Regarding ament_libraries_deduplicate implementation

[VRichardJP]

Hi!

I have a few questions as I inspect a few cmake traces, and see that a big share of find_package() time is spent in ament_libraries_deduplicate:

ament_cmake/ament_cmake_libraries/cmake/ament_libraries_pack_build_configuration.cmake

Line 33 in 154f198

if("${_lib}" MATCHES "^(debug|optimized|general)$")

1. Where do the "debug", "optimized" and "general" come from? There are several places where ament macros check whether these keywords exist, but they don't seem to be introduced by ament. Is this some obscure CMake feature? As I check into a few inputs, I can't find any of these keyword.

2. If I understand ament_libraries_pack_build_configuration code correctly, the function parses inputs like libA;libB;debug;libC;libD into libA;libB;debug:libC;libD. Is that correct? If yes, isn't the code overly complex? Wouldn't a simple regex do the job (and way faster?). The unpack function code seems to exactly do that, but does the REGEX REPLACE in a loop (which is not necessary?)

3. In the main loop of ament_libraries_deduplicate here:

ament_cmake/ament_cmake_libraries/cmake/ament_libraries_deduplicate.cmake

Lines 31 to 36 in 154f198

	foreach(_lib ${_packed})
	# remove existing value if it exists
	list(REMOVE_ITEM _unique ${_lib})
	# append value to the end
	list(APPEND _unique ${_lib})
	endforeach()

It seems to me the code could be faster written like this (without loop):

# keep only the last occurence while removing duplicates
list(REVERSE _packed)
list(REMOVE_DUPLICATES _packed)
list(REVERSE _packed)

[VRichardJP]

I managed to successfully build the whole autoware project (~250 packages) with the following implementation (which should do exactly the same):

macro(ament_libraries_deduplicate VAR)
  string(REGEX REPLACE "(^|;)(debug|optimized|general);(.+)" "\\2${AMENT_BUILD_CONFIGURATION_KEYWORD_SEPARATOR}\\3" _packed "${ARGN}")
  list(REVERSE _packed)
  list(REMOVE_DUPLICATES _packed)
  list(REVERSE _packed)
  string(REGEX REPLACE "(^|;)(debug|optimized|general)${AMENT_BUILD_CONFIGURATION_KEYWORD_SEPARATOR}(.+)" "\\2;\\3" ${VAR} "${_packed}")
endmacro()

The macro is way faster with this change. Here is an example on autoware behavior_path_planner package (left before, right after):

Before ament_auto_find_build_dependencies() function took almost 17s, after it only took 4s.