[swift-dev] Reducing StdlibUnittest compile time

[Dmitri Gribenko]

Hi,

StdlibUnittest is an internal library that we use to write tests for the
standard library, and some parts of the runtime and the compiler (for example,
the implementation of bridging for Objective-C collections).  We heavily rely
on these tests to check correctness of the Swift compiler and standard library.

But there is an issue: today, StdlibUnittest takes a long time to build.
The following table shows the time it takes to build StdlibUnittest.

Compiler      Library       StdlibUnittest build time
ReleaseAssert Release       1m 43s
ReleaseAssert ReleaseAssert 4m 26s
DebugAssert   DebugAssert   2m 25s

I investigated the issue a little bit, and I moved the code that tests
sequences and collections into a new library, StdlibCollectionUnittest, which
only gets built when validation tests are run.

Compiler      Library       StdlibUnittest  StdlibCollectionUnittest
                            build time      build time
ReleaseAssert Release       30s             58s
ReleaseAssert ReleaseAssert 1m 10s          1m 20s
DebugAssert   DebugAssert   53s             50s

These time measurements show that the build time of the combined module is
greater than the sum of the build times of the pieces, and much more so when
the optimization is turned on.  We can make a conjecture that the optimizer is
not scaling well with the module size.

The last commit before the library split was
8aebe8d864ea6094355b365e0f505bb117e29304.  It would be great if someone could
look into the compiler issue.

Dmitri

-- 
main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/