-Ursprüngliche Nachricht-
Von: Adriaan van Os [EMAIL PROTECTED]
An: FPC developers' list fpc-devel@lists.freepascal.org
Gesendet: Samstag, 2. Juli 2005 04:01
Jonas Maebe wrote:
Gerhard Scholz wrote:
if really someone does this, it would be nice to have it in
standard (FPC)
mode also.
You will be able to activate it with a preprocessor switch.
I assume it would be very important to have it switchable in the source.
e.g.
p = packed record
a : 0..100 ;
b : boolean ;
end ; { this should be really packed; 1 byte }
q = packed record
a : 0..100 ;
b : boolean ;
end ; { this should be standard packed; 2 bytes }
so both versions could coexist, either by some new {$packrecords extreme} local
compilerdirectives or e.g. STRICT PACKED RECORD or something similar (please: no
new keyword!).
not just packed records, but also packed arrays:
packed array [ 1..2 ] of 0..15 { one byte }
packed array [ 1..8 ] of boolean { one byte }
The implementation of that would be completely separate from the
support required for records. Also, since that is not required for
compatibility, that will have a lower priority (unless someone submits
a clean patch for it, of course)
I am not trying to say that bit-level packing should have high priority
(there are ways to work around it) but, since you mention
compatibility, I would like to note that bit-level packed arrays are
indeed a feature of UCSD, MPW, Think, MetroWerks, and GNU Pascal.
It came into my mind that nearly everything to integrate (for the moment I call
them strict) strict packed records/arrays is already in the compiler: the
trick would be to see such a record or array as an object (some little inline
preprocessor ?) with some static read/write/index properties (should work for
objects (and maybe classes) also). The ref.pdf describes these properties only
for classes, not for objects, but I tried it and found out that the compiler
didn't read the documentation :-) , properties for objects seem to work o.k.
I assume the best point to incorporate this would be when the structure
definition has been read.
Fields, which are not packable, should stay untouched.
A working example, which should make clearer what I mean, is in the appendix.
The resulting code even looks quite compact. But of course in the ref manual it
should be stated quite clear that using bit-packing is punished by additional
execution time. On the other side there are enough situations where you have to
look into bytes and have to to all shifts/masks yourself. Then bit-packing would
be a help. Or you just have huge amounts of small data to store (the usual
example is the array [ 1..100 ] of boolean).
Another issue that comes to my mind is passing components of packed
structures as actual variable parameters. Standard Pascal doesn't allow
it at all, but it is more practical to forbid it only if the packed
component uses bit-packing.
The packed records or arrays, as they are implemented now, allow it to use their
fields as var params (I just tried some simple examples) obviously.
In my proposal, as a side-effect, bit-packed fields are forbidden as var-params,
but fields which are not bit-packable are allowed.
Gerhard
pckrecar.pas
Description: Binary data
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