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[Support] On Windows, add optional support for {rpmalloc|snmalloc|mimalloc}

Authored by aganea on Aug 27 2020, 8:09 AM.

Description

[Support] On Windows, add optional support for {rpmalloc|snmalloc|mimalloc}

This patch optionally replaces the CRT allocator (i.e., malloc and free) with rpmalloc (mixed public domain licence/MIT licence) or snmalloc (MIT licence) or mimalloc (MIT licence). Please note that the source code for these allocators must be available outside of LLVM's tree.

To enable, use cmake ... -DLLVM_INTEGRATED_CRT_ALLOC=D:/git/rpmalloc -DLLVM_USE_CRT_RELEASE=MT where D:/git/rpmalloc has already been git clone'd from https://github.com/mjansson/rpmalloc. The same applies to snmalloc and mimalloc.

When enabled, the allocator will be embeded (statically linked) into the LLVM tools & libraries. This currently only works with the static CRT (/MT), although using the dynamic CRT (/MD) could potentially work as well in the future.

When enabled, this changes the memory stack from:

new/delete -> MS VC++ CRT malloc/free -> HeapAlloc -> VirtualAlloc

to:

new/delete -> {rpmalloc|snmalloc|mimalloc} -> VirtualAlloc

The goal of this patch is to bypass the application's global heap - which is thread-safe thus inducing locking - and instead take advantage of a modern lock-free, thread cache, allocator. On a 6-core Xeon Skylake we observe a 2.5x decrease in execution time when linking a large scale application with LLD and ThinLTO (12 min 20 sec -> 5 min 34 sec), when all hardware threads are being used (using LLD's flag /opt:lldltojobs=all). On a dual 36-core Xeon Skylake with all hardware threads used, we observe a 24x decrease in execution time (1 h 2 min -> 2 min 38 sec) when linking a large application with LLD and ThinLTO. Clang build times also see a decrease in the range 5-10% depending on the configuration.

Differential Revision: https://reviews.llvm.org/D71786