feat: way to only recompile changed files (#2643)

* feat: lazy compile

* refactor: lazy -> only_changed

* refactor: leave the changed function up to the user

* refactor: pass a function, based on @YannickJadoul and @HDembinski's suggestions

* refactor: old -> _old, as it's not intended for users

* docs: slight improvmenent from @rwgk

* docs: Ccache spelling, extra warning about pip caching

Ccache spelling noted by @YannickJadoul

.pre-commit-config.yaml

@@ -87,7 +87,7 @@ repos:
   - id: disallow-caps
     name: Disallow improper capitalization
     language: pygrep
-    entry: PyBind|Numpy|Cmake
+    entry: PyBind|Numpy|Cmake|CCache
     exclude: .pre-commit-config.yaml
 
 - repo: local

docs/compiling.rst

@@ -89,6 +89,36 @@ default number of threads (0 will take the number of threads available) and
 ``max=N``, the maximum number of threads; if you have a large extension you may
 want set this to a memory dependent number.
 
+If you are developing rapidly and have a lot of C++ files, you may want to
+avoid rebuilding files that have not changed. For simple cases were you are
+using ``pip install -e .`` and do not have local headers, you can skip the
+rebuild if a object file is newer than it's source (headers are not checked!)
+with the following:
+
+.. code-block:: python
+
+    from pybind11.setup_helpers import ParallelCompile, naive_recompile
+
+    SmartCompile("NPY_NUM_BUILD_JOBS", needs_recompile=naive_recompile).install()
+
+
+If you have a more complex build, you can implement a smarter function and pass
+it to ``needs_recompile``, or you can use [Ccache]_ instead. ``CXX="cache g++"
+pip install -e .`` would be the way to use it with GCC, for example. Unlike the
+simple solution, this even works even when not compiling in editable mode, but
+it does require Ccache to be installed.
+
+Keep in mind that Pip will not even attempt to rebuild if it thinks it has
+already built a copy of your code, which it deduces from the version number.
+One way to avoid this is to use [setuptools_scm]_, which will generate a
+version number that includes the number of commits since your last tag and a
+hash for a dirty directory. Another way to force a rebuild is purge your cache
+or use Pip's ``--no-cache-dir`` option.
+
+.. [Ccache] https://ccache.dev
+
+.. [setuptools_scm] https://github.com/pypa/setuptools_scm
+
 .. _setup_helpers-pep518:
 
 PEP 518 requirements (Pip 10+ required)

pybind11/setup_helpers.py

@@ -275,7 +275,8 @@ def auto_cpp_level(compiler):
 class build_ext(_build_ext):  # noqa: N801
     """
     Customized build_ext that allows an auto-search for the highest supported
-    C++ level for Pybind11Extension.
+    C++ level for Pybind11Extension. This is only needed for the auto-search
+    for now, and is completely optional otherwise.
     """
 
     def build_extensions(self):
@@ -293,6 +294,23 @@ class build_ext(_build_ext):  # noqa: N801
         _build_ext.build_extensions(self)
 
 
+def naive_recompile(obj, src):
+    """
+    This will recompile only if the source file changes. It does not check
+    header files, so a more advanced function or Ccache is better if you have
+    editable header files in your package.
+    """
+    return os.stat(obj).st_mtime < os.stat(src).st_mtime
+
+
+def no_recompile(obg, src):
+    """
+    This is the safest but slowest choice (and is the default) - will always
+    recompile sources.
+    """
+    return True
+
+
 # Optional parallel compile utility
 # inspired by: http://stackoverflow.com/questions/11013851/speeding-up-build-process-with-distutils
 # and: https://github.com/tbenthompson/cppimport/blob/stable/cppimport/build_module.py
@@ -306,24 +324,42 @@ class ParallelCompile(object):
     This takes several arguments that allow you to customize the compile
     function created:
 
