Macro-Op fusion does not work in 64-bit mode

A FEW DAYS ago, there was some controversy about the new MCW cores and macro-op fusion (MOF).

The short story is that this Intel touted wonder tech would not function in 64-bit mode. The fanbois and tin-foil hat brigade came out in force declaring the MCW cores worthless, but the truth is far from that, it is true that MOF does not work in 64-bit mode, but it hardly affects anything.

Working with Agner Fog(1), who brought you the comprehensive software optimisation manuals, the question of 'does MOF work in 64-bit mode' was looked at. The short answer is no it does not, performance counters for MOF fusion that incremented in 32-bit mode did not increment in 64-bit mode. Basically, like the slides said, it is off in 64-bit mode.

The thing is though, while MOF may be touted as the best thing since sliced bread, it does not cause many performance problems when it is off. It appears that the bottleneck in the CPU is not in that aspect of the pipeline, so its loss has little speed impact. More on this when the testing is complete. µ

(1) He did all the work, I just watched.