[phobos] phobos commit, revision 2186

[Don Clugston]

On 26 November 2010 19:14, Dmitry Olshansky <dmitry.olsh at gmail.com> wrote:
> On 26.11.2010 20:37, Don Clugston wrote:
>>
>> Excellent! I predict this fails on AMD. (This test is a single
>> instruction).
>
> And  the answer is ... yes, it fails :-)

Awesome!!! In terms of Phobos, that's all we need to know -- the
failure is not our fault, so I should just relax the tolerance on the
test.

I'm extremely pleased about this result. "The unit testing in the D
standard library uncovered an CPU issue" is a pretty good story. The
test exists because I was trying to improve the code coverage of the
math library to >95% -- and I was only doing that because DMD has
built-in code coverage. The bug also wouldn't have been uncovered
unless D supported the hex format for floats.


> So what's the next direction, maybe ask the AMD folks?

Good question. I will certainly ask Agner Fog if he's heard of it.
Perhaps somebody may be motivated to find the exact values where AMD
and Intel differ.
One interesting question is what VIA processors do. Do they follow
Intel, or AMD? I don't even know which is more accurate. Probably, the
correctly rounded result is almost exactly between them, so it's only
an error of a fraction of a bit -- it's significant only in that the
processors give different results.

I could only find two references to AMD/Intel differences in the fyl2x
instruction.

This one is from CERN. It's clearly the same issue, although they
don't give any details (no test cases).
http://boinc.berkeley.edu/MOM1MP01.pdf

This one sounds like the same thing, although note that it wasn't confirmed.
http://forums.amd.com/forum/messageview.cfm?catid=29&threadid=135771&enterthread=y

>
>> import std.math;
>>
>> void main()
>> {
>>      assert( yl2x(0x1.0076fc5cc7933866p+40L, LN2) ==
>> 0x1.bba4a9f774f49d0ap+4L, "YL2X fails");
>> }