I'm writing a simple language to embody the concept of copy-on-write, and so that I can learn how to implement it. The language is called COW and it's at
http://japhy.perlmonk.org/COW/ Ben Tilly suggested I contact the Perl6 Internals folk and let you know that this is an important feature that we should try to incorporate into Perl 6. The main reason I bring this up is because I am a regex fanatic, and I hate that damned $& poison. $_ = "_" x 100_000; while (/./g) { } # runs fast But then...! 1 if $&; # ooh, I just set PL_sawampersand, a hideous global flag! $_ = "_" x 100_000; while (/./g) { ... } # runs real REAL slow The reason the second code is slow is because PL_sawampersand being TRUE makes the regex engine COPY THE STRING it is matched against, so it can provide $`, $&, and $'. COPYING 100,000 byte strings SUCKS. If PL_sawampersand is FALSE, the string is not copied, but a pointer to it is used. This is "dangerous", but since PL_sawampersand is FALSE, it "should" be "safe". Now I'll make it unsafe: $_ = "japhy"; /../; $_ = "tilly"; eval q{ print $& }; # ooh, eval '' hides the $& from compile-time! That prints "ti" instead of "ja", due to the use of pointers. We're lucky the pointer was still valid. :/ So what's copy-on-write? Basically, it's the use of a pointer until that use becomes unsafe. # COW code -- you might need to read the # cow.pod documentation to fully grok this # v_strncpy(dst,src,len) ABSOLUTELY copies # len bytes from src to dst, regardless of # null bytes x = "japhy"; # v_strncpy(SvSTR(sv_x), "japhy", 5) y = cow x; # this does a lot of stuff... # SvCOW_on(sv_x) # copy-on-write # SvDEPa(sv_x, sv_y) # add a dependent # # SvMAGIC_on(sv_y) # y is magical # SvMGs(sv_y, MGt_COW, sv_x) # type=COW, obj=sv_x # SvSTR(sv_y) = SvSTR(sv_x) # use of a pointer print y; # access SvSTR(y) which is a pointer to SvSTR(x) x = "tilly"; # since SvCOW(sv_x) is true... # SvCOW_off(sv_x) # v_strncpy(SvSTR(sv_y), SvSTR(sv_x), SvSTRLEN(sv_x)) # SvDEPr(sv_x, sv_y) # remove dependent # v_strncpy(SvSTR(sv_x), "tilly", 5) # # SvMAGIC_off(sv_y) # remove magic flag # SvMGc(sv_y) # clear magic And that's that. The language will take me a while to develop and all, but I'm interested in helping out as much as I can to integrate COW in Perl 6 -- it'll be a good idea, I feel. -- japhy, perl hacker unordinaire!
