Bitmask vs bitfields

[lithiumcat]

Hi,

This question seems to come up quite often, however I haven't managed
to find an answer relevant to my case.

I often use binary flags, and I have alaways used a "bitmask"
technique, that is using #defined or const int powers of two, and
using the following primitives :
/* declaration and initizalisation of flags */ int flags = 0;
/* setting flag */ flags |= FLAG_1;
/* resseting flag */ flags &= ~FLAG_2;
/* conditional setting or resetting */ if (condition) flags |= FLAG_3;
else flags &= ~FLAG_3;
/* testing flags */ if ((flags & FLAG_4) || !(flags & FLAG_5)) { ... }
/* copy of the flag set */ other_flags = flags;

These are the only operations I ever use, they are only internal
representations. When I store or load or exchange data, I use a human-
readable text format, which basically boils down to setting or testing
flags with the above primitives.

I recently came across the "bitfield" concept, so an alernate solution
would be:
/* declaration and initizalisation of flags */
struct {
unsigned flag1 : 1;
unsigned flag2 : 1;
unsigned flag3 : 1;
unsigned flag4 : 1;
unsigned flag5 : 1;
} flags;
/* setting flag */ flags.flag1 = 1;
/* resetting flag */ flags.flag2 = 0;
/* condition setting or resetting */ flags.flag3 = (condition);
/* testing flags */ if (flags.flag4 || !flags.flag5) { ... }
/* copy fo the flag set */ other_flags = flags;

My first question is, is the last line correct? structure assignment
is a new concept for me, and I'm not familiar with it.

Are these codes portable? I have read quite a lot of portability
warning when using bitfields, however considering the limited set of
operations I used (in particular, the internal representation of the
bitfield is never taken into accoutn), I can't see any problem in it.

Is there any efficiency difference between the two methods? Of course,
it depends on the platform and the compiler, but there might be a rule
of thumb. I have read that the bitfield is usually less efficient,
however I can't see while a compiler with optimization turned on would
produce a different code than with the bitmask method.

With my naive point of view, I can see a few advantages to the
bitfield method : I find the code much more readable (especially for
testing), there is no namespace problems, and I don't have to care
whether or not I'm using more bits than the machine word or not (I
already had to use 34 flags on a 32-bits platform with 32-bits int and
long, and it was quite painful) so in that sense it seems more
portable than the bitmask method.

Could you please help me choosing between the two methods?

Thanks in advance.

 

[Ian Collins]

/* copy fo the flag set */ other_flags = flags;

My first question is, is the last line correct? structure assignment
is a new concept for me, and I'm not familiar with it.

Yes, structures are copied byte for byte.

Are these codes portable? I have read quite a lot of portability
warning when using bitfields, however considering the limited set of
operations I used (in particular, the internal representation of the
bitfield is never taken into accoutn), I can't see any problem in it.

Strictly, no. But in practice I have never had a problem using them.

Is there any efficiency difference between the two methods? Of course,
it depends on the platform and the compiler, but there might be a rule
of thumb. I have read that the bitfield is usually less efficient,
however I can't see while a compiler with optimization turned on would
produce a different code than with the bitmask method.

That you will have to measure on your platform.

With my naive point of view, I can see a few advantages to the
bitfield method : I find the code much more readable (especially for
testing), there is no namespace problems, and I don't have to care
whether or not I'm using more bits than the machine word or not (I
already had to use 34 flags on a 32-bits platform with 32-bits int and
long, and it was quite painful) so in that sense it seems more
portable than the bitmask method.

Could you please help me choosing between the two methods?

It's down to you and your requirements. I prefer bitfields for the
reasons you mention.

 

[Ben Bacarisse]

I often use binary flags, and I have alaways used a "bitmask"
technique, that is using #defined or const int powers of two, and
using the following primitives :
/* declaration and initizalisation of flags */ int flags = 0;
/* setting flag */ flags |= FLAG_1;
/* resseting flag */ flags &= ~FLAG_2;
/* conditional setting or resetting */ if (condition) flags |= FLAG_3;
else flags &= ~FLAG_3;
/* testing flags */ if ((flags & FLAG_4) || !(flags & FLAG_5)) { ... }
/* copy of the flag set */ other_flags = flags;

These are the only operations I ever use, they are only internal
representations. When I store or load or exchange data, I use a human-
readable text format, which basically boils down to setting or testing
flags with the above primitives.

I recently came across the "bitfield" concept, so an alernate solution
would be:
/* declaration and initizalisation of flags */
struct {
unsigned flag1 : 1;
unsigned flag2 : 1;
unsigned flag3 : 1;
unsigned flag4 : 1;
unsigned flag5 : 1;
} flags;
/* setting flag */ flags.flag1 = 1;
/* resetting flag */ flags.flag2 = 0;
/* condition setting or resetting */ flags.flag3 = (condition);
/* testing flags */ if (flags.flag4 || !flags.flag5) { ... }
/* copy fo the flag set */ other_flags = flags;

My first question is, is the last line correct? structure assignment
is a new concept for me, and I'm not familiar with it.

