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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.

4:28 PM · Jun 2, 2021Twitter Web App

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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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