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Re: [PATCH 2/3] powerpc: POWER8 fmod optimization
- From: Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>
- To: Joseph Myers <joseph at codesourcery dot com>
- Cc: "GNU C. Library" <libc-alpha at sourceware dot org>
- Date: Wed, 12 Nov 2014 16:29:17 -0200
- Subject: Re: [PATCH 2/3] powerpc: POWER8 fmod optimization
- Authentication-results: sourceware.org; auth=none
- References: <54627990 dot 1060805 at linux dot vnet dot ibm dot com> <alpine dot DEB dot 2 dot 10 dot 1411112120500 dot 12642 at digraph dot polyomino dot org dot uk> <54637EFF dot 2040302 at linux dot vnet dot ibm dot com> <alpine dot DEB dot 2 dot 10 dot 1411121613030 dot 23958 at digraph dot polyomino dot org dot uk>
On 12-11-2014 14:19, Joseph Myers wrote:
> On Wed, 12 Nov 2014, Adhemerval Zanella wrote:
>> Indeed I overlook it, as I am seeing now with a more extensive testing.
>> Checking now, it seems using a FP division operation in the loop to
>> correct it not the best strategy to achieve better performance.
> And there are other problems with the division I didn't notice previously.
> If the exponent difference is large enough, the division may overflow.
> And you're assuming the result of (round to next integer toward 0 (divide
> and round in current rounding mode)) is the same as (divide in infinite
> precision, round once to an integer toward 0) - but depending on the
> arguments and rounding mode, the result of the division may be rounded
> away from 0 to an integer with magnitude greater than the desired result.
Yes, I noted that my naive approach is far from correct. I'm dropping this patch,
thanks for the review.