As anybody who has ever sat through an "allyesconfig" build - or a
build using a distributor configuration - understands, there is a
of code in the kernel. Most of the time, creating the perfect
kernel is a matter of excluding code until the size becomes reasonable. So
it's a rare kernel build that actually compiles a majority of the code
found in the tree.
The kernel configuration mechanism makes it possible to perform this
selection. Part of this mechanism is wired into the build system; it
allows source files to be passed over entirely if they contain nothing of
interest. The other half, though, is implemented with preprocessor symbols
and conditional compilation. Kernel developers may be discouraged from
using #ifdef, but there are still a lot of conditional blocks in
Sometimes, the logic which leads to the inclusion or exclusion of a
specific block is complex and not at all clear. There are many
configuration options in the kernel, and they can depend on each other in
complicated ways. As a result, dead code - code which will not be compiled
regardless of the selected configuration - may escape notice for years.
Dead code adds noise to the
source tree and, since nobody ever runs it, it is more than likely to
contain bugs. If that code is re-enabled or copied, those bugs could
spread through the tree in surprising ways.
So it would be good to be able to identify dead code and get it out of the
tree. The newly-released undertaker tool
was designed to do a number of types of static analysis, including dead
code identification. Developers can run it on their own to find dead
blocks in specific files; there is also a
web interface which allows anybody to browse through the tree and find
the dead sections. That should lead to patches hauling away the bodies and
cleaning up the tree, which is a good thing.
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