Commits on Oct 4, 2024

1. perf: Reduce compile time by trimming template expansion in IBA …

   I was profiling the builds and saw that modules with lots of template
   expansion dominate the compile time. For example,
   imagebufalgo_pixelmath.cpp alone took 290s to compile on my 2020
   MacbookPro (!), and imagebufalgo_addsub.cpp took 84 seconds.
   
   This is all due to the combnatorics of expanding IBA templates via the
   DISPATCH macros in imagebufalgo_util.h separately for every type that
   the arguments can be. But I claim that most combinations are rarely if
   ever used. I mean, how often does anybody need IBA::add() to add an
   int8 image to an int16 image? So this PR rewrites those macros to
   simplify the cases as follows:
   
   * The common pixel data types are float, half, unint8, and uint16.
   
   * Specialized versions are fully expanded only when the result and
     input images are one of these types. Images not of one of those
     types are first automatically converted to float to make them reduce
     to a common case. That makes uncommon pixel data types like (signed)
     int16 not expand the template, but rather convert to and from float
     and use the float specializations.
   
   * For binary and ternary operations (those with 2 or 3 image inputs),
     if the pixel types of the inputs doesn't match, we make sure they
     both are converted to float. So, for example, we don't need a
     specialized version that adds a half image to a uint16 image -- just
     convert them to float and use the common case. But we do specalize
     if the two inputs are both the same common case, such as adding two
     uint16 images.
   
   * Assume that commonly, the result image will either be float, or will
     be the same pixel data type as the inputs. Other combinations trigger
     assignment to a temporary float IB, then copying with convertion to
     the uncommon use-supplied result buffer.
   
   * Additionally, we gut down on a little bit more templating by moving
     some "deep" methods from the type-templated ImageBuf::Iterator to its
     type-generic non-templated base class IteraterBase.
   
   The net result of all this is an awful lot less template expansion.
   With this in place, my laptop compiles imagebufalgo_pixelmath.cpp
   in 97s (vs 290 before) and imagebufalgo_addsub.cpp in 26s (from 84).
   It takes a big bite out of all the iba files, and reduces project-wide
   compile time by over 10%, around 30s out of 300 for a fresh, uncached,
   optimized build with 16 threads.
   
   Signed-off-by: Larry Gritz <lg@larrygritz.com>

   

   lgritz committed Oct 4, 2024
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