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Re: [Bug-gnubg] sse and sigmoid bug

From: Philippe Michel
Subject: Re: [Bug-gnubg] sse and sigmoid bug
Date: Mon, 2 Jul 2012 22:34:07 +0200 (CEST)
User-agent: Alpine 2.00 (BSF 1167 2008-08-23)

On Mon, 2 Jul 2012, Joseph Heled wrote:

While "importing" the sse code to gnubg-nn I discovered an error in the
sigmoid implementation. The calculation made by the see code did not match
the one from the straight C code. (It was relatively easy for me to catch
since sse can be toggled during run time in gnubg-nn)

Since this results in bad evaluations, I am surprised it has not been
detected in gnubg.

From the differences between the SSE code in gnubg and gnubg-nn, it looks
like you mean the cases when the input is higher than 10, isn't it ?

For these cases, my understanding is that the combination of the _mm_min_ps(xin, tens.ps); at the beginning of sigmoid_positive_ps() and the 101th entry in the lookup table does what is needed.

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