Is geekbench accurate at comparing across different architectures?
Generally, yes. The benchmarks are calculating/computing the same things on both architectures, and the various sub-benchmarks are based on real-world non-trivial computational problems rather than microbenchmarks that are easily manipulated by instruction set differences.
Unless they are written in optimised Asm I would not compare across architectures, because there is considerable leeway in compilers and my experience has shown that a good human can often beat a compiler solidly --- at least on x86.
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