[llvm-dev] (Thin)LTO llvm build

[Mehdi Amini via llvm-dev]

> On Sep 16, 2016, at 6:13 PM, Carsten Mattner <carstenmattner at gmail.com> wrote:
> 
> On Sat, Sep 17, 2016 at 2:07 AM, Teresa Johnson via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> 
>> Yes and to add on - the ThinLTO backend by default will
>> kick off  std::thread::hardware_concurrency # of threads, which I'm finding is
> 
> Is it just me or does that sound not very intuitive or at least a
> little unexpected?
> It's good that it uses the resources eagerly, but in terms of build systems this
> is certainly surprising if there's no control of that parameter via
> make/ninja/xcode.

You can control the parallelism used by the linker, but the option is linker dependent
(On MacOS:   -Wl,-mllvm,-threads=1)

> 
>> too much for machines with hyperthreading. If that ends up being an issue I can
>> give you a workaround (I've been struggling to find a way that works on various
>> OS and arches to compute the max number of physical cores to fix this in the source).
> 
> I've been using ninja -jN so far. I suppose when building with ThinLTO I should
> run ninja -j1. Would that
> 
> What's the workaround?

Seems like you missed my previous email: : cmake -DLLVM_PARALLEL_LINK_JOBS=1
Also, ninja is parallel by default, so no need to pass -j.

This way you get nice parallelism during the compile phase, and ninja will issue only one link job at a time.

— 
Mehdi