JavaScript on the GPU

If you don’t get this ref...shame on you

Jarred Nicholls
@jarrednicholls
jarred@webkit.org

Work @ Sencha
Web Platform Team

Doing webkitty things...

Co-Author
W3C Web Cryptography
API

What I’ll blabber about today
Why JavaScript on the GPU
Running JavaScript on the GPU

What’s to come...

Why JavaScript on the GPU?

Better question:
Why a GPU?

Why JavaScript on the GPU?

Better question:
Why a GPU?

A: They’re fast!
(well, at certain things...)

GPUs are fast b/c...
Totally different paradigm from CPUs
Data parallelism vs. Task parallelism
Stream processing vs. Sequential processing
GPUs can divide-and-conquer
Hardware capable of a large number of “threads”
e.g. ATI Radeon HD 6770m:
480 stream processing units == 480 cores
Typically very high memory bandwidth
Many, many GigaFLOPs

GPUs don’t solve all problems
Not all tasks can be accelerated by GPUs
Tasks must be parallelizable, i.e.:
Side effect free
Homogeneous and/or streamable
Overall tasks will become limited by Amdahl’s Law

Experiment
Code Name “LateralJS”

LateralJS

Our Mission
To make JavaScript a ﬁrst-class citizen on all GPUs
and take advantage of hardware accelerated
operations & data parallelization.

Our Options
OpenCL Nvidia CUDA
AMD, Nvidia, Intel, etc. Nvidia only
A shitty version of C99 C++ (C for CUDA)
No dynamic memory Dynamic memory
No recursion Recursion
No function pointers Function pointers
Terrible tooling Great dev. tooling
Immature (arguably) More mature (arguably)

Why not a Static Compiler?
We want full JavaScript support
Object / prototype
Closures
Recursion
Functions as objects
Variable typing
Type Inference limitations
Reasonably limited to size and complexity of “kernel-
esque” functions
Not nearly insane enough

Why an Interpreter?
We want it all baby - full JavaScript support!
Most insane approach
Challenging to make it good, but holds a lot of promise

Oh the agony...
Multiple memory spaces - pointer hell
No recursion - all inlined functions
No standard libc libraries
No dynamic memory
No standard data structures - apart from vector ops
Buggy ass AMD/Nvidia compilers

Multiple Memory Spaces
In the order of fastest to slowest:
space description
very fast
private stream processor cache (~64KB)
scoped to a single work item
fast
local ~= L1 cache on CPUs (~64KB)
scoped to a single work group
slow, by orders of magnitude
global ~= system memory over slow bus
constant available to all work groups/items
all the VRAM on the card (MBs)

Memory Space Pointer Hell
global uchar* gptr = 0x1000;
local uchar* lptr = (local uchar*) gptr; // FAIL!
uchar* pptr = (uchar*) gptr; // FAIL! private is implicit

0x1000

global local private

0x1000 points to something different
depending on the address space!

Memory Space Pointer Hell
Pointers must always be fully qualiﬁed
Macros to help ease the pain

#define GPTR(TYPE) global TYPE*
#define CPTR(TYPE) constant TYPE*
#define LPTR(TYPE) local TYPE*
#define PPTR(TYPE) private TYPE*

No Recursion!?!?!?
No call stack
All functions are inlined to the kernel function

uint factorial(uint n) {
if (n <= 1)
return 1;
else
return n * factorial(n - 1); // compile-time error
}

memcpy?
strcpy?
strcmp?
etc...

Implement our own
#define MEMCPY(NAME, DEST_AS, SRC_AS)
DEST_AS void* NAME(DEST_AS void*, SRC_AS const void*, uint);
DEST_AS void* NAME(DEST_AS void* dest, SRC_AS const void* src, uint size) {
DEST_AS uchar* cDest = (DEST_AS uchar*)dest;
SRC_AS const uchar* cSrc = (SRC_AS const uchar*)src;
for (uint i = 0; i < size; i++)
cDest[i] = cSrc[i];
return (DEST_AS void*)cDest;
}
PTR_MACRO_DEST_SRC(MEMCPY, memcpy)

Produces
memcpy_g memcpy_gc memcpy_lc memcpy_pc
memcpy_l memcpy_gl memcpy_lg memcpy_pg
memcpy_p memcpy_gp memcpy_lp memcpy_pl

No dynamic memory
No malloc()
No free()
What to do...

Yes! dynamic memory
Create a large buffer of global memory - our “heap”
Implement our own malloc() and free()
Create a handle structure - “virtual memory”
P(T, hnd) macro to get the current pointer address

GPTR(handle) hnd = malloc(sizeof(uint));
GPTR(uint) ptr = P(uint, hnd);
*ptr = 0xdeadbeef;
free(hnd);

Ok, we get the point...
FYL!

