[GRASS-dev] [GRASS GIS] #3069: Evaluate branches in r.mapcalc if-function only when needed

GRASS GIS trac at osgeo.org
Sun Jun 19 21:47:06 PDT 2016

#3069: Evaluate branches in r.mapcalc if-function only when needed
 Reporter:  wenzeslaus                           |      Owner:  grass-dev@…
     Type:  enhancement                          |     Status:  new
 Priority:  normal                               |  Milestone:  7.3.0
Component:  Raster                               |    Version:  svn-trunk
 Keywords:  r.mapcalc, r3.mapcalc,               |        CPU:  Unspecified
  optimization, expression, evaluation, if-      |
  statement, if-function                         |
 Platform:  Unspecified                          |
 The ''r.mapcalc'' expression `x = if(0, rand(0, 2), 3)` takes about the
 same time to evaluate as `x = rand(0, 2) + if(0, 2, 3)`, although the
 true-branch of the if-statement is never used. As far as I understand,
 this is given by fact that `if` is actually a function in ''r.mapcalc'',
 so all the arguments are evaluated before the function is called.

 It would be a nice enhancement if the values of if-function arguments
 would be computed only when needed, i.e. `x = if(0, rand(0, 2), 3)` would
 take same time as `x = if(0, 1, 3)` because `rand(0, 2)` is never used.
 This would allow for some optimizations when part of the output raster is
 already determined and there is no need to perform the computation, for
 example a moving window calculation which needs to be performed only when
 the central cell has a certain value. Another case when this would be
 advantageous is when both branches (arguments) are costly to compute,
 currently both are computed and one thrown away.

 Here is the minimal example:

 g.region rows=10000 cols=10000
 time r.mapcalc "x1 = rand(0, 2) + rand(0, 2) + rand(0, 2) + if(0, 2, 3)"
 time r.mapcalc "x2 = 2 + if(0, rand(0, 2) + rand(0, 2) + rand(0, 2), 3)"

 In both case, I get something like 14.5 s and for the expression with
 trivial unused branch (`x = 2 + if(0, 2, 3)`) I get less than 6 s.

Ticket URL: <https://trac.osgeo.org/grass/ticket/3069>
GRASS GIS <https://grass.osgeo.org>

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