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V8 by example: A journey through the compilation pipeline by Ujjwas Sharma at FrontCon 2019

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V8 is complicated. Things change way too fast and it’s really hard to keep track of what’s the fastest way of doing every specific action. But not anymore. Join me, a V8 contributor, on a journey through the compilation pipeline of V8 and understand how it all works under the hood. We’ll take the example of a popular JavaScript builtin method and find that what does and does not trigger de-optimization. By the end of the talk, you will have a fairly decent idea of how builtins are written inside the V8 compilation pipeline, and how to make sure you always take the fast path, no matter what.

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@ryzokuken V8 by Example A journey through the compilation pipeline 1
@ryzokuken Ujjwal Sharma (@ryzokuken) ● Node.js – Core Collaborator ● Electron.js – Maintainer ● Contribute to the JS/Node.js ecosystem ○ V8 ○ TC39 ○ libuv ○ … ● Student ● Google Summer of Code ● Speaker 2
@ryzokuken@ryzokuken What is V8? 3
@ryzokuken@ryzokuken4
@ryzokuken@ryzokuken How does V8 work? 5
@ryzokuken@ryzokuken6 Source
@ryzokuken@ryzokuken7 Source Parser
@ryzokuken@ryzokuken8 Source Parser
@ryzokuken@ryzokuken9 Source IIFE?
@ryzokuken@ryzokuken10 Source IIFE? Eager
@ryzokuken@ryzokuken11 Source IIFE? Lazy Eager
@ryzokuken@ryzokuken12 Source IIFE? * Eventually Eager Lazy
@ryzokuken@ryzokuken13 Source IIFE? Eager Lazy AST + Scopes
@ryzokuken@ryzokuken14 Source Parser AST + Scopes
@ryzokuken@ryzokuken15 Source Parser AST + Scopes Ignition
@ryzokuken@ryzokuken16 Source Parser AST + Scopes Ignition Bytecode
@ryzokuken@ryzokuken17 Source Parser AST + Scopes Ignition Bytecode
@ryzokuken@ryzokuken18 But interpreters are so slow! Idea: Let’s not be slow. Let’s use a JIT compiler.
@ryzokuken@ryzokuken19
@ryzokuken@ryzokuken20 Source Parser AST + Scopes Ignition Bytecode Turbofan + profiling data OPTIMIZE!
@ryzokuken@ryzokuken21 Source Parser AST + Scopes Ignition Bytecode Turbofan Optimized Code DEOPTIMIZE!
@ryzokuken@ryzokuken22 Source Parser AST + Scopes Ignition Bytecode Turbofan Optimized Code
@ryzokuken@ryzokuken How you feel about everything. Let’s take a couple steps back.23
@ryzokuken@ryzokuken How does V8 work? 24
@ryzokuken@ryzokuken How does V8 work? a compiler 25
@ryzokuken@ryzokuken Source Code Parser Abstract Syntax Tree Assembler Machine Code (R) Compiler Assembly Program Linker Machine Code (T) 26
@ryzokuken@ryzokuken Is it simple enough yet? 27
@ryzokuken@ryzokuken Source Code Parser Abstract Syntax Tree Assembler Machine Code (R) Compiler Assembly Program Linker Machine Code (T) 28
@ryzokuken@ryzokuken Source Code Parser Abstract Syntax Tree Compiler Machine Code (T) 29
@ryzokuken@ryzokuken30 BAD NEWS: V8 isn’t this simple
@ryzokuken@ryzokuken Because ECMAScript is slow by default, that’s why. 31
@ryzokuken@ryzokuken Idea: Let’s not be slow. BUT HOW? Speculative Optimization 32
@ryzokuken@ryzokuken But wait… What even is Speculative Optimization? 33
@ryzokuken@ryzokuken Speculative Optimization [spek-yuh-ley-tiv, -luh-tiv][op-tuh-muh-zey-shuh n] Noun. Guessing what is about to happen based on what has happened. 34
@ryzokuken@ryzokuken Speculative Optimization [spek-yuh-ley-tiv, -luh-tiv][op-tuh-muh-zey-shuh n] Noun. Guessing what is about to happen based on what has happened. Making assumptions about possible future inputs based on previous inputs. 35
@ryzokuken@ryzokuken Top three reasons why a JavaScript function might need optimization: 1. Dynamic Types 2. Dynamic Types 3. Dynamic Types Alright, but how does it help? 36
@ryzokuken@ryzokuken Source Code Parser Abstract Syntax Tree Compiler Machine Code (T) 37
@ryzokuken@ryzokuken Source Code Parser Abstract Syntax Tree 1. Baseline Interpreter 2. Optimizing Compiler Machine Code (T) 38
@ryzokuken@ryzokuken AST Baseline Interpreter Optimizing Compiler Bytecode Optimized Code optimize deoptimize 39
@ryzokuken@ryzokuken Compiler Baseline Interpreter Optimizing Compiler 40
@ryzokuken@ryzokuken AST Baseline Interpreter Optimizing Compiler Bytecode Optimized Code optimize deoptimize 41
@ryzokuken@ryzokuken AST Bytecode Optimized Code optimize deoptimize 42
@ryzokuken@ryzokuken43 Okay, that’s great. But how does it even work? Let’s consider an example.
