On 3/14/2013 3:42 PM, Sean Silva wrote:
On Thu, Mar 14, 2013 at 12:30 AM, Joshua Cranmer <[email protected]
<mailto:[email protected]>> wrote:
On 3/13/2013 10:05 PM, Sean Silva wrote:
While I'm all for the idea of improving the plugin API, I
think that a modest reduction in boilerplate is not
sufficiently compelling to foist a new plugin API on people
who already have existing code. The funny thing about
boilerplate is that it's easy to copy-paste, so it doesn't
really impede people from achieving their goals since they can
just copy the code that already works. The primary problem of
boilerplate is that it has the effect of deterring newbies,
and that issue can be easily combated with improved
documentation, which avoids breaking every external plugin and
tutorial on plugins.
One thing that REALLY sucks with the current approach is the need
to specify clang -Xclang -load -Xclang <plugin tarball> -Xclang
-add-plugin <plugin name> -Xclang -plugin-<name>-arg -Xclang
<blah> ...
With the new approach, the command line is clang
-fplugin=<tarball> -fplugin-arg-<name>-<arg>=<blah>, which is a
much shorter command line and can actually be passed into
CFLAGS/CXXFLAGS without driving libtool bonkers (I detest the need
for wrapper scripts just to pass arguments) and also eliminates
warnings whenever people use $(CXX) $(CXXFLAGS) as the linker.
Realistically a tiny script might be a better long-term design,
allowing e.g. "clang++ `clang-plugin-config myPlugin.so arg1 arg2`
foo.cpp". Remember that the primary advantage of plugins vs
libtooling/libclang is that they are run as part of a build process,
meaning that in reality these command lines are meant to be generated
by a "configure" step and not by hand. So really the "user
friendliness" is determined by "how easy is it to integrate a clang
plugin into my build", and not by the exact commandline syntax per se.
This kind of script could also serve as a useful layer of indirection
and "user friendliness", e.g. it could recognize a "CLANG_PLUGIN_PATH"
or other niceties that would be dubious to add to clang itself.
A "tiny script" sounds to me a much worse option: you now have to run
two processes per compiler invocation instead of one (and on Windows,
process creation can take hundreds of milliseconds, so it's a noticeable
slowdown). Also, you have to find more binaries to run it: if I specify
CXX via a path, how should a build system know where to run
clang-plugin-config from? You could guess by looking up the dirname of
CXX and hoping it's there, but you are also advocating using shell
scripts to represent CXX in another email, which renders this approach
impossible.
If you really want to immediately push plugins forward in a big way,
it would be monumental to set up a buildbot that runs a clang plugin
that does extra checking that isn't really appropriate for being
integrated as a diagnostic into the compiler proper. For example, a
plugin that warns on incorrect uses of dyn_cast<>. For maximum effect
this should be developed in-tree (probably in clang-tools-extra. Even
though it has "tools" in the name, I don't think anybody would be
opposed to developing plugins in there). It should also have an easy
way for people in our community to come up with and implement good
extra checks and get them integrated into that buildbot.
I am working on adding a compiler static checker plugin to Mozilla that
would check the guarantees our old dehydra plugin used to check: a "must
override" annotation (all subclasses must provide their own
implementation of this method), a "stack class" annotation (this class
cannot be allocated from the heap), and a warning that gets emitted
every time you emit a static initializer.
The changes in this patch retain almost all of the same
functionality as the original plugin approach (including the
ability to do things like add custom compile passes via the
RegisterPass statics) while wrapping it in a much saner wrapper.
My opposition to the current patch is that it does not provide enough
value to our users to compensate for the inconvenience that it will
cause them (by breaking their code). My opposition is not technical; I
don't doubt that your approach here is an improvement from a purely
technical standpoint.
The current plugin approach presumes that it is a pure consumer of the
AST, which isn't a viable option in my opinion. One thing I would like
to do in the future is be able to map Decls in the AST to functions
emitted in the LLVM IR, which is completely impossible under the current
architecture. Note also that I'm not removing the current (more or less
broken) plugin architecture, so I'm not compelling people to switch.
Rather, this is about enabling future changes that permit plugins to not
take the view that they happen independently of code generation.
--
Joshua Cranmer
News submodule owner
DXR coauthor
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