I'm a Perl-internals newbie, so the following may be invalid, inaccurate, or just plain wrong.. but here's what seems to be going on, from what I can tell: 
%h_delete = %h_list = %h_undef = (0 .. 50000);

delete @h_delete{keys %h_delete};
%h_list = ();
undef %h_undef;

use Devel::Peek;

Dump \%h_delete;
Dump \%h_list;
Dump \%h_undef;

[download]

Output: 


SV = RV(0x830eb38) at 0x838d3a4
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x8171818
  SV = PVHV(0x8171d20) at 0x8171818
    REFCNT = 2
    FLAGS = (SHAREKEYS)
    IV = 0
    NV = 0
    ARRAY = 0x40376008
    KEYS = 0
    FILL = 0
    MAX = 32767
    RITER = -1
    EITER = 0x0

SV = RV(0x830eb38) at 0x838d3a4
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x81765cc
  SV = PVHV(0x8171d50) at 0x81765cc
    REFCNT = 2
    FLAGS = (SHAREKEYS)
    IV = 0
    NV = 0
    ARRAY = 0x403d8008
    KEYS = 0
    FILL = 0
    MAX = 32767
    RITER = -1
    EITER = 0x0

SV = RV(0x830eb38) at 0x838d3a4
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x81765fc
  SV = PVHV(0x8171d80) at 0x81765fc
    REFCNT = 2
    FLAGS = (SHAREKEYS)
    IV = 0
    NV = 0
    ARRAY = 0x0
    KEYS = 0
    FILL = 0
    MAX = 7
    RITER = -1
    EITER = 0x0

[download]

All the hashes have KEYS = 0, and you would think Perl would be smart enough to notice this when you call keys. The only difference in the 3rd one (%h_undef) is that it has no ARRAY pointer (with MAX set accordingly). I would guess that keys travels that entire array looking for keys, and that's where the slowdown is. This seems to be the case from looking at Perl_hv_iternext_flags. A definitive explanation from someone more versed in the internals is welcome ;)

I don't see why the KEYS = 0 case couldn't be optimized; perhaps you should file a bug and see!

blokhead

In reply to Re: undef speedup ?! by blokhead
in thread undef speedup ?! by powerman

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