On Mon, 13 Dec 2010 12:33:33 +0000
Jonathan Kimmitt <jonat...@kimmitt.co.uk> wrote:
>
> > A C compiler would optimize this to a right shift. Changing that to
> > 'Int64.shift_right n 1' speeds up the code.
>
> Sorry to be a pedant, but this is not correct. The optimisation is
> only possible when the arguments are unsigned integers
That particular program never used negative integers.
> which I don't
> think is specifiable when working in OCAML
You are right, there is no way to tell ocaml that.
>
> # Int64.shift_right (-2L) 1;;
> - : int64 = -1L (So far, so good)
> # Int64.div (-1L) 2L;;
> - : int64 = 0L (Good)
> # Int64.shift_right (-1L) 1;;
> - : int64 = -1L (Duh)
It is still possible to avoid the division, gcc generates this:
movq %rdi, %rax
shrq $63, %rax
addq %rdi, %rax
sarq %rax
Or a better example with division by 8:
leaq 7(%rdi), %rax
testq %rdi, %rdi
cmovns %rdi, %rax
sarq $3, %rax
And division by non-power of two integers can optimized by replacing it
with multiplication with its inverse (which gcc and llvm don't do for
signed divisions, only for unsigned ones):
http://www.hackersdelight.org/HDcode/magic.c.txt
http://www.hackersdelight.org/HDcode/magicu.c.txt
Best regards,
--Edwin
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