[PATCH] D71775: [ThreadPool] On Windows, extend usage to all CPU sockets and all NUMA groups

[Alexandre Ganea via Phabricator via cfe-commits]

aganea added a comment.

@thakis I think this is a side-effect of implementing `computeHostNumPhysicalCores()` for Windows, which previously returned -1, which in turn made `llvm::heavyweight_hardware_concurrency()` previously behave like `llvm::hardware_concurrency()` (on Windows only).
At first sight, Linux seems to report the same thing as Windows (2 cores per "module"). If I read this <https://askubuntu.com/questions/240235/identical-cores-in-proc-cpuinfo> correctly, the behavior on Linux for a AMD Opteron is/was the same as it is currently on Windows after this patch, that is, only one core out of two would be used for `llvm::heavyweight_hardware_concurrency()`.

I think this is a wider problem outside the scope of this patch, which is, some users want to use 100% of the cores/hyper-threads when using ThinLTO. Currently there's no way. An algorithm using `llvm::heavyweight_hardware_concurrency()` explicitly states "I only want one core out of two". It'd be nice if we had a cmd-line flag to override this, to say "I want to use all hyper-threads/cores/modules". `/opt:lldltojobs=all`? The impact is small on a modern Intel Skylake CPU, but it might be better depending on the CPU (AMD Bulldozer).

F11420112: image.png <https://reviews.llvm.org/F11420112>


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