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bit fields in defstruct
Date: 15 Apr 86 00:17 EST
Can someone tell me why Common Lisp defstruct doesn't support bit fields?
This is a Zetalisp feature that I make heavy use of. The equivalent feature
in Common Lisp would look something like:
((low-bar (byte 8 0))
(mid-bar (byte 8 8) #x00 :read-only t) ;filled by constructor
(high-bar (byte 8 12) #x1a))
This makes THINGIE a vector with 4 elements (counting the structure name);
the second element is an integer with default value #x1a0000. The structure
has 5 access functions:
The access function for -BAR slots expands to a LDB, and the SETF macro
expands to a DPB. The knowledge that three of the slots of a THINGIE are
bit fields of the same integer is hidden from the rest of the program.
The reason I need this feature is I am building thousands of instances of a
particular structure, and the difference between representing half a dozen
bit fields with one integer versus six integers is significant in terms of
memory cost. I don't think a user should build his own access functions for
bit fields; I think defstruct should do that for him.
(1) Syntax. What do you think about
((low-bar 0 :byte-field (byte 8 0))
(mid-bar #x00 :byte-field (byte 8 8) :read-only t)
(high-bar #x1A :byte-field (byte 8 12)))
instead of yours? Yes, I know the default value must be specified, but
this syntax is consistent with non-nested syntax. I propose that a
value of NIL means that it isn't filled in (and isn't forced to zero
(2) Performance. Users should be wary that doing such things has
efficiency issues depending on the boundaries between fixnum and bignum.
That is, since the fields are integers, a user could accidentally go
consing a lot of bignums, where a normal defstruct wouldn't.
[3, In your example, mid-bar and high-bar overlap. mid-bar gets filled
in by the constructor, and high-bar has a default. How are these
merged, since they overlap?]
PS: I suppose if defstruct were highly optimizing one could declare six
fields of :TYPE BIT and expect them to be packed into one word. This
doesn't solve everything though; in some cases (like when communicating
with external programs) one wants to specify the exact postion of each bit
field in a word. Also, one might want to have overlapping bit fields in the
same word, such as three 1-bit fields and a 3-bit field that covers them.
Indeed, this is one of the motivations for Symbolics extending defstruct
along these lines. Normal users don't need this; it is usually used for
interfacing to hardware device registers, and sometimes network packets
that have overlapping fields themselves.