On Monday, 16 May 2016 at 10:25:33 UTC, Andrei Alexandrescu wrote:
On 5/16/16 12:37 AM, Walter Bright wrote:
Me, I think of that as "Who cares that you paid $$$ for an 80
bit CPU,
we're going to give you only 64 bits."
I'm not sure about this. My understanding is that all SSE has
hardware for 32 and 64 bit floats, and the the 80-bit hardware
is pretty much cut-and-pasted from the x87 days without anyone
really looking in improving it. And that's been the case for
more than a decade. Is that correct?
I'm looking for example at
http://nicolas.limare.net/pro/notes/2014/12/12_arit_speed/ and
see that on all Intel and compatible hardware, the speed of
80-bit floating point operations ranges between much slower and
disastrously slower.
I think it's time to revisit our attitudes to floating point,
which was formed last century in the heydays of x87. My
perception is the world has moved to SSE and 32- and 64-bit
float; the "real" type is a distraction for D; the whole let's
do things in 128-bit during compilation is a time waster; and
many of the original things we want to do with floating point
are different without a distinction, and a further waste of our
resources.
It is a bit ironic that we worry about autodecoding (I'll
destroy that later) whilst a landslide loss of speed and
predictability in floating point math doesn't raise an eyebrow.
Andrei
The AMD64 programmer's manual discourages the use of x87:
"For media and scientific programs that demand floating-point
operations, it is often easier and more
powerful to use SSE instructions. Such programs perform better
than x87 floating-point programs,
because the YMM/XMM register file is flat rather than
stack-oriented, there are twice as many
registers (in 64-bit mode), and SSE instructions can operate on
four or eight times the number of
floating-point operands as can x87 instructions. This ability to
operate in parallel on multiple pairs of
floating-point elements often makes it possible to remove local
temporary variables that would
otherwise be needed in x87 floating-point code."
