Messages in this thread | | | From | Denys Vlasenko <> | Date | Sat, 10 Jan 2015 22:09:24 +0100 | Subject | Re: [PATCH 3/4] x86: open-code register save/restore in trace_hardirqs thunks |
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On Sat, Jan 10, 2015 at 10:02 PM, Andy Lutomirski <luto@amacapital.net> wrote: > On Sat, Jan 10, 2015 at 12:42 PM, Borislav Petkov <bp@alien8.de> wrote: >> On Sat, Jan 10, 2015 at 12:17:13PM -0800, Andy Lutomirski wrote: >>> I asked this once, and someone told me that push/pop has lower >>> throughput. I find this surprising. >> >> Implicit dependency on %rsp probably. The MOVs allow you to start more >> stuff out-of-order I'd guess... > > AIUI modern CPUs have fancy stack engines that match call/ret pairs, > and presumably they can speculate rsp values across multiple pushes > and pops very quickly.
Yes, stack engine hangs off the pipeline right after decode stage.
> Also, don't compilers generally use push and pop to save and restore > callee-saved registers? I think that function calls are common enough > that the CPU vendors would have made these sequences fast.
Compilers can't predict which functions are hottest. Using mov's would bloat prologues by about factor of 5.
I think using push/pop is okay. In the very hottest code paths you may want to prefer mov's.
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