Bill Baxter wrote:
On Fri, Dec 11, 2009 at 9:34 PM, Don <[email protected]> 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
Almost same here. Program was amazingly slow until I figured there were
NaNs involved. It would be great if we could eliminate that behavior.
Andrei