-    envvar: Set an environment variable to control the compilation threads, like NPY_NUM_BUILD_JOBS
+    envvar:
-    default: 0 will automatically multithread, or 1 will only multithread if the envvar is set.
+        Set an environment variable to control the compilation threads, like
-    max: The limit for automatic multithreading if non-zero
+        NPY_NUM_BUILD_JOBS
+    default:
+        0 will automatically multithread, or 1 will only multithread if the
+        envvar is set.
+    max:
+        The limit for automatic multithreading if non-zero
+    needs_recompile:
+        A function of (obj, src) that returns True when recompile is needed.  No
+        effect in isolated mode; use ccache instead, see
+        https://github.com/matplotlib/matplotlib/issues/1507/
+
+    To use::
 
-    To use:
         ParallelCompile("NPY_NUM_BUILD_JOBS").install()
-    or:
+
+    or::
+
         with ParallelCompile("NPY_NUM_BUILD_JOBS"):
             setup(...)
+
+    By default, this assumes all files need to be recompiled. A smarter
+    function can be provided via needs_recompile.  If the output has not yet
+    been generated, the compile will always run, and this function is not
+    called.
     """
 
-    __slots__ = ("envvar", "default", "max", "old")
+    __slots__ = ("envvar", "default", "max", "_old", "needs_recompile")
 
-    def __init__(self, envvar=None, default=0, max=0):
+    def __init__(self, envvar=None, default=0, max=0, needs_recompile=no_recompile):
         self.envvar = envvar
         self.default = default
         self.max = max
-        self.old = []
+        self.needs_recompile = needs_recompile
+        self._old = []
 
     def function(self):
         """
@@ -360,6 +396,8 @@ class ParallelCompile(object):
                     src, ext = build[obj]
                 except KeyError:
                     return
+
+                if not os.path.exists(obj) or self.needs_recompile(obj, src):
                     compiler._compile(obj, src, ext, cc_args, extra_postargs, pp_opts)
 
             try:
@@ -391,8 +429,8 @@ class ParallelCompile(object):
         return self
 
     def __enter__(self):
-        self.old.append(distutils.ccompiler.CCompiler.compile)
+        self._old.append(distutils.ccompiler.CCompiler.compile)
         return self.install()
 
     def __exit__(self, *args):
-        distutils.ccompiler.CCompiler.compile = self.old.pop()
+        distutils.ccompiler.CCompiler.compile = self._old.pop()

pybind11/setup_helpers.pyi

@@ -1,7 +1,7 @@
 # IMPORTANT: Should stay in sync with setup_helpers.py (mostly checked by CI /
 # pre-commit).
 
-from typing import Any, Iterator, Optional, Type, TypeVar, Union
+from typing import Any, Callable, Iterator, Optional, Type, TypeVar, Union
 from types import TracebackType
 
 from distutils.command.build_ext import build_ext as _build_ext  # type: ignore
@@ -33,12 +33,23 @@ def auto_cpp_level(compiler: distutils.ccompiler.CCompiler) -> Union[int, str]:
 class build_ext(_build_ext):  # type: ignore
     def build_extensions(self) -> None: ...
 
+def no_recompile(obj: str, src: str) -> bool: ...
+def naive_recompile(obj: str, src: str) -> bool: ...
+
 T = TypeVar("T", bound="ParallelCompile")
 
 class ParallelCompile:
+    envvar: Optional[str]
+    default: int
+    max: int
+    needs_recompile: Callable[[str, str], bool]
     def __init__(
-        self, envvar: Optional[str] = None, default: int = 0, max: int = 0
+        self,
-    ): ...
+        envvar: Optional[str] = None,
+        default: int = 0,
+        max: int = 0,
+        needs_recompile: Callable[[str, str], bool] = no_recompile,
+    ) -> None: ...
     def function(self) -> Any: ...
     def install(self: T) -> T: ...
     def __enter__(self: T) -> T: ...