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

[Don Stewart]

Ah ha! Excellent!

Much apprciated.

twhitehead:
> 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
> 
> 
> 


> _______________________________________________
> Glasgow-haskell-users mailing list
> Glasgow-haskell-users at haskell.org
> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users