Ian Cutress replied on the article comments that LTCG is indeed used. With LTCG those strange results make sense - it's spending a lot of time on just 4 threads by default - actually majority of the time is on one thread for the Chromium case, it hits some current limitations of the VC++ compiler regarding CPU/memory usage that makes scaling worse for Chromium (but not for smaller programs or with non-LTCG builds). Increasing the number of threads from the default of 4 is possible, but will not help here. The frontend (parsing) work is well parallelized by Ninja, it's probably the reason why the Threadrippers do end up ahead of the faster single-core Intel CPUs.
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