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Improve x86-64 timer performance #2926
On x86-64 a significant amount of processor time is currently being spent on
This PR tackles both:
I haven't looked for a good microbenchmark that isolates the effect, but I've been getting decent time improvements on NPB (mainly FT) and Quantum Espresso.
@hjelmn people use MPI_Wtime() to time code so I didn't want to introduce a regression by making it less accurate in some weird case where the processor is particularly crafty when reordering instructions.