@ryzokuken@ryzokuken44 function add(x, y) { return x + y; } console.log(add(1, 2));
@ryzokuken@ryzokuken45 Not too intuitive? I’ve got you covered, fam. Let’s see how the Abstract Syntax Tree actually looks.
@ryzokuken@ryzokuken46 [generating bytecode for function: add] --- AST --- FUNC at 12 . KIND 0 . SUSPEND COUNT 0 . NAME "add" . PARAMS . . VAR (...) (mode = VAR) "x" . . VAR (...) (mode = VAR) "y" . RETURN at 22 . . ADD at 31 . . . VAR PROXY parameter[0] (...) (mode = VAR) "x" . . . VAR PROXY parameter[1] (...) (mode = VAR) "y"
@ryzokuken@ryzokuken47 [generating bytecode for function: add] --- AST --- FUNC at 12 . KIND 0 . SUSPEND COUNT 0 . NAME "add" . PARAMS . . VAR (...) (mode = VAR) "x" . . VAR (...) (mode = VAR) "y" . RETURN at 22 . . ADD at 31 . . . VAR PROXY parameter[0] (...) (mode = VAR) "x" . . . VAR PROXY parameter[1] (...) (mode = VAR) "y" FUNC
@ryzokuken@ryzokuken48 [generating bytecode for function: add] --- AST --- FUNC at 12 . KIND 0 . SUSPEND COUNT 0 . NAME "add" . PARAMS . . VAR (...) (mode = VAR) "x" . . VAR (...) (mode = VAR) "y" . RETURN at 22 . . ADD at 31 . . . VAR PROXY parameter[0] (...) (mode = VAR) "x" . . . VAR PROXY parameter[1] (...) (mode = VAR) "y" FUNC
@ryzokuken@ryzokuken49 [generating bytecode for function: add] --- AST --- FUNC at 12 . KIND 0 . SUSPEND COUNT 0 . NAME "add" . PARAMS . . VAR (...) (mode = VAR) "x" . . VAR (...) (mode = VAR) "y" . RETURN at 22 . . ADD at 31 . . . VAR PROXY parameter[0] (...) (mode = VAR) "x" . . . VAR PROXY parameter[1] (...) (mode = VAR) "y" FUNC NAME “add”
@ryzokuken@ryzokuken50 [generating bytecode for function: add] --- AST --- FUNC at 12 . KIND 0 . SUSPEND COUNT 0 . NAME "add" . PARAMS . . VAR (...) (mode = VAR) "x" . . VAR (...) (mode = VAR) "y" . RETURN at 22 . . ADD at 31 . . . VAR PROXY parameter[0] (...) (mode = VAR) "x" . . . VAR PROXY parameter[1] (...) (mode = VAR) "y" FUNC NAME “add” PARAMS
@ryzokuken@ryzokuken51 [generating bytecode for function: add] --- AST --- FUNC at 12 . KIND 0 . SUSPEND COUNT 0 . NAME "add" . PARAMS . . VAR (...) (mode = VAR) "x" . . VAR (...) (mode = VAR) "y" . RETURN at 22 . . ADD at 31 . . . VAR PROXY parameter[0] (...) (mode = VAR) "x" . . . VAR PROXY parameter[1] (...) (mode = VAR) "y" FUNC NAME “add” PARAMS VAR “x”
@ryzokuken@ryzokuken52 [generating bytecode for function: add] --- AST --- FUNC at 12 . KIND 0 . SUSPEND COUNT 0 . NAME "add" . PARAMS . . VAR (...) (mode = VAR) "x" . . VAR (...) (mode = VAR) "y" . RETURN at 22 . . ADD at 31 . . . VAR PROXY parameter[0] (...) (mode = VAR) "x" . . . VAR PROXY parameter[1] (...) (mode = VAR) "y" FUNC NAME “add” PARAMS VAR “x” VAR “y”
@ryzokuken@ryzokuken53 [generating bytecode for function: add] --- AST --- FUNC at 12 . KIND 0 . SUSPEND COUNT 0 . NAME "add" . PARAMS . . VAR (...) (mode = VAR) "x" . . VAR (...) (mode = VAR) "y" . RETURN at 22 . . ADD at 31 . . . VAR PROXY parameter[0] (...) (mode = VAR) "x" . . . VAR PROXY parameter[1] (...) (mode = VAR) "y" FUNC NAME “add” PARAMS RETURN VAR “x” VAR “y”
@ryzokuken@ryzokuken54 [generating bytecode for function: add] --- AST --- FUNC at 12 . KIND 0 . SUSPEND COUNT 0 . NAME "add" . PARAMS . . VAR (...) (mode = VAR) "x" . . VAR (...) (mode = VAR) "y" . RETURN at 22 . . ADD at 31 . . . VAR PROXY parameter[0] (...) (mode = VAR) "x" . . . VAR PROXY parameter[1] (...) (mode = VAR) "y" FUNC NAME “add” PARAMS RETURN VAR “x” VAR “y” ADD
