[ET Trac] [Einstein Toolkit] #387: Fortran interface to LoopControl broken in stable Carpet

[Einstein Toolkit]

#387: Fortran interface to LoopControl broken in stable Carpet
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  Reporter:  rhaas   |       Owner:  rhaas     
      Type:  defect  |      Status:  review    
  Priority:  major   |   Milestone:  ET_2011_05
 Component:  Carpet  |     Version:            
Resolution:          |    Keywords:            
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Comment (by eschnett):

 This expression rounds down to the next multiple of lc_di, assuming that
 lc_di is a power of 2. This is necessary for vectorisation; for example, a
 2-vector of an 8-byte quantity can only be efficiently loaded and stored
 if it is located at an address that is a multiple of 16.

 If lc_di is a power of 2, then -lc_di is a mask that sets some of the
 lowest bits to zero, and lc_ipos & -lc_di is the rounded-down multiple of
 lc_di. The difference to lc_ipos is then the offset to this location.

 The expression lc_ipos & -lc_di is in this case equivalent to
 lc_ipos/lc_di*lc_di, but is evaluated much faster.

 This is standard C and C++. The inline variable declarations may require
 choosing C99 semantics.

 Without vectorisation, lc_di is 1, and lc_ioffset is always 0.

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Ticket URL: <https://trac.einsteintoolkit.org/ticket/387#comment:4>
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