> The data which needs to be stored along with the buffer data, can be > stored as either a header or a footer. The size of this header needs to be > a multiple of 16 (or possibly even 8) bytes, so that the real buffer > which follows would be correctly aligned. I'm not sure if this applies for > a footer.
> A footer might allow us to to tack 5-8 bytes on to the end of an > allocation, which might not always go 'over' the 16 byte rounding-up limit > we currently have in place. As long as we're not just-below the 16 byte > barrier on most of our allocations, this shouldn't waste any more memory. My current code (see http://www.mail-archive.com/perl6-internals@perl.org/msg09196.html) requires one flag byte within the buffer itself, provided the buffer is always guaranteed to be large enough to hold a pointer, which the current 16-byte allocation scheme ensures. To implement substrings requires an additional pointer (or offset) in the string header. I was assuming that we only want to implement COW for strings, as non-string buffers are generally speaking not under our control. I still think that the cheapest implementation for the flag byte is a footer, as we don't need to worry about alignment. This is actually a zero-cost option as far as memory allocation is concerned, as the current allocation scheme always allocates at least one extra byte. Variable-sized string headers are not really an option; if the overhead of an additional pointer is a problem, my first inclination would be to combine the chartype and encoding pointers into a single vtable entry. Dan's recent addition of (re)allocate_about conflicts with my current footer implementation, so I do not have a synced-up version of my COW code at present. These functions have also re-introduced the string growth problem (see http://www.mail-archive.com/perl6-internals@perl.org/msg09322.html); the patch and sample program below can be used to illustrate this. -- Peter Gibbs EmKel Systems Index: core.ops =================================================================== RCS file: /home/perlcvs/parrot/core.ops,v retrieving revision 1.130 diff -u -r1.130 core.ops --- core.ops 24 Apr 2002 20:31:39 -0000 1.130 +++ core.ops 28 Apr 2002 10:59:35 -0000 @@ -2366,6 +2366,18 @@ =cut +=item B<debug>(in STR) + +Print string header information for debugging purposes. + +=cut + +inline op debug(in STR) { + printf("%p -> %p; buflen %lu, bufused %lu, flags %lx\n", + $1, $1->bufstart, $1->buflen, $1->bufused, $1->flags); + goto NEXT(); +} + ############################################################################ ### set S0, "test" set S1, S0 debug S1 clone S2, S1 clone S1, S2 clone S2, S1 clone S1, S2 clone S2, S1 clone S1, S2 debug S1 end
