On 21.11.2010 20:18, David Simcha wrote:
Can others on this mailing list please submit info about their CPUs
(manufacturer and core type) and whether the unit tests pass? My
working hypothesis (mostly because I can't think of anything else
that's at all plausible) is that this discrepancy is somehow
hardware-related. I'll start and give an example of what I'm looking
for:
Intel Penryn: Pass
AMD Brisbane: Fail
I myself, very concerned with FP code producing slightly different
results on AMD and Intel,
so here are my results (exactly the same exe used through out):
AMD Phenom II: Fail (Win7 64bit)
AMD Neo: Fail (Win7 32bit)
Core 2 Duo: Pass (WinVista 32bit)
I have a bad feeling about it...
On 11/21/2010 1:19 AM, Don Clugston wrote:
On 21 November 2010 05:48, David Simcha<[email protected]> wrote:
More research into this issue: I compiled the unittest.exe
executable on my
main (desktop) computer, ran it under my primary OS (Win7 64) and it
failed.. I then ran the exact same executable (no recompile) on my
Linux
Partition (Ubuntu 10.10 64) using Wine and it failed.
I then ran the exact same executable on my laptop on my primary OS
(also
Win7 64) and it passed. I ran it on my laptop's Linux partition
under Wine
(Ubuntu 10.10 32) and it passed.
The only difference between the two systems that might account for
this is
that the laptop has an Intel Penryn CPU, whereas the desktop as an AMD
Brisbane CPU. Does anyone know whether different x86 CPUs can produce
subtly different floating point results when executing the exact
same code?
Alternatively, is it possible that some processor-specific
optimizations to
some function getting called by Don's code could be causing slightly
different results?
That's _very_ interesting. The code in question doesn't use the C
runtime at all.
If it's the same exe, then the difference can only lie in the CPU or
in the environment.
Eg, if it starts with 80-bit floats disabled.
But the fact that every other test passes on your system, makes that
seem unlikely.
Does the failing system have execution protection enabled?
The only documented floating point difference between AMD and Intel
that I know of, is that AMD
raises the invalid exception when loading an 80-bit NaN, but Intel
doesn't. BTW I found that difference
myself, and added it to Wikipedia. That difference is not relevant here.
If the CPU itself is responsible for the difference, that's a CPU bug.
BTW this test was present in Tango for years, and nobody ever reported
this issue before.
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