[cfe-dev] RFC: prototype of clang-scan-deps, faster dependency scanning tool for explicit modules and clangd

[Alex L via cfe-dev]

Hi,


Bruno (CCed), Duncan (CCed) and I have been exploring if we can migrate
some of our clients to explicit modules. As part of this work Duncan and I
developed a new prototype dependency scanning service tool
(clang-scan-deps) that computes the set of file dependencies for a
particular compiler invocation using some optimizations that are outlined
below. This tool makes the non-modular dependency scanning up to 10 times
faster for particular workloads (e.g. llc target, 1542 C++ files) on one of
our machines, when compared to parallel invocations of clang with -Eonly.
We are still in the early stages of proper modules support, but our initial
crude prototype can get up to 4x when run on the first 1000 files from
clang’s compilation database for a build of LLVM with modules turned on.


We still run the full Clang preprocessor. Here’s what we do to reduce its
workload:

   - Minimize sources by stripping away unused tokens. We keep only the
   interesting PP directives (#define, #if, #include, etc.), i.e. those that
   might impact the set of dependencies.
   - Assume the filesystem is immutable for one run of the service, and
   cache the files and their minimized contents in memory in a global cache.
   - Skip over excluded preprocessor ranges by bumping up the buffer
   pointer in the lexer instead of lexing the skipped tokens.


We intend to upstream this service in the upcoming months. We also would
like to integrate this service into Clangd as part of our migration to
Clangd to help us determine a good compilation command for a header file
from a set of known compilation invocations.


I posted a very rough WIP patch on Phabricator (
https://reviews.llvm.org/D53354). It’s based on LLVM checkout r343343.
Please take a look if you’re interested.

Duncan, Bruno and I will be at the LLVM dev meeting. We are interested in
discussing this prototype and collecting feedback from anyone who might be
interested in this work.


Thanks,

Alex
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