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I mean, that's one way to implement a polymorphic inline cache…

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Saagar Jha

@_saagarjha

May 30, 2021

Looks like Apple’s A15 (?) chip will ship with the objc_msgSend branch predictor: opensource.apple.com/source/objc4/o

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Code snippet from runtime/objc-bp-assist.h objc4-824. Mildly truncated but you get the gist:

// Compute the control register value. For more detail on what
// the various pieces mean, see Apple ISA Extensions to ARMv8:
// version H14, chapter 23, "Software assisted Branch Prediction:
// objc_msgSend".
uint64_t enable = 0x8000000000000000ULL;
uint64_t mask = 0x0000000000000000ULL;
uint64_t classPtrRegister = 16; // Class pointer is loaded into x16
uint64_t selectorRegister = 1; // Selector is in x1.

uint64_t controlRegisterValue = enable;
controlRegisterValue |= mask;
controlRegisterValue |= classPtrRegister << ARM64_REG_BP_OBJC_CTL_EL1_AR_ClassPtr_shift;
controlRegisterValue |= selectorRegister << ARM64_REG_BP_OBJC_CTL_EL1_AR_Selector_shift;
controlRegisterValue |= indirectBranchOffset << ARM64_REG_BP_OBJC_CTL_EL1_Br_Offset_shift;

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Tony "Abolish ICE" Arcieri

@bascule

Jun 2, 2021

Replying to

@pcwalton

A decade ago, Java 7 added InvokeDynamic, a programmable API to HotSpot’s JIT which could provide PICs, method inlining, and guard-based cache invalidation at call site granularity. In 2021, Apple is putting it in hardware. I’ve heard this one before and it’s called Jazelle DBX.

Tony "Abolish ICE" Arcieri

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and

I guess this is the difference between HotSpot’s aggressively JIT-oriented strategy (where all dispatch is virtual until inlined) versus the largely static “choose your own dispatch” strategy used by AOT “JITs” like ObjC and C#

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I still think that the Android JIT approach is better than the iOS AOT approach for performance, security concerns notwithstanding, because speculative devirtualization enables so many powerful optimizations

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and

Android doesn't really do anything fancy based on JIT compilation. It has fairly sophisticated devirtualization but it works with AOT compilation. ART JIT is essentially just an in-memory variant of the AOT compiler for a limited amount of hot code that hasn't been AOT compiled.

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and

Interpreter / JIT generate profiles which are saved and used to decide what they should AOT compile in the background. Google has started distributing pre-generated profiles so AOT compilation can start working right away. Cold code, unused code, etc. doesn't end up AOT compiled.

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and

Not really much distinction between ART AOT and JIT compilation. JIT can generate non-PIC code as a tiny micro-optimization. It prefers AOT compiled code paged from storage once it's available. Main purpose of ART JIT is avoiding wasting storage for cold/unused AOT compiled code.

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and

Once you've heavily used an app for a few days, it ends up with the same end result as AOT compiling the whole thing from the beginning but without a bunch of compiled code for the massive amount of the app that's not actually used on your device and with your own usage of it.

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and

They definitely have a very smart compiler system, GC and overall runtime but the JIT is deliberately very stupid and essentially exists to obsolete itself over time with AOT compiled code. They decided that doing sophisticated JIT was a waste of memory, cache, battery life, etc.

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and

Part of what makes the GC smart is that it uses a different mode in the foreground vs. background and the OS will go out of the way to heap compact background apps. It also has integration into malloc for native code to purge malloc dirty pages (or even caches) in the background.

4:51 PM · Jun 2, 2021Twitter Web App

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and

It works a lot differently on a device with 512M memory compared to a Pixel. A low memory device relies a lot on things like GC scheduled in sync with rendering frames, etc. On a Pixel 5 with 8GB of memory, it's not like the foreground app needs to care much about wasting memory.