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Date: Thu, 8 Nov 2012 15:07:11 +0100
From: Roman Divacky <rdivacky@freebsd.org>
To: Bruce Evans <brde@optusnet.com.au>
Subject: Re: svn commit: r242715 - head/sys/conf
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> - clang fails to optimize division by (integer) 2 into multiplication
>    by (floating point with the correct type) 0.5 in the case of long
>    doubles on one supported arch (i386 IIRC).  Both compilers optimize
>    all other cases, including floats and doubles on all arches.  clang
>    with -ffast-math optimizes the broken case, but -ffast-math is
>    generally unsafe and not even fast; it shouldn't be necessary for
>    this, since this is one of the few floating point strength reductions
>    that is always safe.  This detail is not documented for either clang
>    or gcc.  clang doesn't even document the existence of -ffast-math in
>    its man page.
> 
>    Several uncommitted libm functions use the spelling of (integer) 2
>    to avoid ifdefs and/or different code to get the type right for 0.5.
>    The result is that the case of long doubles on i386 is pessimized.
>    clang has many other efficiency and ABI problems on i386, including
>    the next one.

This just got fixed upstream

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20121105/155388.html

We will get it with the next import of llvm/clang.

Thank you! Roman