Align benchmark::State to a cacheline.

[ckennelly]

This can avoid interference with neighboring objects and stabilize
benchmark results.

[oontvoo]

@dominichamon Another reason to have absl dependency :) Would make this PR so much simpler.

[dmah42]

do you have the results from a run with and without this change?

[ckennelly]

@dominichamon I only have an internal microbenchmark, which demonstrated higher variability across runs (especially when combined with a change to TCMalloc) without the alignment attribute.

[LebedevRI]

This seems plausible (although hard to tell without a test), but is this only about the State?
What about ThreadTimer, ThreadManager, PerfCountersMeasurement?

[chfast]

Can you use std::hardware_destructive_interference_size from C++17 if available?

[dmah42]

This seems plausible (although hard to tell without a test), but is this only about the State? What about ThreadTimer, ThreadManager, PerfCountersMeasurement?

we could try to run some tests, but i don't think it should block this PR (which i totally missed).

or just align all the things.

[dmah42]

Can you use std::hardware_destructive_interference_size from C++17 if available?

not in the public header. soon.

[dmah42]

yeah your way is better because it separates the definition of the cacheline size from the macro that uses it.

[LebedevRI]

Not really sure if this outermost guard is needed, but then i don't know what set of per-cpu defines MSVC provides.

[LebedevRI]

Yay, thank you!