[lldb-dev] Coming up: Preliminary C/Objective-C modules support in the expression parser

Zachary Turner zturner at google.com
Thu Dec 4 17:33:13 PST 2014

Yes, but for starters we dont have a windows build bot yet (ive been
working on that), and secondly buildbots shouldnt be used as a way to
bypass the review process.

For example, Kates change to IOHandler recently is small compared to this
one, but it still was looked at by people prior to comitting.

It's possible these changes won't break anything, but still it doesn't seem
unreasonable to ask that particularly huge changes like this at least have
a day or two of being made public before going in.
On Thu, Dec 4, 2014 at 5:25 PM <jingham at apple.com> wrote:

> I'm confused.  Isn't that what the build bots are for?  Seems better to
> rely on the immediate feedback you get from build bots for "other host"
> breakages than waiting around to see if anybody is going to grab your patch
> and apply it and try to build, etc...
> Jim
> > On Dec 4, 2014, at 5:18 PM, Zachary Turner <zturner at google.com> wrote:
> >
> > Would it be possible to put patches up for review for at least a couple
> days BEFORE comitting? There's a pretty good chance this will break a build
> somewhere, and an average chance that the break won't be that easy to fix.
> And having broken builds for a couple of days doesn't seem reasonable.
> >
> > Im not sure how thoroughly we can review it, but at least we can prevent
> broken builds this way.
> > On Thu, Dec 4, 2014 at 5:10 PM Sean Callanan <scallanan at apple.com>
> wrote:
> > Rationale
> >
> > A historical pain point in the expression parser has been using
> functions for which the type is not completely known from debug information.
> > This is the case e.g. for functions from the C standard library, and
> also for Objective-C methods in frameworks.
> > It’s also unfortunate when handy types, enums, etc. are invisible just
> because your DWARF doesn’t happen to contain them.
> > I’m doing something about that, for OS X – but it should generalize to
> other platforms.
> >
> > Clang modules: the basics
> >
> > The code I’m about to commit adds support for Clang modules to the
> expression parser.  Clang modules are described in much more detail here:
> >       http://clang.llvm.org/docs/Modules.html
> > but here is a short introduction:
> >
> > A group of header files is encapsulated in a module, which is provided
> with a module.map file.
> > On OS X, this module.map is typically inside the corresponding framework.
> > Clang reads module.map and the corresponding headers and produces a
> compiled module.
> > This compiled module is essentially a .pch; it provides full information
> about all APIs, types, etc. defined in the module.
> >
> > LLDB support
> >
> > In order to use Clang modules, LLDB must:
> >
> >       • Know where they live (from Clang’s perspective, this is the
> “sysroot”) and what compilation flags to use when parsing them;
> >       • Know which ones the user wants;
> >       • Compile them with its built-in Clang instance; and
> >       • Use the information found in the compiled modules as appropriate.
> >
> > LLDB accomplishes each of these goals in the following ways:
> >
> >       • Platforms can now return the appropriate Clang flags that tell
> the module importer where to find modules for the current platform.
> Explicit support is enabled in PlatformDarwin.  Other platforms can opt
> into this by returning true from Platform::SupportsModules() and adding the
> appropriate compilation options when Platform::
> AddClangModuleCompilationOptions() is called.  If they return false,
> there should be no change in behavior.
> >       • LLDB adds preprocessor callbacks to Clang that catch @import
> directives.  When such a directive is found, LLDB directs its built-in
> Clang to import the named module.
> >       • Modules are imported into a separate compiler instance (with its
> own AST context) encapsulated in ClangModulesDeclVendor so that we can be
> careful about what we actually import into expressions.  Information in
> DWARF will often take precedence, for instance.
> >       • ClangExpressionDeclMap – the code responsible for finding
> entities Clang asks about while it parses expressions – is being extended
> to load information from modules as appropriate.  The initial commit simply
> searches for functions (e.g., printf()) but I will be adding functionality
> rapidly.
> >
> > What the upcoming patches do
> >
> >       • ClangModulesDeclVendor.h/.cpp implements the portions of LLDB
> responsible for driving the modules compiler.  This should be
> platform-generic code, although in practice we may need to tweak it to make
> sure it is flexible enough to handle everything.
> >       • TypeVendor has been changed to DeclVendor, so that the
> Objective-C runtime can share the same method signatures with the Clang
> module importer.  Places that used TypeVendor now use DeclVendor, and I’ve
> tweaked the APIs for getting types from decls to make this transition
> smooth.
> >       • Platform (as mentioned above) now can return the flags necessary
> to tell Clang where modules live.  PlatformDarwin has a bunch of new code
> to find these; I also have a default implementation that you can try out if
> you want, but unless you know what a module.map is I would hold off on this.
> >       • The LLDB bundle on Mac OS X will now also include the
> compiler-specific headers Clang requires to compile standard library
> headers (e.g., tgmath.h, stdarg.h).  Host can find the location of these
> headers; on non-OS X hosts, we’ll need to put them in some sensible place.
> >       • ClangExpressionParser.cpp now sets up the appropriate context to
> intercept @import directives.
> >       • ClangExpressionDeclMap.cpp now searches modules (if available)
> for functions if it doesn’t find them in DWARF.
> >       • Targets now vend their ClangModulesDeclVendors as appropriate.
> >
> > Timeline and priorities
> >
> > I’m going to start committing today, but if any of these commits breaks
> anything for you, please let me know.
> > My priority is getting this working on OS X first.  If parts of my code
> look platform-myopic, please let me know, though – I really want to see
> debugging with modules working on other platforms too.
> > Of course if any of this breaks any build, let me know immediately and
> I’ll get on it.
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