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Re: [patch] Change swdiv evaluation order (x86), remove pass_convert_to_rsqrt

From: Paolo Bonzini <bonzini at gnu dot org>
To: Michael Matz <matz at suse dot de>
Cc: gcc-patches at gcc dot gnu dot org
Date: Wed, 28 Oct 2009 21:38:29 +0100
Subject: Re: [patch] Change swdiv evaluation order (x86), remove pass_convert_to_rsqrt
References: <Pine.LNX.4.64.0910282000470.15566@wotan.suse.de>

On 10/28/2009 08:37 PM, Michael Matz wrote:

transform 0.0 == sqrt(0/x) ->  rsqrt(x/0) == NaN.  Clearly something
for -fnonsense-math-optimizations, not for -funsafe.

Well, the theory probably was that rsqrt(x/0) == rsqrt(inf) == 1/sqrt(inf) == 0. The problem is that exactly because of -funsafe-math-optimizations the last step is bogus (and actually because rsqrt is computed with tables even the second one does not work).

Paolo

References:
- [patch] Change swdiv evaluation order (x86), remove pass_convert_to_rsqrt
  - From: Michael Matz

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