[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:
{{{
#!bash
g.region rows=10000 cols=10000
time r.mapcalc "x1 = rand(0, 2) + rand(0, 2) + rand(0, 2) + if(0, 2, 3)"
seed=1
time r.mapcalc "x2 = 2 + if(0, rand(0, 2) + rand(0, 2) + rand(0, 2), 3)"
seed=1
}}}
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>
More information about the grass-dev
mailing list