Tacit Murky

What instruction? There is no «NRAC» in the original x86 or any of its additions (including x87). Besides, if some code uses any blocked or unavailable instruction, the CPU should generate #UD or other exception. So, it must be either microcode workaround for a HW bug or some other explanation. Well, then there is no reason for keeping it that way for 2 core generations, if it just slows things down. And you haven't answered about other fixes in your program.

Stilt, can you explain, what exactly your patch is doing? I suppose, you write some values into some MSR's to manage low-level fixes and bug workarounds, that have been inserted in the microcode by the BIOS/AGESA on boot. Switching these off may speed up something, but it's dangerous, if the (originally patched) hardware flaw haven't been fixed yet. And what are this fixes? In particular, that is NRAC, TBM («trailing bit manipulation» instructions?) and Stack Special? Officially, Bulldozer (all steppings and revisions) doesn't have TBM, but Piledriver does.