Re: [Caml-list] Int32 vs float unboxing

[Dmitry Bely <dmitry.bely@gmail.com>]

On Mon, Aug 22, 2011 at 11:58 AM, Alain Frisch <alain.frisch@lexifi.com> wrote:
> On 08/22/2011 09:19 AM, Dmitry Bely wrote:
>>
>> In the code below "s" reference is unboxed in sum_float loop, but not
>> in sum_int32. Why?
>>
>> let sum_int32 v =
>>   let s = ref 0l in
>>   for i=0 to (Array.length v)-1 do
>>     s := Int32.add !s (Array.unsafe_get v i)
>>   done;
>>   Int32.add !s Int32.zero
>>
>> let sum_float v =
>>   let s = ref 0. in
>>   for i=0 to (Array.length v)-1 do
>>     s := !s +. (Array.unsafe_get v i)
>>   done;
>>   !s +. 0.
>
> A random guess: since adding Int32.zero is the identity (contrary to adding
> 0.), the compiler is clever enough to optimize away this useless operation;
> unfortunately, this optimization breaks the unboxing pass.

Probably no. If you replace

   Int32.add !s Int32.zero

with

   Int32.add !s Int32.one

s is still boxed. BTW, even zero addition is not optimized away.

- Dmitry Bely