Questions arising from the console log below:

1. Why does reading bits from already allocated memory cause a memory allocation?

   Yes, I am indirecting through a substr ref, but I'm only reading.

2. Why does unpack '%32b*', $v; fail?

   It works just fine here:

   [0] Perl> print unpack '%32b*', chr( 0b10101010 );;
   4
   [download]

Any relevant info gratefully received. Other workarounds for the vec > 2**31 offsets bug?

The following is an annotated console log from my perl REPL.

C:\test>p1  ## Start new session 2.5 MB allocated to process

## Create a 2**32 bit bitvector
[0] Perl> $v = chr(0); $v x= 512*1024*1024;;
## Just over 512 MB allocated to process

## As vec cannot address offsets > 2**31, 
## create a reference to the second half of the bitvector
[0] Perl> $r = \substr $v, 256*1024*1024;;
## No further memory allocated; still just over 512 MB allocated to pr
+ocess

## Set some bits in the first byte of the second half of the vector
## As we can't use offsets that size, address the bits as a byte (offs
+et/8)
[0] Perl> vec( $v, 256*1024*1024, 8 ) = 0b10101010;;
## No further memory allocated; still just over 512 MB allocated to pr
+ocess

## Now check that we can see the bits we just set 
## via the reference we created (offsets == actual offset - 2**31 avoi
+ding the bug
[0] Perl> print vec( $$r, $_, 1 ) for 0..7;;
0
1
0
1
0
1
0
1

## Yes! we can see the bits. (They are in the reverse order to intuiti
+on, but that's okay!)

######## BUT ... the memory usage jumped from 512MB to somewhat over 7
+68 MB!                 ########
## Q1 ## Why? Why does reading bits from already allocated memory caus
+e a memory allocation? ## Q1 ##

## Check the length of the vector
[0] Perl> print length $v;;
536870912
## Still 512 MB

## Try counting the bits
[0] Perl> print unpack "%32b*", $v;;
0
## Q2a ## ZERO! WTF? ## Q2a ##

## Try counting them via the reference
[0] Perl> print unpack "%32b*", $$r;;
0
## Q2b ## ZERO! WTF? ## Q2b ##

## Check the byte we set is non-zero
[0] Perl> print vec( $v, 256*1024*1024, 8 );;
170
## Still set. 170 == 0b10101010 

## Set a bi in the bottom half of the vector without reference tricker
+y
[0] Perl> vec( $v, 7, 1 ) = 1;;

## Now count the bits again
[0] Perl> print unpack "%32b*", $v;;
0
## Q2c ## WTF?

## Check the bit was actually set
[0] Perl> print vec( $v, 7, 1 );;
1
## Yup!

## Check the bits either side are unset
[0] Perl> print vec( $v, 6, 1 );;
0
[0] Perl> print vec( $v, 8, 1 );;
0
## Yup!

## Try doing the same thing again to see it anything changed.
## (Maybe Perl was busy taking a telemarketeers phone call)
[0] Perl> print unpack "%32b*", $v;;
0
## Q2d ## NOPE! Just doesn;t see the bits? ## Q2d ##

## Check via the reference (again).
[0] Perl> print unpack "%32b*", $$r;;
0
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