Yes, that is right, PCHs would probably introduce some additional
dependencies for some object files, and if those PCHs become bloated
over time, then you can expect this to be expressed as diminishing time
savings.
This does imply that maintaining PCHs will require at least some work.
__
Transcribed by my voice-enabled refrigerator, please pardon chilly messages.
On Wed, 15 Feb 2017, Neil Conway wrote:
On Tue, Feb 14, 2017 at 11:28 AM, Jeff Coffler
<jeff.coff...@microsoft.com.invalid> wrote:
For efficiency purposes, if a header file is included by 50% or more of the
source files, it should be included in the precompiled header. If a header is
included in fewer than 50% of the source files, then it can be separately
included (and thus would not benefit from precompiled headers). Note that this
is a guideline; even if a header is used by less than 50% of source files, if
it's very large, we still may decide to throw it in the precompiled header.
It seems like this would have the effect of creating many false
dependencies: if file X doesn't currently include header Y but Y is
included in the precompiled header, the symbols in Y will now be
visible when X is compiled. It would also mean that X would need to be
recompiled when Y changes.
Related: the current policy is that headers and implementation files
should try to include all of their dependencies, without relying on
transitive includes. For example, if foo.cpp includes bar.hpp, which
includes <vector>, but foo.cpp also uses <vector>, both foo.cpp and
bar.hpp should "#include <vector>". Adopting precompiled headers would
mean making an exception to this policy, right?
I wonder if we should instead use headers like:
<- mesos_common.h ->
#include <a>
#include <b>
#include <c>
<- xyz.cpp, which needs headers "b" and "d" ->
#include "mesos_common.h>
#include <b>
#include <d>
That way, the fact that "xyz.cpp" logically depends on <b> (but not
<a> or <c>) is not obscured (in other words, Mesos should continue to
compile if 'mesos_common.h' is replaced with an empty file). Does
anyone know whether the header guard in <b> _should_ make the repeated
inclusion of <b> relatively cheap?
Neil
