[Bug c++/45510] Bug with anonymous unions and bit-fields
--- Comment #9 from runipg at broadcom dot com 2010-09-03 14:45 --- Subject: Re: Bug with anonymous unions and bit-fields Thank you so much. You can close this bug if you wish. -Runip On 03-Sep-2010, at 3:47 AM, paolo dot carlini at oracle dot com gcc-bugzi...@gcc.gnu.org wrote: --- Comment #8 from paolo dot carlini at oracle dot com 2010-09-03 10:46 --- If you look at the actual Standard, both alignment and allocation of bit-fields are implementation defined. Thus, as far as I can see, at best we are talking about non-portable implementation defined behavior. If you want my advice, try to stay away from those tricks, with and without anonymous struct. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45510 --- You are receiving this mail because: --- You reported the bug, or are watching the reporter. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45510
[Bug c++/45510] Bug with anonymous unions and bit-fields
--- Comment #10 from runipg at broadcom dot com 2010-09-03 19:19 ---
Subject: Re: Bug with anonymous unions and bit-fields
Your suggestion works:
struct bfc {
union {
struct {
unsigned int a : 1,
b : 4;
};
unsigned int data;
};
};
void
testc()
{
cout __func__ endl;
bfc bf0;
bf0.data = 0;
cout bf0.data= hex bf0.data endl;
cout bf0.a= bf0.a endl;
cout bf0.b= bf0.b endl;
bf0.a = 1;
cout bf0.data= hex bf0.data endl;
cout bf0.a= bf0.a endl;
cout bf0.b= hex bf0.b endl;
bf0.b = 1;
cout bf0.data= hex bf0.data endl;
cout bf0.a= bf0.a endl;
cout bf0.b= hex bf0.b endl;
}
Output is
testc
bf0.data=0
bf0.a=0
bf0.b=0
bf0.data=1
bf0.a=1
bf0.b=0
bf0.data=3
bf0.a=1
bf0.b=1
---
I guess the fundamental issue which confused me was:
struct bfa {
union {
unsigned int a : 1,
b : 4;
unsigned int data;
};
};
that it appeared to be a union of 2 integers (one bit-field) rather than
the implementation version where the union is of 3 integers
(two-bitfields). If the standard does not say anything specific about
it, then I guess the compiler is free to do whatever it wants.
In fact, I tried a slight variation on bfa, where I tried to force
alignment on the first integer packing.
struct bfd {
union {
unsigned int a : 1,
b : 4,
: 0; //force alignment
unsigned int data;
};
};
However this gives the same result as bfa.
So I guess semantically it is very confusing to a user. Perhaps this
part of the language should be tightened.
Thanks Paolo!
-Runip
paolo dot carlini at oracle dot com wrote:
--- Comment #8 from paolo dot carlini at oracle dot com 2010-09-03 10:46
---
If you look at the actual Standard, both alignment and allocation of
bit-fields
are implementation defined. Thus, as far as I can see, at best we are talking
about non-portable implementation defined behavior. If you want my advice, try
to stay away from those tricks, with and without anonymous struct.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45510
[Bug c++/45510] Bug with anonymous unions and bit-fields
--- Comment #12 from runipg at broadcom dot com 2010-09-03 21:12 ---
Subject: Re: Bug with anonymous unions and bit-fields
Okay, I guess I was confused by struct or union semantics. Thanks!
jakub at gcc dot gnu dot org wrote:
--- Comment #11 from jakub at gcc dot gnu dot org 2010-09-03 20:53
---
I don't see anything confusing about it. If you have:
union { int a, b, c; } u;
u.a overlaps u.b and u.c as well, and the same applies to bitfields. union
isn't struct, see ISO C99 6.7.2.1/5. The standard is clear on this.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45510
[Bug c++/45510] Bug with anonymous unions and bit-fields
--- Comment #3 from runipg at broadcom dot com 2010-09-03 01:03 --- Subject: Re: Bug with anonymous unions and bit-fields That was fast and interesting that two other compilers behave the same. Unfortunately I don't have access to any other compiler. I am simply befuddled by the behavior though. I wouldn't file a bug, if I thought it was frivolous. paolo dot carlini at oracle dot com wrote: --- Comment #2 from paolo dot carlini at oracle dot com 2010-09-03 00:02 --- Without having seriously looked into your code, I note that two completely different, closed source compilers (ICC and SunStudio) leads to the same behavior as GCC at runtime. Does your code actually work (behaves like you expect) somewhere? -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45510
[Bug c++/45510] Bug with anonymous unions and bit-fields
--- Comment #7 from runipg at broadcom dot com 2010-09-03 03:31 ---
Subject: Re: Bug with anonymous unions and bit-fields
Wow, So there is a diff inside and outside a struct?
This is very counter-intuitive but I am to accept your explanation if that's
what the language says.
Thanks so much for looking into it and responding. Sorry if it is not a bug, I
googled for anything similar before filing it.
-Runip
On 02-Sep-2010, at 6:11 PM, pinskia at gcc dot gnu dot org
gcc-bugzi...@gcc.gnu.org wrote:
--- Comment #5 from pinskia at gcc dot gnu dot org 2010-09-03 01:10
---
union {
unsigned int a : 1,
b : 4;
unsigned int data;
};
This is an union of three elements each over lapping, that is a:1 overlaps
with
b:4 and data. So this is expected behavior as far as I can tell.
--
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