Yes, it is fine, but you need have named your struct or you won't be
able to declare the other_flags variable.

Are these codes portable? I have read quite a lot of portability
warning when using bitfields, however considering the limited set of
operations I used (in particular, the internal representation of the
bitfield is never taken into accoutn), I can't see any problem in
it.

Yes. Portability is affected by things like the largest bitfield that
is allowed, and the mapping onto values actual bit positions. You
case is OK.

Is there any efficiency difference between the two methods? Of course,
it depends on the platform and the compiler, but there might be a rule
of thumb. I have read that the bitfield is usually less efficient,
however I can't see while a compiler with optimization turned on would
produce a different code than with the bitmask method.

Do you care about such fine differences? If you do, the only recourse
is to measure and see.

With my naive point of view, I can see a few advantages to the
bitfield method : I find the code much more readable (especially for
testing), there is no namespace problems, and I don't have to care
whether or not I'm using more bits than the machine word or not (I
already had to use 34 flags on a 32-bits platform with 32-bits int and
long, and it was quite painful) so in that sense it seems more
portable than the bitmask method.

Could you please help me choosing between the two methods?

The main disadvantage (that I can see) is that bits can't be tested
in sets. If you are happy to abandon that usage, then you must also
ask why you don't just have a struct with char fields. This has a few
(slight) advantages over using bit fields and it is unlikely that
the wasted space matters except where the program needs lots of copies
of the struct.

 

[Ian Collins]

The main disadvantage (that I can see) is that bits can't be tested
in sets. If you are happy to abandon that usage, then you must also
ask why you don't just have a struct with char fields. This has a few
(slight) advantages over using bit fields and it is unlikely that
the wasted space matters except where the program needs lots of copies
of the struct.

Or the bits map to hardware.

 

[Ian Collins]

Or the bits map to hardware.

Or bits in some other externally defined object.

 

[lithiumcat]

Do you care about such fine differences? If you do, the only recourse
is to measure and see.

I don't care about fine differences, when speed is really an issue
portability is no longer a concern and I measure to get the optimal
code
for the concerned platform.

However I do care about large differences. For example something like
"generally bitfield are vastly less effeciently implemented than
bitmask
because so few people use them" would have been enough to make me
reconsider the use of bitfields. I'm glad this is not the case.

The main disadvantage (that I can see) is that bits can't be tested
in sets. If you are happy to abandon that usage, then you must also
ask why you don't just have a struct with char fields. This has a few
(slight) advantages over using bit fields and it is unlikely that
the wasted space matters except where the program needs lots of copies
of the struct.

I've seldom needed to test a set of flags, and when it was the case it
was never more than two or three flags, so I don't mind abandonning
that.

I haven't thought about a struct of char fields, because I still can't
see the advantages over a bitfield, except the small speed gain of not
having to perform bit shifts under the scenes. Actually that might be
more efficient than the bitmask method too, I should keep that in mind
next time I have a speed-critical program. But is there any other
advantage?

 

[Ben Bacarisse]

Or bits in some other externally defined object.

Yes, may pedant hat must have fallen off. I meant "if you are happy
to abandon the idea of bits".

 

[Ben Bacarisse]

I've seldom needed to test a set of flags, and when it was the case it
was never more than two or three flags, so I don't mind abandonning
that.

I haven't thought about a struct of char fields, because I still can't
see the advantages over a bitfield, except the small speed gain of not
having to perform bit shifts under the scenes. Actually that might be
more efficient than the bitmask method too, I should keep that in mind
next time I have a speed-critical program. But is there any other
advantage?

In "the old days" some compilers did support bit fields, but I can't
image there are any like that left. There will be a few situations
where writing and then reading back a struct full of chars will be
portable where a struct of bitfields is not (not many, but a few).

 

[Ian Collins]

Why "no?" Bit-fields are part of the language, the width
specified (1 bit) is supported on all implementations, and even
the implementation's discretion to make plain-int bit-fields
either signed or unsigned has been explicitly overridden with
the `unsigned' keyword. What's non-portable?

I thought the order of the bits and support for bigger than unsigned
char sized fields was implementation defined.

 

[Ian Collins]

The arrangement of the bits in the "storage unit" is
implementation-defined, as is the size of the S.U. itself.
But that doesn't matter if you're treating the bit-fields
individually (as the O.P. was); it's only an issue if you're
trying to manipulate them en masse or make them match an
externally-imposed format or something of that kind.

That was my point. Other's experience may differ, but most of my uses
of bit-fields have been in drivers to map registers and protocols to map
bits in packets.

 

[lawrence.jones]

I thought the order of the bits and support for bigger than unsigned
char sized fields was implementation defined.

The order of the bits is, but implementations must support bit fields up
to the width of an object with the same type as the bit field, so int
bit fields up to 15 bits are portable as are unsigned bit fields up to
16 bits.

-Larry Jones

At times like these, all Mom can think of is how long she was in
labor with me. -- Calvin