UVector overallocating for (Word/Int)(8/16/32)

[Tyson Whitehead]

I believe the arrays for (Word/Int)(8/16/32) are currently taking eight, four, 
and two times, respectively, as much memory as actually required.  That is,

newMBU n = ST $ \s1# ->
  case sizeBU n (undefined::e) of {I# len#          ->
  case newByteArray# len# s1#   of {(# s2#, marr# #) ->
  (# s2#, MBUArr n marr# #) }}

sizeBU (I# n#) _ = I# (wORD_SCALE n#)
wORD_SCALE   n# = scale# *# n# where I# scale# = SIZEOF_HSWORD

(sizeBU is a class member, but all the instances for (Word/Int)(8/16/32) are 
as given above, and SIZEOF_HSWORD is defined as 8 in MachDeps.h on my x86_64)

which would seems to always allocate memory assuming an underlying alignment 
that is always eight bytes.  It seems like the readWord(8/16/32)Array# 
functions may have once operated that way, but, when I dumped the assembler 
associated with them under ghc 6.8.2 (both native and C), I get

readWord8Array
  leaq 16(%rsi),%rax
  movzbl (%rax,%rdi,1),%ebx
  jmp *(%rbp)

readWord16Array
  leaq 16(%rsi),%rax
  movzwl (%rax,%rdi,2),%ebx
  jmp *(%rbp)

readWord32Array
  leaq 16(%rsi),%rax
  movl (%rax,%rdi,4),%ebx
  jmp *(%rbp)

readWord64Array
  leaq 16(%rsi),%rax
  movq (%rax,%rdi,8),%rbx
  jmp *(%rbp)

which is using alignments of one, two, four, and eight bytes respectively.

I'll attach a patch (which I haven't tested beyond compiling and looking at 
the generated assembler).

Cheers!  -Tyson



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