Re: [boost] sorting floats by casting to integer

Subject: Re: [boost] sorting floats by casting to integer
From: Steven Ross (spreadsort_at_[hidden])
Date: 2009-07-06 16:51:58

On Mon, Jul 6, 2009 at 1:48 PM, Steven Ross <spreadsort_at_[hidden]> wrote:

>
>
> On Mon, Jul 6, 2009 at 11:07 AM, Vladimir Prus <vladimir_at_[hidden]>wrote:
>
>> > Yes, I guess I was using denormalized numbers for the 23-bit case, but
>> it's
>> > interesting that there is such a huge speed difference, and I doubt it's
>> > because they're denormalized; there is no reason to special-case
>> > denormalized sorting.
>>
>> It is well possible that comparing denormalized numbers is slow.
>
>
>  Taking 10M elements with my 23 bits of randomness case, and making the top
> exponent bit 1, so that the data is normalized:
>      if(!(float_mem_cast<float, int>(array[v]) & 0x7f800000)) {
>        //Make the top exponent bit high
>        CAST_TYPE temp = 0x80000000 | float_mem_cast<float, int>(array[v]);
>        memcpy(&(array[v]), &temp, sizeof(DATATYPE));
>        //printf("new %e\n", array[v]);
>      }
>
> I see std::sort take 83.527s, as compared to 1.226s when sorted as
> integers.  denormalized numbers are not the problem.
>
That was the sign bit.  Sorry.  With that fixed (0x40000000), the change
does appear to be an issue with denormalized numbers.
Instead, I'm seeing 3X speedups on modest-sized tests.

I still have a problem: what's a good floating-point benchmark?