class_docstr: Abstract base class to define the interface that must be implemented
  by real compiler classes.  Also has some utility methods used by
  several compiler classes.
  
  The basic idea behind a compiler abstraction class is that each
  instance can be used for all the compile/link steps in building a
  single project.  Thus, attributes common to all of those compile and
  link steps -- include directories, macros to define, libraries to link
  against, etc. -- are attributes of the compiler instance.  To allow for
  variability in how individual files are treated, most of those
  attributes may be varied on a per-compilation or per-link basis.
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docstr: Decide which source files must be recompiled.
  
  Determine the list of object files corresponding to 'sources',
  and figure out which ones really need to be recompiled.
  Return a list of all object files and a dictionary telling
  which source files can be skipped.
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code: def _prep_compile(self, sources, output_dir, depends=None):
      """Decide which source files must be recompiled.
  
      Determine the list of object files corresponding to 'sources',
      and figure out which ones really need to be recompiled.
      Return a list of all object files and a dictionary telling
      which source files can be skipped.
      """
      # Get the list of expected output (object) files
      objects = self.object_filenames(sources, output_dir=output_dir)
      assert len(objects) == len(sources)
  
      # Return an empty dict for the "which source files can be skipped"
      # return value to preserve API compatibility.
      return objects, {}
  
imports: import sys
  import os
  import re
  import warnings
  from .errors import CompileError, LinkError, UnknownFileError, DistutilsPlatformError, DistutilsModuleError
  from .spawn import spawn
  from .file_util import move_file
  from .dir_util import mkpath
  from ._modified import newer_group
  from .util import split_quoted, execute
  from ._log import log
label_desc: Use image node_id 21 for calling the CCompiler obj's underlying member method code with example usage: obj._prep_compile(sources, output_dir, depends) and returns: objects,  dict