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Re: soft-fp: support after-rounding tininess detection
- From: Richard Henderson <rth at redhat dot com>
- To: "Joseph S. Myers" <joseph at codesourcery dot com>, Richard Henderson <rth at twiddle dot net>
- Cc: libc-alpha at sourceware dot org
- Date: Wed, 12 Feb 2014 08:32:24 -0800
- Subject: Re: soft-fp: support after-rounding tininess detection
- Authentication-results: sourceware.org; auth=none
- References: <Pine dot LNX dot 4 dot 64 dot 1402030023100 dot 7179 at digraph dot polyomino dot org dot uk> <52FA97E6 dot 6020000 at twiddle dot net> <Pine dot LNX dot 4 dot 64 dot 1402112217080 dot 11759 at digraph dot polyomino dot org dot uk> <52FB8992 dot 1010601 at twiddle dot net> <Pine dot LNX dot 4 dot 64 dot 1402121517450 dot 5339 at digraph dot polyomino dot org dot uk>
On 02/12/2014 07:27 AM, Joseph S. Myers wrote:
> On Wed, 12 Feb 2014, Richard Henderson wrote:
>
>> But it still makes no sense to me. E.g. rounding up,
>>
>> X_f = 111...1100
>>
>> with the shift sees no carry into OVERFLOW, but then the subsequent
>> "real" rounding does see carry into (OVERFLOW>>1). So now we have
>> underflow signaled for a result that's not subnormal.
>
> Yes, that's correct: for both before-rounding and after-rounding tininess
> detection you get underflow signaled for some cases where the final result
> is not subnormal or zero.
...
> In the x86 manual I have to hand (Intel 64 and
> IA-32 Architectures Software Developer's Manual Combined Volumes: 1, 2A,
> 2B, 2C, 3A, 3B and 3C Order Number: 325462-047US June 2013), this is
> described in "4.9.1.5 Numeric Underflow Exception (#U)" (presumably also
> at similar locations in other versions of the manuals).
>
Ok then.
r~