Most SV structures are greater than 12 bytes, and I could reduce the overhead commensurately if (for example) there was only ever one SvNULL allocated in the system and all other SV's were multiples of 16 or 20 bytes?

But more than one SvNULL will be created, as the following shows: 

$ perl -MDevel::Peek -we '$a = undef; $b = undef; Dump($a); Dump($b)'
SV = NULL(0x0) at 0x8191dc4
  REFCNT = 1
  FLAGS = ()
SV = NULL(0x0) at 0x8191ddc
  REFCNT = 1
  FLAGS = ()

[download]
And note that their addresses are 24 bytes apart, which doesn't suggest any alignment on 16 or 20 byte intervals. This: 
$ perl -MDevel::Peek -we '$a = 1; $b = 2; Dump ($a); Dump ($b)'
SV = IV(0x8192edc) at 0x8191dbc
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 1
SV = IV(0x8192ee4) at 0x8191de0
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 2

[download]
creates two SVs 36 bytes apart, which suggest you can't use anything larger than 12.

In reply to Re: Circular reference testing. by Anonymous Monk
in thread Circular reference testing. by BrowserUk

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