Don't do stack squeezing during context switches in single-threaded programs to guarantee determinism in allocations

As #4450 (closed) and #8611 (closed) show, stack squeezing in the RTS makes allocation numbers between two different runs of the same binary non-deterministic, because its effect is depending on when context switches are bound to happen.

A short-term solution might be to deactivate stack squeezing for vulnerable benchmarks with ~~+RTS -Z~~ +RTS -V0 like in D5460, but IMO a more elegant solution would be to only deactivate stack squeezing in threadPause calls that happen due to context switches. Would you agree?

Edited Jan 16, 2020 by Sebastian Graf

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