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* [Bug c++/43976] New: warning about increased alignment during casting printed even though variable is properly aligned
@ 2010-05-03 14:40 thiago at kde dot org
2010-05-04 17:27 ` [Bug middle-end/43976] " pinskia at gcc dot gnu dot org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: thiago at kde dot org @ 2010-05-03 14:40 UTC (permalink / raw)
To: gcc-bugs
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Source: https://bugs.webkit.org/show_bug.cgi?id=38045
Given the following code:
$ cat test.cpp
struct Foo
{
char __attribute__((aligned(4))) c[sizeof(int)];
};
char junk;
Foo f;
int main()
{
int *i = reinterpret_cast<int *>(&f.c);
*i = 0;
}
When compiled for ARM produces the warning:
$ arm-none-linux-gnueabi-g++ -Wcast-align -O3 -fsyntax-only /tmp/test.cpp
test.cpp:11: warning: cast from 'char (*)[4]' to 'int*' increases required
alignment of target type
Note that we requested that Foo::c be aligned to 4 bytes, which is the required
alignment for int. The assembly dump of the build confirms that the alignment
was honoured:
junk:
.space 1
.space 3
.type f, %object
.size f, 4
f:
.space 4
According to Dirk Müller, the following code is at fault (I have no idea where
it's from):
/* Warn about possible alignment problems. */
if (STRICT_ALIGNMENT && warn_cast_align
&& (complain & tf_warning)
· && !VOID_TYPE_P (type)
· && TREE_CODE (TREE_TYPE (intype)) != FUNCTION_TYPE
· && COMPLETE_TYPE_P (TREE_TYPE (type))
· && COMPLETE_TYPE_P (TREE_TYPE (intype))
· && TYPE_ALIGN (TREE_TYPE (type)) > TYPE_ALIGN (TREE_TYPE (intype)))
· warning (OPT_Wcast_align, "cast from %qT to %qT "
"increases required alignment of target type", intype, type);
As it only verifies the aligment of the type in question (char*), not of the
variable in question (&f.c).
--
Summary: warning about increased alignment during casting printed
even though variable is properly aligned
Product: gcc
Version: 4.4.1
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: thiago at kde dot org
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: arm-none-linux-gnueabi
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43976
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/43976] warning about increased alignment during casting printed even though variable is properly aligned
2010-05-03 14:40 [Bug c++/43976] New: warning about increased alignment during casting printed even though variable is properly aligned thiago at kde dot org
@ 2010-05-04 17:27 ` pinskia at gcc dot gnu dot org
2010-05-04 19:40 ` thiago at kde dot org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2010-05-04 17:27 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from pinskia at gcc dot gnu dot org 2010-05-04 17:27 -------
I think the warning is correct as the resulting type of &f.c is a pointer to an
array and that array is of type char[4] which has an alignment of 1. Yes f.c
has an alignment of 4 but the array type has an alignment of 1.
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|c++ |middle-end
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43976
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/43976] warning about increased alignment during casting printed even though variable is properly aligned
2010-05-03 14:40 [Bug c++/43976] New: warning about increased alignment during casting printed even though variable is properly aligned thiago at kde dot org
2010-05-04 17:27 ` [Bug middle-end/43976] " pinskia at gcc dot gnu dot org
@ 2010-05-04 19:40 ` thiago at kde dot org
2010-05-05 19:18 ` hp at gcc dot gnu dot org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: thiago at kde dot org @ 2010-05-04 19:40 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from thiago at kde dot org 2010-05-04 19:40 -------
Well, technically f.c has a type of 'char (__attribute__((aligned(4)))) [4]'.
Anyway, the point is that the variable is properly aligned, so the warning is
superfluous.
Or, in other words, we need a way to tell GCC "I know this is properly aligned,
so don't tell me there's a problem". The -Wcast-align warning is very useful to
catch real mistakes, but there's no way to weed out the false positives.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43976
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/43976] warning about increased alignment during casting printed even though variable is properly aligned
2010-05-03 14:40 [Bug c++/43976] New: warning about increased alignment during casting printed even though variable is properly aligned thiago at kde dot org
2010-05-04 17:27 ` [Bug middle-end/43976] " pinskia at gcc dot gnu dot org
2010-05-04 19:40 ` thiago at kde dot org
@ 2010-05-05 19:18 ` hp at gcc dot gnu dot org
2010-05-05 19:40 ` thiago at kde dot org
2010-09-08 10:02 ` ibolton at gcc dot gnu dot org
4 siblings, 0 replies; 6+ messages in thread
From: hp at gcc dot gnu dot org @ 2010-05-05 19:18 UTC (permalink / raw)
To: gcc-bugs
------- Comment #3 from hp at gcc dot gnu dot org 2010-05-05 19:17 -------
The struct Foo looks like an odd way to try and express
union Foo
{
int i;
char c[sizeof (int)];
};
but that doesn't work either, for my 4.3.1 arm-linux build.
