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Do pad name string buffers need to be pointer-aligned?

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Father Chrysostomos
November 26, 2014 06:20
Do pad name string buffers need to be pointer-aligned?
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On the sprout/pn branch, I have created a pad name struct:

struct padname {
    char *      xpadn_pv;
    HV *        xpadn_ourstash;
    union {
        HV *    xpadn_typestash;
        CV *    xpadn_protocv;
    } xpadn_type_u;
    U32         xpadn_low;
    U32         xpadn_high;
    U32         xpadn_refcnt;
    int         xpadn_gen;
    U8          xpadn_len;
    U8          xpadn_flags;

which is 5.25 pointers large on 64-bit platforms.

And, to allocate a string buffer as part of the same chunk of memory:

struct padname_with_str {
    struct padname      xpadn_padname;
    char                xpadn_str[1];

Now this allocates the 'padname_with_str' with six empty octets before
the string.  So for short pad names we use 7 pointers.

I could fiddle things a bit to get the string buffer to begin at
xpadn_flags+1.  That would get us down to 6 pointers for four-char
names like $self, or even 5 (or 6 for longer names) if we can merge
typestash and ourstash, as I suggested in another thread ('our
Type $thing').

A couple of years ago someone pointed out that memcmp is faster at
pointer-aligned addresses.  But how much faster?  Is it noticeable?
Note that this would only happen during pad name lookup at com-
pile time.

If I count them correctly, we currently have 3 + 2 + 4 (sv head +
string + xpvnv) = 9 pointers allocated for most pad names, and 11 for
our/typed/mysub pad names.  So either way I am still saving memory.
But I need advice on the best way to proceed.

The two pointers for the string buffer are shown by LEN = 10 below.

$ ./perl -Ilib -e 'use B; use Devel::Peek; Dump B::svref_2object(sub {my$self})->PADLIST->ARRAYelt(0)->ARRAYelt(1)->object_2svref'
SV = IV(0x7fff2c0780d0) at 0x7fff2c0780e0
  REFCNT = 1
  RV = 0x7fff2c02feb8
  SV = PVNV(0x7fff2c06c728) at 0x7fff2c02feb8
    REFCNT = 2
    FLAGS = (POK,pPOK)
    IV = 0
    NV = 6.83282644849413e-312
    PV = 0x7fff2bc21d78 "$self"\0
    CUR = 5
    LEN = 10

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