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As mentioned in the introductory section, modules reduce the compiler's
workload by performing em(h + s) instead of em(h * s) header compilations when
em(s) sources each including tt(h) header files are compiled. Furthermore,
using modules no longer requires the use of include guards or other defensive
measures to avoid repeated inclusion of headers, which is particularly
unwelcome in bf(C++) as bf(C++) header files typically not only declare
functions and variables, but also define class interfaces and templates
(cf. chapter ref(TEMPLATES) and beyond).
So what's the gain? In this final section about modules we compare the
compilation times of a relatively large program (ti(bisonc++), cf.
lurl(http://fbb-git.gitlab.io/bisoncpp/)), consisting of almost 30
classes.
Compilation is performed by tt(clang++-7), using three variants:
itemization(
it() Plain compilation, using the compiler options
verb(--std=c++2a -Wall -O2 -pthread)
it() Compilation using a module. The module tt(bisoncpp) was first created
using the compiler options
verb(-fmodules --std=c++2a -Wall -O2 -pthread)
and merely compiling tt(main.cc). The generated module was
tt(bisoncpp-2VBAIMWQCR12B.pcm), whose size was 13 MB. Following the
module's construction the tt(main.o) file was removed and the program
was built using the above options (so including tt(-fmodules)).
it() Finally, the program was compiled not using modules, but using
precompiled headers. The precompiled headers were created as part of
the compilation process. The total size of all precompiled headers was
MB, which is fairly large, but as the precompiled headers are only
used for the compilation they can be removed once the program has been
compiled.
)
Compilating tt(bisonc++) using the plain source files (no module, no
precompiled headers) required slightly more than 7 minutes. The bf(time)(1)
program reported:
verb( 442.894u 25.751s 7:57.73 98.0% 0+0k 6416+14560io 31pf+0w)
It took Gnu's tt(g++) compiler a comparable amount of time to compile the
sources:
verb( 467.053u 32.818s 8:20.39 99.8% 0+0k 1064+26264io 10pf+0w)
This comparison is important, as we'll see below.
Next, after first constructing the program's module, and then timing the full
program's construction bf(time) reported:
verb( 420.305u 23.804s 7:29.43 98.8% 0+0k 0+39672io 0pf+0w)
A somewhat unexpected (and disappointing) result. There is a minimal gain
in compilation time, but it's marginal. In fact at a second attempt a
compilation time of 458 seconds was reported. Some variation is to be
expected, but the compilation times of both methods clearly don't notably
differ.
Finally using precompiled headers. For this test Gnu's tt(g++) compiler was
used, as tt(clang) doesn't automatically use available precompiled headers
(cf.
url(clang's users manual)(
https://clang.llvm.org/docs/UsersManual.html#using-a-pch-file)). At the
start of the compilation process the headers used by tt(bisonc++'s) almost 500
source files were first precompiled (the time it took to precompile the
headers is included in the time reported below). Now bf(time) reported:
verb( 138.040u 18.447s 2:38.57 98.6% 0+0k 32+2245104io 0pf+0w)
The total size of the precompiled headers is definitely exceeding the size
of the module tt(bisoncpp-2VBAIMWQCR12B.pcm): 1.1 GB. But then, the
compilation time is reduced to about 1/3rd of the other two compilation
methods.
Considering the above results the added benefit of using modules isn't
immediately clear. Using modules doesn't result in an observable gain in
compilation time, and so the main advantage appears to be that include guards
are no longer required. Personally, I've em(never) experienced problems with
using include guards. Using of long, capitalized preprocessor identifiers
has never been a problem, and has never resulted in colliding identifiers.
On the other hand, modules are a new element of bf(C++) and the tt(clang)
documentation explicitly states that the compiler's implementation and module
definition language is subject to future changes. In that respect, modules are
definitely an interesting addition to bf(C++) that is absolutely worth
monitoring.
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