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Bit-fields vs integral promotions

 
 
Thurston Manson
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      08-23-2007
Suppose I'm using an implementation where an int is 16 bits. In the
program below, what function is called in the first case, and what is
called in the second case? Also, if there is a difference between C89
and C99, I would like to know. I have tried with different compilers,
and I see some differences. Before I file a bug report with my C
vendor, I would like to know what the correct behavior is.

struct S
{
unsigned a:4;
unsigned b:16;
};

foo();
bar();

main()
{
struct S s;
s.a = s.b = 0;

if (s.a - 5 < 0) foo();
else bar();

if (s.b - 5 < 0) foo();
else bar();
}

 
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CBFalconer
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      08-23-2007
Thurston Manson wrote:
>
> Suppose I'm using an implementation where an int is 16 bits. In
> the program below, what function is called in the first case, and
> what is called in the second case? Also, if there is a difference
> between C89 and C99, I would like to know. I have tried with
> different compilers, and I see some differences. Before I file a
> bug report with my C vendor, I would like to know what the correct
> behavior is.
>
> struct S {
> unsigned a:4;
> unsigned b:16;
> };
>
> foo();
> bar();
>
> main() {
> struct S s;
> s.a = s.b = 0;
> if (s.a - 5 < 0) foo();
> else bar();
> if (s.b - 5 < 0) foo();
> else bar();
> }


This won't even compile. foo and bar are never defined. main
should return an int (required in C99). The return is missing
(required in C90). You are comparing signed and unsigned values.
No compiler bugs have been evidenced.

--
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Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>


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Jack Klein
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      08-24-2007
On Thu, 23 Aug 2007 18:08:29 +0200 (CEST), Thurston Manson
<(E-Mail Removed)> wrote in comp.lang.c:

> Suppose I'm using an implementation where an int is 16 bits. In the
> program below, what function is called in the first case, and what is
> called in the second case? Also, if there is a difference between C89
> and C99, I would like to know. I have tried with different compilers,
> and I see some differences. Before I file a bug report with my C
> vendor, I would like to know what the correct behavior is.
>
> struct S
> {
> unsigned a:4;
> unsigned b:16;
> };
>
> foo();
> bar();
>
> main()


Since you mentioned C99, I'll point out that the two declarations and
the definition of main() above are illegal in C99. Implicit int was
removed from the language in C99.

> {
> struct S s;
> s.a = s.b = 0;
>
> if (s.a - 5 < 0) foo();
> else bar();
>
> if (s.b - 5 < 0) foo();


Before you file a bug report with your C vendor, I'd suggest
considering that the "correct behavior" is for the programmer to
write:

if ((int)s.b - 5 < 0) foo();

....or:

if ((unsigned)s.b - 5 < 0) foo();

....and make the desired behavior explicit.

> else bar();
> }


This would probably be better asked on comp.std.c, by the way. I'm
cross posting this reply there, to see what comment it elicits.

As for an unsigned bit-field holding 16 bits on a platform where int
has 16 bits, this is quite clear. It must promote to unsigned int,
therefore "s.b - 5" becomes (unsigned int)(UINT_MAX - 4), otherwise
known as 65531.

The promotion of an unsigned int bit-field with fewer bits than the
number of value bits in a signed int is more controversial.

Unfortunately, the wording of the C90 standard concerning bit-fields
is obscure and rather ambiguous. The wording of the C99 standard has
changed and added more specifics, mainly due to the addition of _Bool
and the <stdint.h> integer types, but at least it specifically
mentions bit-fields in the integer promotions. Some feel that it is
still ambiguous, but I think it is pretty clear.