@ryzokuken@ryzokuken55 [generating bytecode for function: add] --- AST --- FUNC at 12 . KIND 0 . SUSPEND COUNT 0 . NAME "add" . PARAMS . . VAR (...) (mode = VAR) "x" . . VAR (...) (mode = VAR) "y" . RETURN at 22 . . ADD at 31 . . . VAR PROXY parameter[0] (...) (mode = VAR) "x" . . . VAR PROXY parameter[1] (...) (mode = VAR) "y" FUNC NAME “add” PARAMS RETURN VAR “x” VAR “y” ADD PROXY “x”
@ryzokuken@ryzokuken56 [generating bytecode for function: add] --- AST --- FUNC at 12 . KIND 0 . SUSPEND COUNT 0 . NAME "add" . PARAMS . . VAR (...) (mode = VAR) "x" . . VAR (...) (mode = VAR) "y" . RETURN at 22 . . ADD at 31 . . . VAR PROXY parameter[0] (...) (mode = VAR) "x" . . . VAR PROXY parameter[1] (...) (mode = VAR) "y" FUNC NAME “add” PARAMS RETURN VAR “x” VAR “y” ADD PROXY “x” PROXY “y”
@ryzokuken@ryzokuken57 Scope Resolution FUNC NAME “add” PARAMS RETURN VAR “x” VAR “y” ADD PROXY “y”PROXY “x”
@ryzokuken@ryzokuken58 Let’s step through the bytecode. Hope you like Assembly. Once this is done, we can generate bytecode.
@ryzokuken@ryzokuken59 [generated bytecode for function: add] Parameter count 3 Frame size 0 12 E> 0x38d5f59df42 @ 0 : a5 StackCheck 22 S> 0x38d5f59df43 @ 1 : 25 02 Ldar a1 31 E> 0x38d5f59df45 @ 3 : 34 03 00 Add a0, [0] 35 S> 0x38d5f59df48 @ 6 : a9 Return Constant pool (size = 0) Handler Table (size = 0)
@ryzokuken@ryzokuken60 [generated bytecode for function: add] Parameter count 3 Frame size 0 12 E> 0x38d5f59df42 @ 0 : a5 StackCheck 22 S> 0x38d5f59df43 @ 1 : 25 02 Ldar a1 31 E> 0x38d5f59df45 @ 3 : 34 03 00 Add a0, [0] 35 S> 0x38d5f59df48 @ 6 : a9 Return Constant pool (size = 0) Handler Table (size = 0)
@ryzokuken@ryzokuken61 [generated bytecode for function: add] Parameter count 3 Frame size 0 12 E> 0x38d5f59df42 @ 0 : a5 StackCheck 22 S> 0x38d5f59df43 @ 1 : 25 02 Ldar a1 31 E> 0x38d5f59df45 @ 3 : 34 03 00 Add a0, [0] 35 S> 0x38d5f59df48 @ 6 : a9 Return Constant pool (size = 0) Handler Table (size = 0)
@ryzokuken@ryzokuken62 [generated bytecode for function: add] Parameter count 3 Frame size 0 12 E> 0x38d5f59df42 @ 0 : a5 StackCheck 22 S> 0x38d5f59df43 @ 1 : 25 02 Ldar a1 31 E> 0x38d5f59df45 @ 3 : 34 03 00 Add a0, [0] 35 S> 0x38d5f59df48 @ 6 : a9 Return Constant pool (size = 0) Handler Table (size = 0)
@ryzokuken@ryzokuken63 StackCheck Ldar a1 Add a0, [0] Return
@ryzokuken@ryzokuken64 StackCheck Ldar a1 Add a0, [0] Return
@ryzokuken@ryzokuken65 StackCheck Ldar a1 Add a0, [0] Return
@ryzokuken@ryzokuken66 StackCheck Ldar a1 Add a0, [0] Return
@ryzokuken@ryzokuken67 Two important things happen here. 1. Addition: It’s complicated. 2. Profiling: Feedback Vectors help.
@ryzokuken@ryzokuken68 StackCheck Ldar a1 Add a0, [0] Return
@ryzokuken@ryzokuken69 What about that Speculative Optimization thingie he was talking about earlier? Now that we finally have the baseline code running, let’s put that profiling data to good use. But wait...
@ryzokuken@ryzokuken70 Oh, also, remember that time when I said addition in JavaScript was complicated?
@ryzokuken@ryzokuken71
@ryzokuken@ryzokuken72 Let me break it down for you.
@ryzokuken@ryzokuken73 Number String Object
@ryzokuken@ryzokuken74
@ryzokuken@ryzokuken75 Remember Feedback Vectors? Let’s see how they really look to see how it all works. Awesome! But how do we make that assumption?
