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Re: [PATCH] Fix static-binary lazy FPU context allocation

On Thu, 22 Aug 2013, Rich Felker wrote:

> >  We have an issue with FPU control word initialization in static binaries.  
> > We do it unconditionally, defeating any lazy FPU context allocation the OS 
> > may implement.
> Could you elaborate on why this initialization needs to take place at
> all? Under what conditions would the kernel give an incorrect initial
> control word?

 There's no "incorrect" control word.  There's one kernel default for each 
particular target (and possibly ABI variation on that target).  Our 
default value of __fpu_control is meant to match the kernel default so if 
we use it -- fine.

 The symbol can however be overridden by a program wishing to use a 
non-default FPU execution mode.  In that case we respect that value and 
install it in hardware explicitly (allocating the FPU context right away 
as a side effect) as the kernel won't do that for us.  We need to do this 
early on so that static initialisers see the right FPU execution mode.

 That's at least my understanding how this variable works.


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