[webkit-dev] Replacing the JSC classic C++ interpreter with a llint generated C++ interpreter

[Mark Lam]

As part of an effort to simplify future development, the JSC team is deprecating the classic C++ interpreter (and will delete it shortly on Sept 24).  In its place, we will use the LLINT (low level interpreter) with the C++ back-end (see https://bugs.webkit.org/show_bug.cgi?id=91052) to generate a llint C++ interpreter for new ports that do not support the JITs yet.

In order to deprecate the classic C++ interpreter, we will need your help to convert your ports to use the llint (if you are using JSC for your port).  I will lay out some details below on how this conversion works:


Why deprecate the classic C++ interpreter?
=================================
The llint is where active development will take place as we add new JIT and runtime enhancements.  Having the classic C++ interpreter around only slows down the development effort.  The classic C++ interpreter is also prone to experience bit-rot, and hence will easily get buggy over time.


How will you bring up a new platform without the classic C++ interpreter?
========================================================
You will have to add the llint offline ASM build phase to generate the llint C++ interpreter (see below for details on the llint offline build phase).  Alternatively, you can generate the llint C++ interpreter on a port that already supports it, and use the generated LLIntAssembly.h from there to bootstrap your port.  It is preferred that every port supports the llint natively because this will enable you to get the latest bug fixes and performance enhancements.


What is the performance of the llint C++ interpreter?
========================================
The performance of the new llint C++ interpreter is currently slower than the classic C++ interpreter, but within approximately 10% on average.  The goal of the llint C++ interpreter is not to achieve the highest performance, but to provide a functional C++ interpreter to help bootstrap new ports.  As such, we don't plan to spend a lot of time on optimizing it.

sunspider results:
                   classic C++ interpreter       llint C++ interpreter
<arithmetic> *     18.3123+-0.1593          !    18.6551+-0.1095        ! definitely 1.0187x slower


v8-real results:
                   classic C++ interpreter       llint C++ interpreter
<geometric> *      15.31772+-0.22338        !    16.92580+-0.09735      ! definitely 1.1050x slower


How is the llint built?
================
Here's a summary of the steps:

Step 1: Generate LLIntDesiredOffsets.h

mkdir -p "${BUILT_PRODUCTS_DIR}/LLIntOffsets/"

/usr/bin/env ruby "${SRCROOT}/offlineasm/generate_offset_extractor.rb" "${SRCROOT}/llint/LowLevelInterpreter.asm" "${BUILT_PRODUCTS_DIR}/LLIntOffsets/LLIntDesiredOffsets.h"

Step 2: Build LLIntOffsetsExtractor from LLIntDesiredOffsets.h (from step 1) and Source/JavaScriptCore/llint/LLIntOffsetsExtrator.cpp using the cross-compiler for your target.

LLIntOffsetsExtractor is supposed to be an executable binary for your target port.  However, we will only be using it to extract offsets that we need for the next step, and won't be running it.

Step 3: Generate LLIntAssembly.h

/usr/bin/env ruby JavaScriptCore/offlineasm/asm.rb JavaScriptCore/llint/LowLevelInterpreter.asm ${BUILT_PRODUCTS_DIR}/JSCLLIntOffsetsExtractor LLIntAssembly.h || exit 1

LLIntAssembly.h provides the body of the interpreter, and will be #included in Source/JavaScriptCore/llint/LowLevelInterpreter.cpp to be built into JSC.


How to get JSC to build with the llint C++ interpreter?
========================================

In Platform.h (or equivalent), set the following settings:

#define ENABLE_JIT 0
#define ENABLE_LLINT 1
#define ENABLE_CLASSIC_INTERPRETER 0

This combination of settings will ENABLE(LLINT_C_LOOP) which builds the llint C++ interpreter.  Eventually, ENABLE_CLASSIC_INTERPRETER won't be needed when the classic C++ interpreter gets deprecated and deleted.


What do you need to do for your port?
=============================
The Mac port already works with the llint.  I'm planning to fix the Windows port to also work with the llint.  On Sept 24, we plan to delete the classic C++ interpreter.  This means your port will cease to build if you are still relying on the classic C++ interpreter then.

Please look into migrating your port to use the llint C++ interpreter (if not the Assembly one) within the next few weeks.  If you encounter issues and need some help getting the llint to work for your port, please feel free to contact me.  I will do my best to help you out.


Thanks.

Regards,
Mark
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