I'll start with C99 (which is easier to quote from the PDF file):

====
6.3.1 Arithmetic operands
6.3.1.1 Boolean, characters, and integers
1 Every integer type has an integer conversion rank defined as
follows:
— No two signed integer types shall have the same rank, even if they
have the same representation.
— The rank of a signed integer type shall be greater than the rank of
any signed integer type with less precision.
— The rank of long long int shall be greater than the rank of long
int, which shall be greater than the rank of int, which shall be
greater than the rank of short int, which shall be greater than the
rank of signed char.
— The rank of any unsigned integer type shall equal the rank of the
corresponding signed integer type, if any.
— The rank of any standard integer type shall be greater than the rank
of any extended integer type with the same width.
— The rank of char shall equal the rank of signed char and unsigned
char.
— The rank of _Bool shall be less than the rank of all other standard
integer types.
— The rank of any enumerated type shall equal the rank of the
compatible integer type (see 6.7.2.2).
— The rank of any extended signed integer type relative to another
extended signed integer type with the same precision is
implementation-defined, but still subject to the other rules for
determining the integer conversion rank.
— For all integer types T1, T2, and T3, if T1 has greater rank than T2
and T2 has greater rank than T3, then T1 has greater rank than T3

2 The following may be used in an expression wherever an int or
unsigned int may be used:
— An object or expression with an integer type whose integer
conversion rank is less than the rank of int and unsigned int.
— A bit-field of type _Bool, int, signed int, or unsigned int.

If an int can represent all values of the original type, the value is
converted to an int; otherwise, it is converted to an unsigned int.

These are called the integer promotions. All other types are unchanged
by the integer promotions.
====

Notice the last bullet point in paragraph 2, that specifies that
bit-fields of int, signed int, unsigned int, and, in C99 only, _Bool,
participate in the integer promotions. This is followed by the
definition of integer promotions, namely that if all values of the
original type are within the range of signed int,

Next, in C99, we get to the definition of bit-fields, 6.7.2.1 P9:

====
A bit-field is interpreted as a signed or unsigned integer type
consisting of the specified number of bits.
====

Now to tie this together, we have 6.2.6.2 P6:

====
The precision of an integer type is the number of bits it uses to
represent values, excluding any sign and padding bits. The width of an
integer type is the same but including any sign bit; thus for unsigned
integer types the two values are the same, while for signed integer
types the width is one greater than the precision.
====

So in C99, even though "A bit-field of type _Bool, int, signed int, or
unsigned int" participates in the integer promotions, it is converted
to signed in if "an int can represent all values of the original
type", but there has been quite some discussion about whether "the
original type" means either:

1. The "original type" of an unsigned int bit-field is unsigned int,
regardless of the fact that the number of bits used is less than the
value bits in a signed int, so an unsigned int bit-field always
promotes to unsigned int. Even though all the values that can
actually fit into a small unsigned int bit-field are within the range
of values that an int can hold.

2. The "original type" is the user-defined "unsigned integer type
consisting of the specified number of bits."

There seems to be less ambiguity when precision is factored in. Since
the unsigned int bit-field is "interpreted" as consisting of the
specified number of bits, its precision is that number of bits.

And there is the clear statement that "the rank of a signed integer
type shall be greater than the rank of any signed integer type with
less precision" together with "the rank of any unsigned integer type
shall equal the rank of the corresponding signed integer type."

So s.a has a precision of 4, and signed int on the implementation you
specified has a precision of 15, which means it has a lesser rank than
signed or unsigned int, and therefore should promote to signed int
because all the values that can fit in an unsigned int with a
precision of 4 are in the range of signed int.

There has been some argument that this chain of reasoning falls apart
because it is based on the word "interpreted", which is not actually
defined by the standard, at least not as used in this context.

For C90, things are even muddier.

The entire section on the integer promotions 6.2.1.1 is:

====
A char, a short int, or an int bit-field, or their signed or
unsigned varieties, or an object that has enumeration type, may be
used in an expression wherever an int or unsigned int may be used. If
an int can represent all values of the original type, the value is
converted to an int; otherwise it is converted to an unsigned int.
These are called the integral promotions.

The integral promotions preserve value including sign. As
discussed earlier, whether a ``plain'' char is treated as signed is
implementation-defined.
=====

The statement about bit-fields is nearly the same as C99, "A bit-field
is interpreted as an integral type consisting of the specified number
of bits."

