math-emu issue with fp divide
Kumar Gala
galak@kernel.crashing.org
Fri Jun 13 15:50:00 GMT 2008
On Jun 12, 2008, at 11:24 PM, David Miller wrote:
> From: Kumar Gala <galak@kernel.crashing.org>
> Date: Thu, 5 Jun 2008 08:38:44 -0500 (CDT)
>
>> Now that I'm digging into this a bit I'm thinking my issue has to
>> do with
>> the fix you put in place from back in Aug 2007 (commit
>> 405849610fd96b4f34cd1875c4c033228fea6c0f):
>>
>> [MATH-EMU]: Fix underflow exception reporting.
>>
>> 2) we ended up rounding back up to normal (this is the case where
>> we set the exponent to 1 and set the fraction to zero), this
>> should set inexact too
>> ...
>>
>> Another example, "0x0.0000000000001p-1022 / 16.0", should signal
>> both
>> inexact and underflow. The cpu implementations and ieee1754
>> literature is very clear about this. This is case #2 above.
>>
>> I'm not clear from your commit comment on what actual number
>> 0x0.0....01p-1022 is?
>
> I haven't been able to look closely at this yet but I think I
> happened to stumble over the test case that lead me to that
> changeset you are referencing here.
>
> The "actual number" is exactly as listed "0x0.0000000000001p-1022",
> I don't know what's so confusing about it :-)))
I don't think I've ever seen the notation before :)
> I think this was distilled by Jakub Jelinek from some glibc test case.
>
> #include <float.h>
> #include <fenv.h>
> #include <stdio.h>
>
> volatile double d = DBL_MIN;
> volatile double e = 0x0.0000000000001p-1022;
> volatile double f = 16.0;
> int
> main (void)
> {
> printf ("%x\n", fetestexcept (FE_UNDERFLOW));
> d /= f;
> printf ("%x\n", fetestexcept (FE_UNDERFLOW));
> e /= f;
> printf ("%x\n", fetestexcept (FE_UNDERFLOW));
> return 0;
> }
Cool, I'll try this out and see what it does on PPC HW and w/my
current EMU. I'll also see if I can work up a test case to show the
issue I've set a patch for.
- k
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