diff options
Diffstat (limited to 'gv.c')
| -rw-r--r-- | gv.c | 59 |
1 files changed, 56 insertions, 3 deletions
@@ -391,6 +391,52 @@ Perl_gv_init_pv(pTHX_ GV *gv, HV *stash, const char *name, U32 flags) gv_init_pvn(gv, stash, name, strlen(name), flags); } +/* Packages in the symbol table are "stashes" - hashes where the keys are symbol + names and the values are typeglobs. The value $foo::bar is actually found + by looking up the typeglob *foo::{bar} and then reading its SCALAR slot. + + At least, that's what you see in Perl space if you use typeglob syntax. + Usually it's also what's actually stored in the stash, but for some cases + different values are stored (as a space optimisation) and converted to full + typeglobs "on demand" - if a typeglob syntax is used to read a value. It's + the job of this function, Perl_gv_init_pvn(), to undo any trickery and + replace the SV stored in the stash with the regular PVGV structure that it is + a shorthand for. This has to be done "in-place" by upgrading the actual SV + that is already stored in the stash to a PVGV. + + As the public documentation above says: + Converting any scalar that is C<SvOK()> may produce unpredictable + results and is reserved for perl's internal use. + + Values that can be stored: + + * plain scalar - a subroutine declaration + The scalar's string value is the subroutine prototype; the integer -1 is + "no prototype". ie shorthand for sub foo ($$); or sub bar; + * reference to a scalar - a constant. ie shorthand for sub PI() { 4; } + * reference to a sub - a subroutine (avoids allocating a PVGV) + + The earliest optimisation was subroutine declarations, implemented in 1998 + by commit 8472ac73d6d80294: + "Sub declaration cost reduced from ~500 to ~100 bytes" + + This space optimisation needs to be invisible to regular Perl code. For this + code: + + sub foo ($$); + *foo = []; + + When the first line is compiled, the optimisation is used, and $::{foo} is + assigned the scalar '$$'. No PVGV or PVCV is created. + + When the second line encountered, the typeglob lookup on foo needs to + "upgrade" the symbol table entry to a PVGV, and then create a PVCV in the + {CODE} slot with the prototype $$ and no body. The typeglob is then available + so that [] can be assigned to the {ARRAY} slot. For the code above the + upgrade happens at compile time, the assignment at runtime. + + Analogous code unwinds the other optimisations. +*/ void Perl_gv_init_pvn(pTHX_ GV *gv, HV *stash, const char *name, STRLEN len, U32 flags) { @@ -435,11 +481,18 @@ Perl_gv_init_pvn(pTHX_ GV *gv, HV *stash, const char *name, STRLEN len, U32 flag } if (SvLEN(gv)) { if (proto) { + /* For this case, we are "stealing" the buffer from the SvPV and + re-attaching to an SV below with the call to sv_usepvn_flags(). + Hence we don't free it. */ SvPV_set(gv, NULL); - SvLEN_set(gv, 0); - SvPOK_off(gv); - } else + } + else { + /* There is no valid prototype. (SvPOK() must be true for a valid + prototype.) Hence we free the memory. */ Safefree(SvPVX_mutable(gv)); + } + SvLEN_set(gv, 0); + SvPOK_off(gv); } SvIOK_off(gv); isGV_with_GP_on(gv); |