r241620 - Wrap clang modules and pch files in an object file container.

[Richard Smith]

On Mon, Jul 13, 2015 at 3:06 PM, Adrian Prantl <aprantl at apple.com> wrote:

> > On Jul 13, 2015, at 2:00 PM, Eric Christopher <echristo at gmail.com>
> wrote:
> >
> > Hi Adrian,
> >
> > Finally getting around to looking at some of this and I think it's going
> in slightly the wrong direction. In general I think begin -able- to put
> modules in object files to simplify wrapping, use, etc is a good thing. I
> think being required to do so is somewhat problematic.
> >
>
> Let me start with that the current infrastructure already allows selecting
> whether you want wrapped modules or not by passing the appropriate
> PCHContainerOperations object to CompilerInstance. Clang currently
> unconditionally uses an object file wrapper, all of clang-tools-extra
> doesn’t. We could easily control the behavior of clang based on a (new)
> command line option.
>
> But.. on a platform with a shared module cache you always have to assume
> that a module once built will eventually be used by a client that wants to
> read the debug info. Think llvm-dsymutil — it does not know and does not
> want to know how to build clang modules, but does want to read all the
> debug info from a clang module.
>
> > Imagine, for example, you have a giant distributed build system...
> >
> > You'd want to create a pile of modules (that may reference/include/etc
> other modules) that aren't don't or may not have debug information as part
> of them (because you might want to build without it or have the debug info
> alongside it as a separate compilation). Waiting on the full build of the
> module including debug is going to adversely affect your overall build time
> and so shouldn't be necessary - especially if you want to be able to have
> information separate ultimately.
> >
> > Make sense?
>
> Not sure if you would be saving much by having the debug info separately,
> from what I’ve measured so far the debug info for a module makes up less
> than 10% of the total size. Admittedly, build-time-wise going through the
> backend to emit the object file is a lot more expensive than just dumping
> the raw PCH. [1]
>
> Yeah, I think wanting to be able to control the behavior is reasonable, we
> just need to be careful what the implications for consumers are. If we add
> a, e.g., an “-fraw-modules” [2] or switch to clang to turn off the object
> file wrapping, I’d strongly suggest that we add the value of this switch to
> the module hash (or add a an optional “-g” to the module file name after
> the hash or something like that) to avoid ugly race conditions between
> debug info and non-debug-info builds of the same module. This way we’d have
> essentially two separate module caches, with and without debug info.
>

That's fine, I think (we don't use a module cache at all in our build
system; it doesn't really make much sense for a distributed build) and most
command-line flag changes already have this effect.


> would that work for you?
> -- adrian
>
> [1] If you want to be serious about building the module debug info in
> parallel to the rest of the build, you could even have a clang-based tool
> import the just-built raw clang module and emit the debug info without
> having to parse the headers again :-)
>

That is what we intend to do :) (Assuming this turns out to actually be
faster than re-parsing; faulting in the entire contents of a module has
much worse locality than parsing.)

[2] -fraw-modules, -fmodule-format-raw, -fmodule-debug-info, ...?
>     I would imagine that the driver enables module debug info when
> "-gmodules” is present and by default on Darwin.


That seems reasonable to me. For the frontend flag, I think a flag to turn
this on or to select the module format makes more sense than a flag to
switch to the raw format.

[One other thing... I think we may have made a mistake by putting the
reader and writer code behind the same interface: it forces tools that want
to read the module format to link against all of LLVM IR, code generation,
and so on, when all they really need is something like libObject.]
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