2 Dec
2022
2 Dec
'22
8:54 p.m.
On 12/2/22 14:39, Paul Gofman wrote:
On 12/2/22 14:32, Zebediah Figura (@zfigura) wrote:
On Fri Dec 2 20:30:47 2022 +0000, **** wrote:
Paul Gofman replied on the mailing list:
On 12/2/22 14:25, Gabriel Ivăncescu (@insn) wrote: > On Fri Dec 2 18:57:30 2022 +0000, Jacek Caban wrote: >>> This should help a bit more, does it make a difference for you? >> My previous test wasn't really good for measuring it. >> I hacked a micro-benchmark, which confirms that the patch improves >> performance a lot. It was visible when doing "real" Vulkan >> vkGetPhysicalDeviceProperties calls in a loop, but even cleaner when I >> changed it further to make Unix side to be no-op. It closes most of >> the >> gap between direct call and __wine_unix_call_dispatcher. Times >> recorded >> for no-op calls: >> - direct call: 5761 >> - unpatched Wine: 13933 >> - ret.diff: 6823 (55% time spent in __wine_unix_call_dispatcher, >> 29% in >> PE vkGetPhysicalDeviceProperties) >> Looks impressive! > @gofman This isn't about setting it in rcx or not, it's about mispairing `call`s and `ret`s, which basically means 100% mispredicted because CPUs are optimized for it, so it couldn't do any speculative execution past the return before. Yes, I figured that much. Yet the attached diff removes the return address from rcx in wine_syscall_dispatcher(), so I thought it makes sense to note that it will break things.Would it help to return to the return address already on the PE stack?
I am sorry, am not sure if I understand... help perf or help anticheat, and how return address on PE stack is related? Also note that:
- ret address in rcx relates to wine_syscall_dispatcher only, not
__wine_unix_call_dispatcher, while it is __wine_unix_call_dispatcher is of the performance concern here;
- I guess that moving the ret address to rcx and push rcx / ret might be
the same performance-wise as pushq 0x70(%rcx), ret.
I mean something like the attached patch. I don't know enough about modern x86 optimization to know if it would help, but it seems like it would at least avoid a memory access?