C90 makes no mention of "rank", or "precision" for integer types based
on number of value bits.

So there are fewer sign posts in C90, all one can go on is one's
opinion about whether the "original type" of "unsigned: 4:" is
"unsigned" or an "interpreted" type that can hold the values from 0 to
16.

My personal opinion is that the intent in both versions is that an
unsigned int bit-field with the same number of value bits as a signed
int, or fewer, should promote to signed int.

I have to agree with posted sentiments that the normative text of the
standard does not actually state that conclusively, although one can
make a stronger case for C99 than for C90.

As for filing a bug report, don't be surprised if your compiler vendor
disagrees with you.

Every compiler I have ever tested this on always promotes unsigned
bit-fields to unsigned int, regardless of the number of bits. This
includes at least Microsoft VC++ 6.0 an 2005 Express, Borland C++
Builder X, Peles C, and MinGW.

So even if you agree that s.a should promote to signed int, be
prepared to hear that the standard is ambiguous, especially C90, and
also that most compilers promote it to unsigned.

--
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Chris Torek
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      08-24-2007
In article <(E-Mail Removed)>
Thurston Manson <(E-Mail Removed)> wrote:
>[assuming 16-bit "int" in the example below]
>... I would like to know what the correct behavior is.
>
>struct S
>{
> unsigned a:4;
> unsigned b:16;
>};

[snippage]
> struct S s;
> s.a = s.b = 0;
>
> if (s.a - 5 < 0) foo();
> else bar();
>
> if (s.b - 5 < 0) foo();
> else bar();


Jack Klein has given an accurate (I believe) description of the
State of the Standard, which I think boils down to "nobody seems
to know for sure, because compilers don't do what the Standard
seems to say they should." (OK, this is a bit oversimplified.
Realistically, it is a problem though.)

Since I am waiting for yet another interminable compile, I will
take this opportunity to gripe about the C standards here. I think
C *should* have been defined with the much simpler rule: "in a
meetup between signed and unsigned, unsigned persists". That is,
given a narrow unsigned [%] type, and a wider signed type, and an
operator that has to find a common type for whatever reason, the
signed type should be converted to unsigned.

If C worked like this, the answer would be clear: since s.a and
s.b are both explicitly "unsigned", they would widen to ordinary
unsigned int, regardless of the relative values of INT_MAX and
UINT_MAX. Then, since s.a and s.b had value 0, the subtraction
would always result in (UINT_MAX + 1 - 5), or (UINT_MAX - 4), an
unsigned number definitely greater than zero, so both tests would
call foo().

Unfortunately, to paraphrase someone I might prefer not to paraphrase,
we have to work with the Standard we have, not the Standard we
might like or wish to have. So in questionable cases like
this, you are probably best off using casts or temporary variables
to force whichever interpretation you wnat.

[%] In order to handle plain char, which may be unsigned (i.e.,
CHAR_MAX == UCHAR_MAX and CHAR_MIN == 0) but should probably expand
to plain, i.e., signed, int, I think the rule should apply only to
things explicitly declared as "unsigned". This probably should
apply to bitfields as well, since the signedness of bitfields is
implementation-defined. Note that this would have some annoying
implications:

- If CHAR_BIT is 16 or more and CHAR_MAX == UINT_MAX, a plain
"char" (which is unsigned on this implementation) widens to
signed "int", which cannot hold all the values in the
original. (Of course, if you explicitly write "unsigned char",
it remains unsigned.)

- Similarly, if the number of bits in a plain "int" is N, the
bitfield:
int field:N;
would widen to a signed int, even if the implementation makes
"plain" ints in bitfields unsigned. (Again, if you used
unsigned int field:N;
it would remain unsigned, because you explicitly used the
"unsigned" keyword.)

(But I do not get to dictate how C should work.)
--
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email: forget about it http://web.torek.net/torek/index.html
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André Gillibert
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      08-24-2007
Jack Klein wrote:

>
> As for an unsigned bit-field holding 16 bits on a platform where int
> has 16 bits, this is quite clear. It must promote to unsigned int,
> therefore "s.b - 5" becomes (unsigned int)(UINT_MAX - 4), otherwise
> known as 65531.


