[llvm-dev] [Progress Update] LLVM Runtimes Subdirectory

Chris Bieneman via llvm-dev llvm-dev at lists.llvm.org
Fri Aug 26 14:45:05 PDT 2016

Hi LLVM-Dev,

Over the past week or so I’ve been working on build system improvements around the LLVM runtimes directory. You may remember this from the LLVM-Dev discussion (http://lists.llvm.org/pipermail/llvm-dev/2016-June/100859.html), or the patch review discussion (https://reviews.llvm.org/D20992) back in June.

Recently, I’ve made a handful of new changes that actually make it useful, and I wanted to blast out this update and encourage people to give it a try.

Let me start with an update on the current state.

First and foremost, this is not currently supported with multi-configuration generators (Visual Studio or Xcode). I’ve done my testing with Ninja, but I expect it will work the same with Make.

Currently the runtimes directory is supported for building compiler-rt and libcxx. You can enable the new build paths either by putting your compiler-rt and libcxx checkouts into the runtimes subdirectory or by symlinking the directories. I do plan to support variables for overriding paths to the runtimes similar to how we support the LLVM projects directory today, but I haven’t gotten there yet (let me know if this prevents you from using the feature and I’ll prioritize it).

With support for libcxx I made the runtimes build generate top-level targets for each runtime project. The runtime project targets are named for the project with leading ‘lib’ prefixes removed (i.e. compiler-rt, cxx). Also note, for the runtimes subdirectory compiler-rt is treated as a separate project from the builtins. One of the current limitations of this system is that I still don’t have a solution for building and running the builtins tests.

I’ve also added support to the runtimes for generating a target that runs all runtime tests from a single lit invocation. This target is exposed in the build as “check-runtimes”, and it is connected as a dependency of LLVM’s “check-all” target. This makes it so “check-all” includes the runtime test, but not under the same lit invocation. I have ideas for how to fix that which I’ll be working on.

The last new feature I’ve added is the ability to expose a set of targets from runtime projects into the top-level generated build files. I’ve connected this new feature into compiler-rt, which allows exposing targets like asan and check-asan through to the top-level build. This last feature has some slight awkwardness and I’m curious to hear opinions on it.

To avoid having to hard-code a list of targets to expose I made the runtime configuration step generate a list of targets to map back. This means that before you can use targets from the runtimes build you need to build enough of the toolchain to configure the runtime project build. What that means is you can’t do the following in a new build tree:

> cmake ...
> ninja asan

Instead you would do:

> cmake ...
> ninja runtimes-configure
> ninja asan

I can’t think of any good ways around this. I’m curious if anyone has ideas, or opinions on whether or not this is an acceptable workflow.

The big items on my to-do list are:
* Running the builtins tests
* Porting libcxxabi and libunwind
* Getting check-all with a single lit invocation
* IDE support
* Supporting specifying the path of runtime libraries

In general I think this is getting to a point where people can give it a test drive and see what they think, and I’m eagerly looking for feedback.


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