linux-kernel - Re: math-emu issue with fp divide

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Message-Id: <F02EB8BE-6780-4C75-ABB9-7F2397457374@kernel.crashing.org>
Date:	Fri, 27 Jun 2008 09:15:40 -0500
From:	Kumar Gala <galak@...nel.crashing.org>
To:	David Miller <davem@...emloft.net>
Cc:	joseph@...esourcery.com, linux-kernel@...r.kernel.org,
	libc-alpha@...rceware.org
Subject: Re: math-emu issue with fp divide


On Jun 12, 2008, at 11:24 PM, David Miller wrote:

> From: Kumar Gala <galak@...nel.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 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;
> }

I tested this on PPC HW fp and kern emu w/my patch and get the same  
results:

0
0
8000000

(8000000 is underflow in the PPC FP status register)

- k
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