[cfe-dev] JumboSupport: making unity builds easier in Clang

[David Chisnall via cfe-dev]

On 10 Apr 2018, at 21:28, Daniel Bratell via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> I've heard (hearsay, I admit) from profiling that it seems the single largest time consumer in clang is template instantiation, something I assume can't easily be prepared in advance.
> 
> One example is chromium's chrome/browser/browser target which is 732 files that normally need 6220 CPU seconds to compile, average 8,5 seconds per file. All combined together gives a single translation unit that takes 400 seconds to compile, a mere 0.54 seconds on average per file. That indicates that about 8 seconds per compiled file is related to the processing of headers.

It sounds as if there are two things here:

1. The time taken to parse the headers
2. The time taken to repeatedly instantiate templates that the linker will then discard

Assuming a command line where all of the relevant source files are provided to the compiler invocation:

Solving the first one is relatively easy if the files have a common prefix (which can be determined by simple string comparison).  Find the common prefix in the source files, build the clang AST, and then do a clone for each compilation unit.  Hopefully, the clone is a lot cheaper than re-parsing (and can ideally share source locations).

The second is slightly more difficult, because it relies on sharing parts of the AST across notional compilation units.

To make this work well with incremental builds, ideally you’d spit out all of the common template instantiations into a separate IR file, which could then be used with ThinLTO.  

Personally, I would prefer to have an interface where a build system can invoke clang with all of the files that need building and the degree of parallelism to use and let it share as much state as it wants across builds.  In an ideal world, clang would record which templates have been instantiated in a prior build (or a previous build step in the current build) and avoid any IRGen for them, at the very least.

Old C++ compilers, predating linker support for COMDATs, emitted templates lazily, simply emitting references to them, then parsing the linker errors and generating missing implementations until the linker errors went away.  Modern C++ compilers generate many instantiations of the same templates and then discard most of them.  It would be nice to find an intermediate point, which worked well with ThinLTO, where templates could be emitted once and be available for inlining everywhere.

David