It's not that clear.
If signed int can hold all the values of unsigned int (IIRC, there are
platforms without "native" unsigned arithmetic for which this is the
case), then, it should be promoted to int.

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Jack Klein
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      08-24-2007
On Fri, 24 Aug 2007 11:03:54 +0200, "André Gillibert"
<(E-Mail Removed)> wrote in comp.std.c:

> Jack Klein wrote:
>
> >
> > As for an unsigned bit-field holding 16 bits on a platform where int
> > has 16 bits, this is quite clear. It must promote to unsigned int,
> > therefore "s.b - 5" becomes (unsigned int)(UINT_MAX - 4), otherwise
> > known as 65531.

>
> It's not that clear.
> If signed int can hold all the values of unsigned int (IIRC, there are
> platforms without "native" unsigned arithmetic for which this is the
> case), then, it should be promoted to int.


Please note that I specifically said "on a platform where int has 16
bits". A platform such as you allude to must have at least 17 bits in
a signed int.

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CBFalconer
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      08-24-2007
Jack Klein wrote:
>

.... snip ...
>
> Please note that I specifically said "on a platform where int has
> 16 bits". A platform such as you allude to must have at least 17
> bits in a signed int.


I don't believe you can have such without having a 17 bit unsigned
int, thus leading to a 18 bit int, and thence to an 18 bit
unsigned, and ....

I don't think use of trap bits can affect this. Unsigned to signed
conversion is always possible because of the modulo unsigned
arithmetic.

--
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<http://cbfalconer.home.att.net>



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André Gillibert
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      08-24-2007
Jack Klein wrote:

> Please note that I specifically said "on a platform where int has 16
> bits". A platform such as you allude to must have at least 17 bits in
> a signed int.
>

Yep. 18 bits machine did exist... I doubt that there has ever been any 18
bits machine with 1 padding bit, and one sign bit, without unsigned
arithmetic.

Anyway, the standard *permits* that, even if it's practically impossible
to find any existing platforms that uses such a model.

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Francis Glassborow
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      08-25-2007
CBFalconer wrote:
> Jack Klein wrote:
> ... snip ...
>> Please note that I specifically said "on a platform where int has
>> 16 bits". A platform such as you allude to must have at least 17
>> bits in a signed int.

>
> I don't believe you can have such without having a 17 bit unsigned
> int, thus leading to a 18 bit int, and thence to an 18 bit
> unsigned, and ....
>
> I don't think use of trap bits can affect this. Unsigned to signed
> conversion is always possible


You may be correct (I do not have time to research the Standard to come
up with a conclusion) but not for the reason you give.

because of the modulo unsigned
> arithmetic.
>

Modulo unsigned arithmetic only guarantees that signed to unsigned is
always possible it has nothing to say about what happens when a
sufficiently large unsigned value is converted to a signed one.


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Richard Heathfield
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      08-25-2007
CBFalconer said:

> Thurston Manson wrote:
>>

<snip>
>>
>> struct S {
>> unsigned a:4;
>> unsigned b:16;
>> };
>>
>> foo();
>> bar();
>>
>> main() {
>> struct S s;
>> s.a = s.b = 0;
>> if (s.a - 5 < 0) foo();
>> else bar();
>> if (s.b - 5 < 0) foo();
>> else bar();
>> }

>
> This won't even compile.


Yes, it will.

> foo and bar are never defined.


So? They are implicitly declared by first use. That's enough for
compilation.

> main should return an int (required in C99).


Clearly this is C90 code.

> The return is missing (required in C90).


Not so. The return status is undefined, but the behaviour of the program
is not. In fact, I think that the TU is strictly conforming. I'm not
saying I approve of omitting the return statement, of course - I don't.

> You are comparing signed and unsigned values.


So?

> No compiler bugs have been evidenced.


Indeed.

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