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.
