Phobos unit testing uncovers a CPU bug

[Dmitry Olshansky]

On 26.11.2010 23:02, Don wrote:
> The code below compiles to a single machine instruction, yet the 
> results are CPU manufacturer-dependent.
> ----
> import std.math;
>
> void main()
> {
>      assert( yl2x(0x1.0076fc5cc7933866p+40L, LN2)
>     == 0x1.bba4a9f774f49d0ap+4L); // Pass on Intel, fails on AMD
> }
> ----
> The results for yl2x(0x1.0076fc5cc7933866p+40L, LN2) are:
>
> Intel:  0x1.bba4a9f774f49d0ap+4L
> AMD:    0x1.bba4a9f774f49d0cp+4L
>
> The least significant bit is different. This corresponds only to a 
> fraction of a bit (that is, it's hardly important for accuracy. For 
> comparison, sin and cos on x86 lose nearly sixty bits of accuracy in 
> some cases!). Its importance is only that it is an undocumented 
> difference between manufacturers.
>
> The difference was discovered through the unit tests for the 
> mathematical Special Functions which will be included in the next 
> compiler release. Discovery of the discrepancy happened only because 
> of several features of D:
>
> - built-in unit tests (encourages tests to be run on many machines)
>
> - built-in code coverage (the tests include extreme cases, simply 
> because I was trying to increase the code coverage to high values)
>
> - D supports the hex format for floats. Without this feature, the 
> discrepancy would have been blamed on differences in the 
> floating-point conversion functions in the C standard library.
>
> This experience reinforces my belief that D is an excellent language 
> for scientific computing.
>
> Thanks to David Simcha and Dmitry Olshansky for help in tracking this 
> down.
Glad to help!
I was genuinely intrigued because not more then a few weeks ago I 
discussed with a friend of mine a possibility of differences in FP 
calculations of AMD vs Intel.
You see, his scientific app yielded different results when working at 
home/at work, which is a frustrating experience. Since that's exactly 
same binary, written in Delphi (no C run-time involved and so on) and 
environment is pretty much the same... I suggested to check CPU vendors 
just in case... of course, different.

In the meantime, I sort of "ported" the test case to M$ c++ inline asm 
and posted it on AMD forums, let's see what they have to say.
http://forums.amd.com/forum/messageview.cfm?catid=319&threadid=142893&enterthread=y 
<http://forums.amd.com/forum/messageview.cfm?catid=319&threadid=142893&enterthread=y>

-- 
Dmitry Olshansky