Neither does moving the attribute just before the ";" (either ones) or before
"char" (the original or the above). Neither in C (changing reinterpret_cast to
a C cast).
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43976
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/43976] warning about increased alignment during casting printed even though variable is properly aligned
2010-05-03 14:40 [Bug c++/43976] New: warning about increased alignment during casting printed even though variable is properly aligned thiago at kde dot org
` (2 preceding siblings ...)
2010-05-05 19:18 ` hp at gcc dot gnu dot org
@ 2010-05-05 19:40 ` thiago at kde dot org
2010-09-08 10:02 ` ibolton at gcc dot gnu dot org
4 siblings, 0 replies; 6+ messages in thread
From: thiago at kde dot org @ 2010-05-05 19:40 UTC (permalink / raw)
To: gcc-bugs
------- Comment #4 from thiago at kde dot org 2010-05-05 19:39 -------
(In reply to comment #3)
> The struct Foo looks like an odd way to try and express
>
> union Foo
> {
> int i;
> char c[sizeof (int)];
> };
Well, that is the objective, but you can't replace int with a non-POD type in
C++98. C++0x relaxes the unions and that's exactly what is intended: delayed
construction.
I used int as an example, but the objective is to have:
template <typename T>
struct Buffer
{
char __attribute__((aligned(__alignof__(T)))) buf[sizeof(T)];
};
[the WebKit WTF::Vector code is a bit more complex than that to get around the
fact that gcc 4.2 can't do aligned(__alignof__(T)), so it partially specialises
alignments of 1, 2, 4, 8, 16, and 64]
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43976
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/43976] warning about increased alignment during casting printed even though variable is properly aligned
2010-05-03 14:40 [Bug c++/43976] New: warning about increased alignment during casting printed even though variable is properly aligned thiago at kde dot org
` (3 preceding siblings ...)
2010-05-05 19:40 ` thiago at kde dot org
@ 2010-09-08 10:02 ` ibolton at gcc dot gnu dot org
4 siblings, 0 replies; 6+ messages in thread
From: ibolton at gcc dot gnu dot org @ 2010-09-08 10:02 UTC (permalink / raw)
To: gcc-bugs
------- Comment #5 from ibolton at gcc dot gnu dot org 2010-09-08 10:02 -------
Confirmed on latest 4.4, 4.5 and 4.6 (trunk).
Related GCC documentation on alignment of structure fields is here:
http://gcc.gnu.org/onlinedocs/gcc-4.5.0/gcc/Variable-Attributes.html#Variable-Attributes
In the short-term, one workaround is to write the code as follows:
#include <stdio.h>
struct Foo
{
char c[sizeof(int)];
} __attribute__((aligned(4)));
char junk;
Foo f;
int main()
{
int *i = reinterpret_cast<int *>(&f);
*i = 0x44434241;
printf("%c %c %c %c", f.c[0], f.c[1], f.c[2], f.c[3]);
}
By aligning the structure Foo to 4 bytes, you can successfully cast a Foo* to
an int* and then initialise all four chars in one go. (Without the type
attribute for the struct Foo, you still get the warning.) My example prints "A
B C D".
FYI: I have tracked down the alleged offending code mentioned in an earlier
comment to build_c_cast() in c-typeck.c.
--
ibolton at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Ever Confirmed|0 |1
Known to fail| |4.4.5 4.5.2 4.6.0
Last reconfirmed|0000-00-00 00:00:00 |2010-09-08 10:02:04
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43976
^ permalink raw reply [flat|nested] 6+ messages in thread
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2010-05-03 14:40 [Bug c++/43976] New: warning about increased alignment during casting printed even though variable is properly aligned thiago at kde dot org
2010-05-04 17:27 ` [Bug middle-end/43976] " pinskia at gcc dot gnu dot org
2010-05-04 19:40 ` thiago at kde dot org
2010-05-05 19:18 ` hp at gcc dot gnu dot org
2010-05-05 19:40 ` thiago at kde dot org
2010-09-08 10:02 ` ibolton at gcc dot gnu dot org
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