261–270 of 367 posts
I wonder if such a direct translation from ARM to another architecture would even be possible given that the instruction set can be changed at runtime (thumb mode). Does anybody know how often typical ARM32 programs execute this mode switching or if such sections can be recognized statically?
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I think I landed in a place where it's basically "the compiler has insufficient information to achieve ideal optimization because some things can only be known at runtime." Which is not exclusively an argument for runtime JIT— it can also be an argument for instrumenting your runtime environment, and feeding that profiling data back to the compiler to help it make smarter decisions the next time. But that's definitel…
The problem with JIT is not all information known at runtime is the correct information to optimize one. In finance the performance critical code path is often the one run least often. That is you have a if(unlikely_condition) {run_time_sensitive_trade();}. In this case you need to tell the compiler to ensure the CPU will have a pipeline stall because of a branch misprediction most of the time to ensure the time that…
Does anyone know the names of the key people behind Rosetta 2? In my experience, exceptionally well executed tech like this tends to have 1-2 very talented people leading. I'd like to follow their blog or Twitter.
I am the creator / main author of Rosetta 2. I don't have a blog or a Twitter (beyond lurking).
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> removing support for non-hidpi displays from MacOS Did that really reduce sales? Consider that the wide availability of crappy low end hardware gave Windows laptops a terrible reputation. Eg https://www.reddit.com/r/LinusTechTips/comments/yof7va/frien...
> Consider that the wide availability of crappy low end hardware gave Windows laptops a terrible reputation. Standard DPI displays are not "crappy low-end hardware"? I don't think there's a single widescreen display which qualifies as hiDPI out there, that more or less doesn't exist: a 5K 34" is around 160 DPI (to say nothing of the downright pedestrian 5K 49" like the G9 or the AOC Agon).
I agree it's kinda sad how few desktop monitors are high dpi. It gets even worse if you limit yourself to low latency monitors.
Anyway I haven't used macos in a while so I'm not sure what you mean by Apple not supporting non-hidpi
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That is correct, the article goes into details why. See the "Apple's Secret Extension" section as well as the "Total Store Ordering" section. The "Apple's Secret Extension" section talks about how the M1 has 4 flag bits and the x86 has 6 flag bits, and how emulating those 2 extra flags would make every add/sub/cmp instruction significantly slower. Apple has an undocumented extension that adds 2 more flag bits to make…
I’m aware of both of these extensions; they’re not actually necessary for most applications. Yes, you trade fidelity with performance, but it’s not that big of a deal. The majority of Rosetta’s performance is good software decisions and not hardware.
"Apple's Secret Extension" isn't even used by Rosetta 2 on Linux (opting for, at least, explicit parity flag calculations rather than reduced fidelity). It's still fast.
TSO is only required for accuracy on multithreaded applications, and the PF and AF flags are basically never used, and, if they are, will usually be used immediately after being set, allowing emulators to achieve reasonable fidelity by only calculating them when used.
There's perhaps a better argument for performance-via-vertical-integration with the flag-manipulation extensions, which I believe Apple created and standardised, but which now anyone can use.
But the reason I wrote this post is that I think most of the ideas are transferable and could help other emulators :)
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The problem with JIT is not all information known at runtime is the correct information to optimize one. In finance the performance critical code path is often the one run least often. That is you have a if(unlikely_condition) {run_time_sensitive_trade();}. In this case you need to tell the compiler to ensure the CPU will have a pipeline stall because of a branch misprediction most of the time to ensure the time that…
How does the compiler arrange for the CPU to mispredict the branch most of the time? I didn't think there were any knobs for the branch predictor other than static ones (e.g. backwards-jumps statically predicted as taken, or PowerPC branch hint bit).
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I eventually changed my opinion into JIT being the only way to make dynamic languages faster, while strong typed ones can benefit from having both AOT/JIT for different kinds of deployment scenarios, and development workflows.
I think I landed in a place where it's basically "the compiler has insufficient information to achieve ideal optimization because some things can only be known at runtime." Which is not exclusively an argument for runtime JIT— it can also be an argument for instrumenting your runtime environment, and feeding that profiling data back to the compiler to help it make smarter decisions the next time. But that's definitel…
This is where the profile guided optimisation comes in – for statically compiled languages, with a caveat being not always straightforward to come up with a set of inputs that will trigger an execution of all possible code paths. One solution is to provide the coverage specifically for the performance critical code paths and let the rest just be.
I remember years ago when Java adjacent research was all the rage, HP had a problem that was “Rosetta lite” if you will. They had a need to run old binaries on new hardware that wasn’t exactly backward compatible. They made a transpiler that worked on binaries. It might have even been a JIT but that part of the memory is fuzzy. What made it interesting here was that as a sanity check they made an A->A mode where they…
Could it be simply because many binaries were produced by much older, outdated optimizers. Or optimized for size. Also, optimizers usually target “most common denominator” so native binaries rarely use full power of current instruction set. Jumping from that peculiar finding to praising runtime JIT feels like a longshot. To me it’s more of an argument towards distributing software in intermediate form (like Apple Bit…
Or distribute it in source form and make compilation part of the install process. Aka, the Gentoo model.
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The problem with JIT is not all information known at runtime is the correct information to optimize one. In finance the performance critical code path is often the one run least often. That is you have a if(unlikely_condition) {run_time_sensitive_trade();}. In this case you need to tell the compiler to ensure the CPU will have a pipeline stall because of a branch misprediction most of the time to ensure the time that…
How does the compiler arrange for the CPU to mispredict the branch most of the time? I didn't think there were any knobs for the branch predictor other than static ones (e.g. backwards-jumps statically predicted as taken, or PowerPC branch hint bit).
http://eel.is/c++draft/dcl.attr.likelihood
Prior to this it was also possible, but required compiler-specific macros like GCC's __builtin_expect, which is used all over the kernel:
One thing that’s interesting to note is that the amount of effort expended here is not actually all that large. Yes, there are smart people working on this, but the performance of Rosetta 2 for the most part is probably the work of a handful of clever people. I wouldn’t be surprised if some of them have an interest in compilers but the actual implementation is fairly straightforward and there isn’t much of the stuff…
Yeah, agreed. I get the impression it's a small team. But there is a long-tail of weird x86 features that are implemented, that give them amazing compatibility, that I regret not mentioning: * 32-bit support for Wine * full x87 emulation * full SSE2 support (generally converting to efficient NEON equivalents) for performance on SIMD code I consider all of these "compatibility", but that last one in particular should…