@ryzokuken@ryzokuken76 function add(x, y) { return x + y; } console.log(add(1, 2));
@ryzokuken@ryzokuken77 function add(x, y) { return x + y; } console.log(add(1, 2)); %DebugPrint(add);
@ryzokuken@ryzokuken78 ... - feedback vector: 0x18bc7711df89: [FeedbackVector] in OldSpace - map: 0x18bc38c00bc1 <Map> - length: 1 - shared function info: 0x18bc7711dc59 <SharedFunctionInfo add> - optimized code/marker: OptimizationMarker::kNone - invocation count: 1 - profiler ticks: 0 - slot #0 BinaryOp BinaryOp:SignedSmall { [0]: 1 } ...
@ryzokuken@ryzokuken79 function add(x, y) { return x + y; } console.log(add(1, 2)); %DebugPrint(add);
@ryzokuken@ryzokuken80 function add(x, y) { return x + y; } console.log(add(1, 2)); console.log(add(1.1, 2.2)); %DebugPrint(add);
@ryzokuken@ryzokuken81 ... - feedback vector: 0xcbd92d1dfe1: [FeedbackVector] in OldSpace - map: 0x0cbd4f880bc1 <Map> - length: 1 - shared function info: 0x0cbd92d1dc59 <SharedFunctionInfo add> - optimized code/marker: OptimizationMarker::kNone - invocation count: 2 - profiler ticks: 0 - slot #0 BinaryOp BinaryOp:Number { [0]: 7 } ...
@ryzokuken@ryzokuken82
@ryzokuken@ryzokuken83 Question: Is there a method to all this? Answer: Yes, it is called the Feedback Lattice.
@ryzokuken@ryzokuken84 None SignedSmall Number BigIntString NumberOrOddball Any
@ryzokuken@ryzokuken85 None SignedSmall Number BigIntString NumberOrOddball Any
@ryzokuken@ryzokuken86 None SignedSmall Number BigIntString NumberOrOddball Any
@ryzokuken@ryzokuken87 None SignedSmall Number BigIntString NumberOrOddball Any
@ryzokuken@ryzokuken88 None SignedSmall Number BigIntString NumberOrOddball Any
@ryzokuken@ryzokuken89 None SignedSmall Number BigIntString NumberOrOddball Any
@ryzokuken@ryzokuken90 None SignedSmall Number BigIntString NumberOrOddball Any
@ryzokuken@ryzokuken91 None SignedSmall Number BigIntString NumberOrOddball Any
@ryzokuken@ryzokuken92 This data in the Feedback Vectors is used to finally optimize your code once a function is hot. When is a function “hot”? Let’s see what actually happens by explicitly triggering optimization.
@ryzokuken@ryzokuken93 function add(x, y) { return x + y; } add(1, 2);
@ryzokuken@ryzokuken94
@ryzokuken@ryzokuken95 function add(x, y) { return x + y; } add(1, 2); // Warm up with SignedSmall feedback. %OptimizeFunctionOnNextCall(add); add(1, 2); // Optimize and run generated code.
@ryzokuken@ryzokuken96 Question: What did we just do?
@ryzokuken@ryzokuken97 Let’s see the optimized code generated by passing the -- print-opt-code flag to d8.
@ryzokuken@ryzokuken98 leaq rbx,[rip+0xfffffff9] movq rbx,[rcx-0x20] testb [rbx+0xf],0x1 jz 0x198104882dfb <+0x3b> movq r10,0x10efbfde0 (CompileLazyDeoptimizedCode) jmp r10 push rbp movq rbp,rsp push rsi push rdi cmpq rsp,[r13+0x11e8] (root (stack_limit)) jna 0x198104882e55 <+0x95> movq rdx,[rbp+0x18] testb rdx,0x1 jnz 0x198104882e7b <+0xbb> movq rcx,[rbp+0x10] testb rcx,0x1 jnz 0x198104882e87 <+0xc7> movq rdi,rcx shrq rdi, 32 movq r8,rdx shrq r8, 32 addl rdi,r8 jo 0x198104882e93 <+0xd3> shlq rdi, 32 movq rax,rdi movq rsp,rbp pop rbp ret 0x18
@ryzokuken@ryzokuken99 leaq rbx,[rip+0xfffffff9] movq rbx,[rcx-0x20] testb [rbx+0xf],0x1 jz 0x198104882dfb <+0x3b> movq r10,0x10efbfde0 (CompileLazyDeoptimizedCode) jmp r10 push rbp movq rbp,rsp push rsi push rdi cmpq rsp,[r13+0x11e8] (root (stack_limit)) jna 0x198104882e55 <+0x95> movq rdx,[rbp+0x18] testb rdx,0x1 jnz 0x198104882e7b <+0xbb> movq rcx,[rbp+0x10] testb rcx,0x1 jnz 0x198104882e87 <+0xc7> movq rdi,rcx shrq rdi, 32 movq r8,rdx shrq r8, 32 addl rdi,r8 jo 0x198104882e93 <+0xd3> shlq rdi, 32 movq rax,rdi movq rsp,rbp pop rbp ret 0x18 Prologue
