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Thomas Schilling authored
This is needed both for per-session parallelism and for allowing multiple concurrent sessions in the same process. With the help of atomicModifyIORef and unsafePerformIO it is also quite fast--an MVar would most likely be slower. On a full compilation of Cabal's head branch it was about 1-2 percent slower, but then overall compilation times varied by about 4 percent, so I think it's worth it.
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