On 02/07/18 09:01, Linus Torvalds wrote: > > Look - much smaller code, and register %rcx isn't used at all. And no > D$ miss on loading that constant (that is a constant depending on > boot-time setup only). > > It's rather more complex, but it actually gives a much bigger win. The > code itself will be much better, and smaller. > > The *infrastructure* for the code gets pretty hairy, though. > > The good news is that the patch already existed to at least _some_ > degree. Peter Anvin did it about 18 months ago. > > It was not really pursued all the way because it *is* a lot of extra > complexity, and I think there was some other hold-up, but he did have > skeleton code for the actual replacement. > > There was a thread on the x86 arch list with the subject line > > Disgusting pseudo-self-modifying code idea: "variable constants" > > but I'm unable to actually find the patch. I know there was at least a > vert early prototype. > > Adding hpa to the cc in the hope that he has some prototype code still > laying around.. > The patchset I have is about 85% complete. It mostly needs cleanup, testing, and breaking into reasonable chunks (it got put on the backburner for somewhat obvious reasons, but I don't think it'll take very long at all to productize it.) The main reason I haven't submitted it yet is that I got a bit overly ambitious and wanted to implement a whole bunch of more complex subcases, such as 64-bit shifts on a 32-bit kernel. The win in that case is actually quite huge, but it is requires data-dependent code patching and not just immediate patching, which requires augmentation of the alternatives framework. -hpa -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
