[CMake] Poor performance of copy_if_different

[Clinton Stimpson]

add_custom_command(
  OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/my.dll
  COMMAND cmake -E copy ${CMAKE_CURRENT_SOURCE_DIR}/my.dll 
${CMAKE_CURRENT_BINARY_DIR}/my.dll
  DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/my.dll
  )

add_custom_target(copy_dll ALL DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/my.dll)

Whether you use 1 custom command per dll, or 1 for all dlls, is up to you.  If 
they usually change together, then you could have fewer custom commands.  In 
that case, the OUTPUT would have multiple files and both DEPENDS would have 
multiple files.

Another alternative is to use add_custom_command(TARGET POST_BUILD ...) if 
these are dlls that you compile yourself and need to copy after the build 
step.


On Tuesday, November 06, 2012 11:23:03 AM Robert Dailey wrote:
> Also, is it safe to add 1 custom command for each DLL copy? Or would
> you somehow only use 1 custom command and copy them all with it? Also
> would I add the custom commands to the custom target?
> 
> On Tue, Nov 6, 2012 at 11:11 AM, Clinton Stimpson <clinton at elemtech.com> 
wrote:
> > On Tuesday, November 06, 2012 11:06:45 AM Robert Dailey wrote:
> >> I use ${CMAKE_COMMAND} -E copy_if_different to copy DLL files to my
> >> binary output directory. The custom target runs this command about
> >> 50-100 times (for that many files).
> >> 
> >> I notice that when all files are already copied, the commands still
> >> run extremely slowly. It takes just as long to copy all the files as
> >> it does to copy none of them.
> >> 
> >> I used the Sysinternals tool called Process Monitor to see what CMake
> >> is doing as it runs copy_if_different. It's opening the full file and
> >> seems to be comparing actual contents instead of something simple,
> >> such as timestamp.
> >> 
> >> I do not need such a thorough check, I simply want the check to see
> >> which timestamp is higher and copy if the source is newer than the
> >> target.
> >> 
> >> Any reason why copy_if_different is so slow? Is my assumption correct?
> >> How can I make it faster?
> > 
> > How about using plain "cmake -E copy ..." and rely on the timestamp check
> > done by a custom command (add_custom_command()).
> > You need to make sure the input/ouput parts of the custom command are set
> > correctly so it can do a timestamp check.
> > 
> > --
> > Clinton Stimpson
> > Elemental Technologies, Inc
> > Computational Simulation Software, LLC
> > www.csimsoft.com
> > --
> > 
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-- 
Clinton Stimpson
Elemental Technologies, Inc
Computational Simulation Software, LLC
www.csimsoft.com