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Re: [PATCH] powerpc: Fix logbl on power7 [BZ# 21280]
- From: Steven Munroe <munroesj at linux dot vnet dot ibm dot com>
- To: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- Cc: Tulio Magno Quites Machado Filho <tuliom at linux dot vnet dot ibm dot com>, libc-alpha at sourceware dot org
- Date: Fri, 28 Apr 2017 13:26:00 -0500
- Subject: Re: [PATCH] powerpc: Fix logbl on power7 [BZ# 21280]
- Authentication-results: sourceware.org; auth=none
- References: <1490043778-30228-1-git-send-email-tuliom@linux.vnet.ibm.com> <50b87cce-2f3a-d84b-bc38-d81fa3623738@linaro.org> <87wpbi4ck0.fsf@totoro.br.ibm.com> <b22f1e69-55c3-88ae-a057-3983a9e89d52@linaro.org>
- Reply-to: munroesj at linux dot vnet dot ibm dot com
On Tue, 2017-03-21 at 17:18 -0300, Adhemerval Zanella wrote:
>
> On 21/03/2017 17:09, Tulio Magno Quites Machado Filho wrote:
> > Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:
> >
> >> On 20/03/2017 18:02, Tulio Magno Quites Machado Filho wrote:
> >>> 1. Fix the results for negative subnormals by ignoring the signal when
> >>> normalizing the value.
> >>> 2. Fix the output when the high part is a power of 2 and the low part
> >>> is a nonzero number with opposite sign. This fix is based on commit
> >>> 380bd0fd2418f8988217de950f8b8ff18af0cb2b.
> >>
> >> It is not a blocker, but based on the complexity of the change and the
> >> optimization real case usage, I wonder if it would be better to just remove
> >> it and use the default implementation instead.
> >
> > I tried to do that before fixing these, but the performance difference is
> > still around 2x.
>
> I would expect it, but the question is do we really care about the performance
> of logbl-ibm128 to maintain a separated implementation for power7+ only?
>
Yes we still care. The power7 optimization avoids LHS by performing the
mask and convert in VSX. and applies to power8, power9 for IBM long
double..
It will be some time before _float128 is fully deployed and until then
customers will notice, if this IBM long double optimization went
missing.