dmd 1.053 and 2.037 release

[Bill Baxter]

On Fri, Dec 11, 2009 at 9:34 PM, Don <nospam at nospam.com> wrote:
> Walter Bright wrote:
>>
>> Don wrote:
>>>
>>> Yeah. Actually the CPU problem is an accepts-invalid bug. It worked on my
>>> Pentium M, but it shouldn't have.
>>> The problem is what DMD does to the "uninitialized assignments".
>>>
>>> float x;
>>>
>>> gets changed into
>>>
>>> float x = double.snan;
>>>
>>> and is implemented with
>>> fld float.snan; fstp x;
>>>
>>> The FLD is triggering the snan. They should be changed into mov EAX,
>>> reinterpret_cast<int>(float.snan); mov x, EAX;
>>
>> Sounds like a good idea.
>>
>>> There's another reason for doing this. On Pentium 4, x87 NaNs are
>>> incredibly slow. More than 250 cycles!!! On AMD and on Pentium 4 SSE2, they
>>> are the same as any other value (about 0.5 cycles). Yet another reason to
>>> hate the P4. But still, this is such a horrific performance killer that we
>>> ought to avoid it.
>>
>> I had no idea that was the case!
>
> I only just discovered it. Every documentation I've seen just said "These
> [cycle count] values are for normal operands. NaNs, infinities, and
> denormals may increase cycle counts considerably." I found a blog of someone
> who'd actually measured it.

I experienced it in a fluid sim I was working on in grad school.  NaNs
were creeping in and performance was terrible.  I thought it was two
problems till I got rid of the NaNs and suddenly performance was ok
too.

--bb