@ryzokuken@ryzokuken100 leaq rbx,[rip+0xfffffff9] movq rbx,[rcx-0x20] testb [rbx+0xf],0x1 jz 0x198104882dfb <+0x3b> movq r10,0x10efbfde0 (CompileLazyDeoptimizedCode) jmp r10 push rbp movq rbp,rsp push rsi push rdi cmpq rsp,[r13+0x11e8] (root (stack_limit)) jna 0x198104882e55 <+0x95> movq rdx,[rbp+0x18] testb rdx,0x1 jnz 0x198104882e7b <+0xbb> movq rcx,[rbp+0x10] testb rcx,0x1 jnz 0x198104882e87 <+0xc7> movq rdi,rcx shrq rdi, 32 movq r8,rdx shrq r8, 32 addl rdi,r8 jo 0x198104882e93 <+0xd3> shlq rdi, 32 movq rax,rdi movq rsp,rbp pop rbp ret 0x18 Check x is Smi
@ryzokuken@ryzokuken101 leaq rbx,[rip+0xfffffff9] movq rbx,[rcx-0x20] testb [rbx+0xf],0x1 jz 0x198104882dfb <+0x3b> movq r10,0x10efbfde0 (CompileLazyDeoptimizedCode) jmp r10 push rbp movq rbp,rsp push rsi push rdi cmpq rsp,[r13+0x11e8] (root (stack_limit)) jna 0x198104882e55 <+0x95> movq rdx,[rbp+0x18] testb rdx,0x1 jnz 0x198104882e7b <+0xbb> movq rcx,[rbp+0x10] testb rcx,0x1 jnz 0x198104882e87 <+0xc7> movq rdi,rcx shrq rdi, 32 movq r8,rdx shrq r8, 32 addl rdi,r8 jo 0x198104882e93 <+0xd3> shlq rdi, 32 movq rax,rdi movq rsp,rbp pop rbp ret 0x18 Check y is Smi
@ryzokuken@ryzokuken102 leaq rbx,[rip+0xfffffff9] movq rbx,[rcx-0x20] testb [rbx+0xf],0x1 jz 0x198104882dfb <+0x3b> movq r10,0x10efbfde0 (CompileLazyDeoptimizedCode) jmp r10 push rbp movq rbp,rsp push rsi push rdi cmpq rsp,[r13+0x11e8] (root (stack_limit)) jna 0x198104882e55 <+0x95> movq rdx,[rbp+0x18] testb rdx,0x1 jnz 0x198104882e7b <+0xbb> movq rcx,[rbp+0x10] testb rcx,0x1 jnz 0x198104882e87 <+0xc7> movq rdi,rcx shrq rdi, 32 movq r8,rdx shrq r8, 32 addl rdi,r8 jo 0x198104882e93 <+0xd3> shlq rdi, 32 movq rax,rdi movq rsp,rbp pop rbp ret 0x18 Smi → Word32 (x)
@ryzokuken@ryzokuken103 leaq rbx,[rip+0xfffffff9] movq rbx,[rcx-0x20] testb [rbx+0xf],0x1 jz 0x198104882dfb <+0x3b> movq r10,0x10efbfde0 (CompileLazyDeoptimizedCode) jmp r10 push rbp movq rbp,rsp push rsi push rdi cmpq rsp,[r13+0x11e8] (root (stack_limit)) jna 0x198104882e55 <+0x95> movq rdx,[rbp+0x18] testb rdx,0x1 jnz 0x198104882e7b <+0xbb> movq rcx,[rbp+0x10] testb rcx,0x1 jnz 0x198104882e87 <+0xc7> movq rdi,rcx shrq rdi, 32 movq r8,rdx shrq r8, 32 addl rdi,r8 jo 0x198104882e93 <+0xd3> shlq rdi, 32 movq rax,rdi movq rsp,rbp pop rbp ret 0x18 Smi → Word32 (y)
@ryzokuken@ryzokuken104 leaq rbx,[rip+0xfffffff9] movq rbx,[rcx-0x20] testb [rbx+0xf],0x1 jz 0x198104882dfb <+0x3b> movq r10,0x10efbfde0 (CompileLazyDeoptimizedCode) jmp r10 push rbp movq rbp,rsp push rsi push rdi cmpq rsp,[r13+0x11e8] (root (stack_limit)) jna 0x198104882e55 <+0x95> movq rdx,[rbp+0x18] testb rdx,0x1 jnz 0x198104882e7b <+0xbb> movq rcx,[rbp+0x10] testb rcx,0x1 jnz 0x198104882e87 <+0xc7> movq rdi,rcx shrq rdi, 32 movq r8,rdx shrq r8, 32 addl rdi,r8 jo 0x198104882e93 <+0xd3> shlq rdi, 32 movq rax,rdi movq rsp,rbp pop rbp ret 0x18 Add x and y Overflow Check
@ryzokuken@ryzokuken105 leaq rbx,[rip+0xfffffff9] movq rbx,[rcx-0x20] testb [rbx+0xf],0x1 jz 0x198104882dfb <+0x3b> movq r10,0x10efbfde0 (CompileLazyDeoptimizedCode) jmp r10 push rbp movq rbp,rsp push rsi push rdi cmpq rsp,[r13+0x11e8] (root (stack_limit)) jna 0x198104882e55 <+0x95> movq rdx,[rbp+0x18] testb rdx,0x1 jnz 0x198104882e7b <+0xbb> movq rcx,[rbp+0x10] testb rcx,0x1 jnz 0x198104882e87 <+0xc7> movq rdi,rcx shrq rdi, 32 movq r8,rdx shrq r8, 32 addl rdi,r8 jo 0x198104882e93 <+0xd3> shlq rdi, 32 movq rax,rdi movq rsp,rbp pop rbp ret 0x18 Result to Smi
@ryzokuken@ryzokuken106 leaq rbx,[rip+0xfffffff9] movq rbx,[rcx-0x20] testb [rbx+0xf],0x1 jz 0x198104882dfb <+0x3b> movq r10,0x10efbfde0 (CompileLazyDeoptimizedCode) jmp r10 push rbp movq rbp,rsp push rsi push rdi cmpq rsp,[r13+0x11e8] (root (stack_limit)) jna 0x198104882e55 <+0x95> movq rdx,[rbp+0x18] testb rdx,0x1 jnz 0x198104882e7b <+0xbb> movq rcx,[rbp+0x10] testb rcx,0x1 jnz 0x198104882e87 <+0xc7> movq rdi,rcx shrq rdi, 32 movq r8,rdx shrq r8, 32 addl rdi,r8 jo 0x198104882e93 <+0xd3> shlq rdi, 32 movq rax,rdi movq rsp,rbp pop rbp ret 0x18 Epilogue
