue;
+ /* An exclusion may apply either to a function declaration,
+ type declaration, or a field/variable declaration, or
+ any subset of the three. */
+ if (TREE_CODE (node) == FUNCTION_DECL
+ && !excl->function)
+ continue;
+
+ if (TREE_CODE (node) == TYPE_DECL
+ && !excl->type)
+ continue;
+
+ if ((TREE_CODE (node) == FIELD_DECL
+ || TREE_CODE (node) == VAR_DECL)
+ && !excl->variable)
+ continue;
+
found = true;
/* Print a note? */
diff --git a/gcc/testsuite/gcc.dg/Wattributes-8.c b/gcc/testsuite/gcc.dg/Wattributes-8.c
new file mode 100644
index 0000000..a4b4c00
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wattributes-8.c
@@ -0,0 +1,38 @@
+/* PR middle-end/84108 - incorrect -Wattributes warning for packed/aligned
+ conflict on struct members
+ { dg-do compile }
+ { dg-options "-Wall -Wattributes" } */
+
+#define ATTR(list) __attribute__ (list)
+#define ASSERT(e) _Static_assert (e, #e)
+
+/* GCC is inconsistent in how it treats attribute aligned between
+ variable and member declarations. Attribute aligned alone is
+ sufficient to reduce a variable's alignment requirement but
+ the attribute must be paired with packed to have the same
+ effect on a member. Worse, declaring a variable both aligned
+ and packed emits a warning. */
+
+/* Avoid exercising this since emitting a warning for these given
+ the requirement for members seems like a misfeature:
+ int a ATTR ((packed, aligned (2))); // -Wattributes
+ int b ATTR ((aligned (2), packed)); // -Wattributes
+ ASSERT (_Alignof (a) == 2);
+ ASSERT (_Alignof (b) == 2); */
+
+int c ATTR ((aligned (2))); // okay (reduces alignment)
+ASSERT (_Alignof (c) == 2);
+
+struct {
+ int a ATTR ((packed, aligned (2))); /* { dg-bogus "\\\[-Wattributes" } */
+ int b ATTR ((aligned (2), packed)); /* { dg-bogus "\\\[-Wattributes" } */
+
+ /* Avoid exercising this since the attribute has no effect yet
+ there is no warning.
+ int c ATTR ((aligned (2))); // missing warning? */
+} s;
+
+ASSERT (_Alignof (s.a) == 2);
+ASSERT (_Alignof (s.b) == 2);
+
+/* ASSERT (_Alignof (s.c) == 4); */
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] avoid warning for members declared both aligned and packed (PR 84108)
2018-02-02 18:58 [PATCH] avoid warning for members declared both aligned and packed (PR 84108) Martin Sebor
@ 2018-02-13 19:04 ` Martin Sebor
2018-02-14 7:14 ` Jeff Law
1 sibling, 0 replies; 3+ messages in thread
From: Martin Sebor @ 2018-02-13 19:04 UTC (permalink / raw)
To: Gcc Patch List
Ping: https://gcc.gnu.org/ml/gcc-patches/2018-02/msg00125.html
On 02/02/2018 11:58 AM, Martin Sebor wrote:
> The design of the attribute exclusion framework includes
> support for different exclusions applying to different
> kinds of declarations (functions, types, and variables
> or fields), but the support is incomplete -- the logic
> to consider these differences is missing. This is
> because the differences are apparently rare. However,
> as the bug below points out, they do exist.
>
> PR middle-end/84108 - incorrect -Wattributes warning for
> packed/aligned conflict on struct members, shows that while
> declaring a non-member variable aligned is enough to reduce
> the its alignment and declaring it both aligned and packed
> triggers a -Wattributes warning:
>
> int a __attribute__((packed, aligned (2))); // -Wattributes
>
> a struct member must be declared both aligned and packed in
> order to have its alignment reduced. (Declaring a member
> just aligned has no effect and doesn't cause a warning).
>
> struct S {
> int b __attribute__((packed, aligned (2)));
> int c __attribute__((aligned (2))); // no effect
> };
>
> As a result of the incomplete logic GCC 8 issues a -Wattributes
> for the declaration of b in the struct.
>
> By adding the missing logic the attached patch lets GCC avoid
> the spurious warning.
>
> I considered adding support for detecting the ineffective
> attribute aligned on the declaration of the member c at
> the same time but since that's not a regression I decided
> to defer that until GCC 9. I opened bug 84185 to track it.
>
> Tested on x86_64-linux with no regressions.
>
> Martin
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] avoid warning for members declared both aligned and packed (PR 84108)
2018-02-02 18:58 [PATCH] avoid warning for members declared both aligned and packed (PR 84108) Martin Sebor
2018-02-13 19:04 ` Martin Sebor
@ 2018-02-14 7:14 ` Jeff Law
1 sibling, 0 replies; 3+ messages in thread
From: Jeff Law @ 2018-02-14 7:14 UTC (permalink / raw)
To: Martin Sebor, Gcc Patch List
On 02/02/2018 11:58 AM, Martin Sebor wrote:
> The design of the attribute exclusion framework includes
> support for different exclusions applying to different
> kinds of declarations (functions, types, and variables
> or fields), but the support is incomplete -- the logic
> to consider these differences is missing.  This is
> because the differences are apparently rare.  However,
> as the bug below points out, they do exist.
>
> PR middle-end/84108 - incorrect -Wattributes warning for
> packed/aligned conflict on struct members, shows that while
> declaring a non-member variable aligned is enough to reduce
> the its alignment and declaring it both aligned and packed
> triggers a -Wattributes warning:
>
>   int a __attribute__((packed, aligned (2)));   // -Wattributes
>
> a struct member must be declared both aligned and packed in
> order to have its alignment reduced.  (Declaring a member
> just aligned has no effect and doesn't cause a warning).
>
>   struct S {
>     int b __attribute__((packed, aligned (2)));
>     int c __attribute__((aligned (2)));           // no effect
> Â Â };
>
> As a result of the incomplete logic GCC 8 issues a -Wattributes
> for the declaration of b in the struct.
>
> By adding the missing logic the attached patch lets GCC avoid
> the spurious warning.
>
> I considered adding support for detecting the ineffective
> attribute aligned on the declaration of the member c at
> the same time but since that's not a regression I decided
> to defer that until GCC 9.  I opened bug 84185 to track it.
>
> Tested on x86_64-linux with no regressions.
>
> Martin
>
> gcc-84108.diff
>
>
> PR middle-end/84108 - incorrect -Wattributes warning for packed/aligned conflict on struct members
>
> gcc/ChangeLog:
>
> PR c/84108
> * attribs.c (diag_attr_exclusions): Consider the exclusion(s)
> that correspond to the kind of a declaration.
>
> gcc/testsuite/ChangeLog:
>
> PR c/84108
> * gcc.dg/Wattributes-8.c: New test.
OK. Sorry for the wait.
Thanks.
Jeff
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2018-02-14 7:14 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2018-02-02 18:58 [PATCH] avoid warning for members declared both aligned and packed (PR 84108) Martin Sebor
2018-02-13 19:04 ` Martin Sebor
2018-02-14 7:14 ` Jeff Law
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).