@ryzokuken@ryzokuken107 Now, let’s try something fun.
@ryzokuken@ryzokuken108 function add(x, y) { return x + y; } add(1, 2); // Warm up with SignedSmall feedback. %OptimizeFunctionOnNextCall(add); add(1, 2); // Optimize and run generated code.
@ryzokuken@ryzokuken109 function add(x, y) { return x + y; } add(1, 2); // Warm up with SignedSmall feedback. %OptimizeFunctionOnNextCall(add); add(1, 2); // Optimize and run generated code. add(1.1, 2.2) // DEOPTIMIZE!
@ryzokuken@ryzokuken110
@ryzokuken@ryzokuken111 Note: I’m obviously kidding. Deoptimization is no laughing matter.
@ryzokuken@ryzokuken112 Let’s see how the code is actually deoptimized by passing the --trace-deopt flag to d8.
@ryzokuken@ryzokuken113 [deoptimizing (DEOPT eager): begin 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> (opt #0) @0, FP to SP delta: 24, caller sp: 0x7ffee7bee248] ;;; deoptimize at <add.js:2:12>, not a Smi reading input frame add => bytecode_offset=0, args=3, height=1, retval=0(#0); inputs: 0: 0x08d97929dbc1 ; [fp - 16] 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> 1: 0x08d9c6b81521 ; [fp + 32] 0x08d9c6b81521 <JSGlobal Object> 2: 0x08d97929da31 ; rdx 0x08d97929da31 <HeapNumber 1.1> 3: 0x08d97929da41 ; [fp + 16] 0x08d97929da41 <HeapNumber 2.2> 4: 0x08d979281749 ; [fp - 24] 0x08d979281749 <NativeContext[247]> 5: 0x08d92b080e19 ; (literal 2) 0x08d92b080e19 <Odd Oddball: optimized_out> translating interpreted frame add => bytecode_offset=0, height=8 0x7ffee7bee240: [top + 72] <- 0x08d9c6b81521 <JSGlobal Object> ; stack parameter (input #1) 0x7ffee7bee238: [top + 64] <- 0x08d97929da31 <HeapNumber 1.1> ; stack parameter (input #2) 0x7ffee7bee230: [top + 56] <- 0x08d97929da41 <HeapNumber 2.2> ; stack parameter (input #3) ------------------------- 0x7ffee7bee228: [top + 48] <- 0x00010d2881e8 ; caller's pc 0x7ffee7bee220: [top + 40] <- 0x7ffee7bee290 ; caller's fp 0x7ffee7bee218: [top + 32] <- 0x08d979281749 <NativeContext[247]> ; context (input #4) 0x7ffee7bee210: [top + 24] <- 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> ; function (input #0) 0x7ffee7bee208: [top + 16] <- 0x08d97929dca1 <BytecodeArray[7]> ; bytecode array 0x7ffee7bee200: [top + 8] <- 0x003900000000 <Smi 57> ; bytecode offset ------------------------- 0x7ffee7bee1f8: [top + 0] <- 0x08d92b080e19 <Odd Oddball: optimized_out> ; accumulator (input #5) [deoptimizing (eager): end 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> @0 => node=0, pc=0x00010d2886c0, caller sp=0x7ffee7bee248, took 1.163 ms]
@ryzokuken@ryzokuken Don’t worry, I got you.114
@ryzokuken@ryzokuken115 [deoptimizing (DEOPT eager): begin 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> (opt #0) @0, FP to SP delta: 24, caller sp: 0x7ffee7bee248] ;;; deoptimize at <add.js:2:12>, not a Smi reading input frame add => bytecode_offset=0, args=3, height=1, retval=0(#0); inputs: 0: 0x08d97929dbc1 ; [fp - 16] 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> 1: 0x08d9c6b81521 ; [fp + 32] 0x08d9c6b81521 <JSGlobal Object> 2: 0x08d97929da31 ; rdx 0x08d97929da31 <HeapNumber 1.1> 3: 0x08d97929da41 ; [fp + 16] 0x08d97929da41 <HeapNumber 2.2> 4: 0x08d979281749 ; [fp - 24] 0x08d979281749 <NativeContext[247]> 5: 0x08d92b080e19 ; (literal 2) 0x08d92b080e19 <Odd Oddball: optimized_out> translating interpreted frame add => bytecode_offset=0, height=8 0x7ffee7bee240: [top + 72] <- 0x08d9c6b81521 <JSGlobal Object> ; stack parameter (input #1) 0x7ffee7bee238: [top + 64] <- 0x08d97929da31 <HeapNumber 1.1> ; stack parameter (input #2) 0x7ffee7bee230: [top + 56] <- 0x08d97929da41 <HeapNumber 2.2> ; stack parameter (input #3) ------------------------- 0x7ffee7bee228: [top + 48] <- 0x00010d2881e8 ; caller's pc 0x7ffee7bee220: [top + 40] <- 0x7ffee7bee290 ; caller's fp 0x7ffee7bee218: [top + 32] <- 0x08d979281749 <NativeContext[247]> ; context (input #4) 0x7ffee7bee210: [top + 24] <- 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> ; function (input #0) 0x7ffee7bee208: [top + 16] <- 0x08d97929dca1 <BytecodeArray[7]> ; bytecode array 0x7ffee7bee200: [top + 8] <- 0x003900000000 <Smi 57> ; bytecode offset ------------------------- 0x7ffee7bee1f8: [top + 0] <- 0x08d92b080e19 <Odd Oddball: optimized_out> ; accumulator (input #5) [deoptimizing (eager): end 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> @0 => node=0, pc=0x00010d2886c0, caller sp=0x7ffee7bee248, took 1.163 ms]
@ryzokuken@ryzokuken116 [deoptimizing (DEOPT eager): begin 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> (opt #0) @0, FP to SP delta: 24, caller sp: 0x7ffee7bee248] ;;; deoptimize at <add.js:2:12>, not a Smi reading input frame add => bytecode_offset=0, args=3, height=1, retval=0(#0); inputs: 0: 0x08d97929dbc1 ; [fp - 16] 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> 1: 0x08d9c6b81521 ; [fp + 32] 0x08d9c6b81521 <JSGlobal Object> 2: 0x08d97929da31 ; rdx 0x08d97929da31 <HeapNumber 1.1> 3: 0x08d97929da41 ; [fp + 16] 0x08d97929da41 <HeapNumber 2.2> 4: 0x08d979281749 ; [fp - 24] 0x08d979281749 <NativeContext[247]> 5: 0x08d92b080e19 ; (literal 2) 0x08d92b080e19 <Odd Oddball: optimized_out> translating interpreted frame add => bytecode_offset=0, height=8 0x7ffee7bee240: [top + 72] <- 0x08d9c6b81521 <JSGlobal Object> ; stack parameter (input #1) 0x7ffee7bee238: [top + 64] <- 0x08d97929da31 <HeapNumber 1.1> ; stack parameter (input #2) 0x7ffee7bee230: [top + 56] <- 0x08d97929da41 <HeapNumber 2.2> ; stack parameter (input #3) ------------------------- 0x7ffee7bee228: [top + 48] <- 0x00010d2881e8 ; caller's pc 0x7ffee7bee220: [top + 40] <- 0x7ffee7bee290 ; caller's fp 0x7ffee7bee218: [top + 32] <- 0x08d979281749 <NativeContext[247]> ; context (input #4) 0x7ffee7bee210: [top + 24] <- 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> ; function (input #0) 0x7ffee7bee208: [top + 16] <- 0x08d97929dca1 <BytecodeArray[7]> ; bytecode array 0x7ffee7bee200: [top + 8] <- 0x003900000000 <Smi 57> ; bytecode offset ------------------------- 0x7ffee7bee1f8: [top + 0] <- 0x08d92b080e19 <Odd Oddball: optimized_out> ; accumulator (input #5) [deoptimizing (eager): end 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> @0 => node=0, pc=0x00010d2886c0, caller sp=0x7ffee7bee248, took 1.163 ms]
@ryzokuken@ryzokuken117 [deoptimizing (DEOPT eager): begin 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> (opt #0) @0, FP to SP delta: 24, caller sp: 0x7ffee7bee248] ;;; deoptimize at <add.js:2:12>, not a Smi reading input frame add => bytecode_offset=0, args=3, height=1, retval=0(#0); inputs: 0: 0x08d97929dbc1 ; [fp - 16] 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> 1: 0x08d9c6b81521 ; [fp + 32] 0x08d9c6b81521 <JSGlobal Object> 2: 0x08d97929da31 ; rdx 0x08d97929da31 <HeapNumber 1.1> 3: 0x08d97929da41 ; [fp + 16] 0x08d97929da41 <HeapNumber 2.2> 4: 0x08d979281749 ; [fp - 24] 0x08d979281749 <NativeContext[247]> 5: 0x08d92b080e19 ; (literal 2) 0x08d92b080e19 <Odd Oddball: optimized_out> translating interpreted frame add => bytecode_offset=0, height=8 0x7ffee7bee240: [top + 72] <- 0x08d9c6b81521 <JSGlobal Object> ; stack parameter (input #1) 0x7ffee7bee238: [top + 64] <- 0x08d97929da31 <HeapNumber 1.1> ; stack parameter (input #2) 0x7ffee7bee230: [top + 56] <- 0x08d97929da41 <HeapNumber 2.2> ; stack parameter (input #3) ------------------------- 0x7ffee7bee228: [top + 48] <- 0x00010d2881e8 ; caller's pc 0x7ffee7bee220: [top + 40] <- 0x7ffee7bee290 ; caller's fp 0x7ffee7bee218: [top + 32] <- 0x08d979281749 <NativeContext[247]> ; context (input #4) 0x7ffee7bee210: [top + 24] <- 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> ; function (input #0) 0x7ffee7bee208: [top + 16] <- 0x08d97929dca1 <BytecodeArray[7]> ; bytecode array 0x7ffee7bee200: [top + 8] <- 0x003900000000 <Smi 57> ; bytecode offset ------------------------- 0x7ffee7bee1f8: [top + 0] <- 0x08d92b080e19 <Odd Oddball: optimized_out> ; accumulator (input #5) [deoptimizing (eager): end 0x08d97929dbc1 <JSFunction add (sfi = 0x8d97929d999)> @0 => node=0, pc=0x00010d2886c0, caller sp=0x7ffee7bee248, took 1.163 ms]
@ryzokuken@ryzokuken118 Pro Tip: If you don’t want your code to be slow, try to stick to certain types. It makes things easier.
@ryzokuken Special Thanks ● Benedikt Meurer (@bmeurer) ● Yang Guo (@hashseed) ● Sathya Gunasekaran (@_gsathya) ● Jakob Gruber (@schuay) ● Sigurd Schneider (@sigurdschn) ● ...and everyone else from the V8 team. ● The organizers. 119
@ryzokuken@ryzokuken Paldies! 120

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Notes de l'éditeur

  1. V8 is Google’s open source high-performance JavaScript and WebAssembly engine, written in C++. It is used in Chormium, the open source browser from Google and it’s derivatives, and in Node.js, among others.
  2. Here we explicitly warm up the x+y site with SignedSmall feedback by passing in two integer values whose sum also fits into the small integer range. Then we tell V8 that it should optimize the function add (with TurboFan) when it’s called the next time, and eventually we call add, which triggers TurboFan and then runs the generated machine code.
  3. The prologue checks whether the code object is still valid or whether some condition changed which requires us to throw away the code object. Once we know that the code is still valid, we build the stack frame and check that there’s enough space left on the stack to execute the code.
  4. Then we start with the body of the function. We load the values of the parameters x and y from the stack (relative to the frame pointer in rbp) and check if both values have Smi representation (since feedback for + says that both inputs have always been Smi so far). This is done by testing the least significant bit. Once we know that they are both represented as Smi, we need to convert them to 32-bit representation, which is done by shifting the value by 32 bit to the right. If either x or y is not a Smi the execution of the optimized code aborts immediately and the deoptimizer restores the state of the function in the interpreter right before the Add.
  5. Then we go on to perform the integer addition on the inputs. We need to test explicitly for overflow, since the result of the addition might be outside the range of 32-bit integers, in which case we’d need to go back to the interpreter, which will then learn Number feedback on the Add. Finally we convert the result back to Smi representation by shifting the signed 32-bit value up by 32 bit, and then we return the value in the accumulator register rax.
  6. As said before, this is not yet the perfect code for this case, since here it would be beneficial to just perform the addition on Smi representation directly, instead of going to Word32, which would save us three shift instructions. But even putting aside this minor aspect, you can see that the generated code is highly optimized and specialized to the profiling feedback. It doesn’t even try to deal with other numbers, strings, big ints or arbitrary JavaScript objects here, but focuses only on the kind of values we’ve seen so far. This is the key ingredient to peak performance for many JavaScript applications.