High-level Architecture
V8 Data Heap

Esprima Parser Stack-based
Interpreter

Host
Host
Host GPUs
Data Serializer &
Marshaller Garbage Collector

Device Mgr

eval(code);
V8 Data Heap
Build JSON AST

Interpreter

Host
Host
Host GPUs
Data Serializer &

Device Mgr

eval(code);
V8 Data Heap
Build JSON AST

Interpreter
Serialize AST
Host
Host
Host JSON => C Structs GPUs
Data Serializer &

Device Mgr

eval(code);
V8 Data Heap
Build JSON AST

Interpreter
Serialize AST
Host
Host
Data Serializer &
Ship to GPU to Interpret

Device Mgr

eval(code);
V8 Data Heap
Build JSON AST

Interpreter
Serialize AST
Host
Host
Data Serializer &
Ship to GPU to Interpret

Device Mgr
Fetch Result

AST Generation

JSON AST
JavaScript Source
(v8::Object)

Lateral AST
Esprima in V8
(C structs)

Embed esprima.js

Resource Generator

$ resgen esprima.js resgen_esprima_js.c

Embed esprima.js

resgen_esprima_js.c
const unsigned char resgen_esprima_js[] = {
0x2f, 0x2a, 0x0a, 0x20, 0x20, 0x43, 0x6f, 0x70, 0x79, 0x72,
0x69, 0x67, 0x68, 0x74, 0x20, 0x28, 0x43, 0x29, 0x20, 0x32,
...
0x20, 0x3a, 0x20, 0x2a, 0x2f, 0x0a, 0x0a, 0
};

Embed esprima.js
ASTGenerator.cpp
extern const char resgen_esprima_js;

void ASTGenerator::init()
{
HandleScope scope;
s_context = Context::New();
s_context->Enter();
Handle<Script> script = Script::Compile(String::New(&resgen_esprima_js));
script->Run();
s_context->Exit();
s_initialized = true;
}

Build JSON AST

e.g.
ASTGenerator::esprimaParse(
"var xyz = new Array(10);"
);

Build JSON AST
Handle<Object> ASTGenerator::esprimaParse(const char* javascript)
{
if (!s_initialized)
init();

HandleScope scope;
s_context->Enter();
Handle<Object> global = s_context->Global();
Handle<Object> esprima = Handle<Object>::Cast(global->Get(String::New("esprima")));
Handle<Function> esprimaParse = Handle<Function>::Cast(esprima-
>Get(String::New("parse")));
Handle<String> code = String::New(javascript);
Handle<Object> ast = Handle<Object>::Cast(esprimaParse->Call(esprima, 1,
(Handle<Value>*)&code));

s_context->Exit();
return scope.Close(ast);
}

Build JSON AST
{
"type": "VariableDeclaration",
"declarations": [
{
"type": "VariableDeclarator",
"id": {
"type": "Identifier",
"name": "xyz"
},
"init": {
"type": "NewExpression",
"callee": {
"name": "Array"
},
"arguments": [
{
"type": "Literal",
"value": 10
}
]
}
}
],
"kind": "var"
}

Lateral AST structs
typedef struct ast_type_st { #ifdef __OPENCL_VERSION__
CL(uint) id; #define CL(TYPE) TYPE
CL(uint) size; #else
} ast_type; #define CL(TYPE) cl_##TYPE
#endif
typedef struct ast_program_st {
ast_type type;
CL(uint) body;
CL(uint) numBody;
Structs shared between
} ast_program; Host and OpenCL

typedef struct ast_identifier_st {
ast_type type;
CL(uint) name;
} ast_identifier;

Lateral AST structs

v8::Object => ast_type
expanded
ast_type* vd1_1_init_id = (ast_type*)astCreateIdentifier("Array");
ast_type* vd1_1_init_args[1];
vd1_1_init_args[0] = (ast_type*)astCreateNumberLiteral(10);
ast_type* vd1_1_init = (ast_type*)astCreateNewExpression(vd1_1_init_id, vd1_1_init_args, 1);
free(vd1_1_init_id);
for (int i = 0; i < 1; i++)
free(vd1_1_init_args[i]);
ast_type* vd1_1_id = (ast_type*)astCreateIdentifier("xyz");
ast_type* vd1_decls[1];
vd1_decls[0] = (ast_type*)astCreateVariableDeclarator(vd1_1_id, vd1_1_init);
free(vd1_1_id);
free(vd1_1_init);
ast_type* vd1 = (ast_type*)astCreateVariableDeclaration(vd1_decls, 1, "var");
for (int i = 0; i < 1; i++)
free(vd1_decls[i]);

Lateral AST structs
astCreateIdentiﬁer
ast_identifier* astCreateIdentifier(const char* str) {
CL(uint) size = sizeof(ast_identifier) + rnd(strlen(str) + 1, 4);
ast_identifier* ast_id = (ast_identifier*)malloc(size);

// copy the string
strcpy((char*)(ast_id + 1), str);

// fill the struct
ast_id->type.id = AST_IDENTIFIER;
ast_id->type.size = size;
ast_id->name = sizeof(ast_identifier); // offset

return ast_id;
}

Lateral AST structs
astCreateIdentiﬁer(“xyz”)
offset ﬁeld value
0 type.id AST_IDENTIFIER (0x01)
4 type.size 16
8 name 12 (offset)
12 str[0] ‘x’
13 str[1] ‘y’
14 str[2] ‘z’
15 str[3] ‘0’

Lateral AST structs
astCreateNewExpression
ast_expression_new* astCreateNewExpression(ast_type* callee, ast_type** arguments, int numArgs) {
CL(uint) size = sizeof(ast_expression_new) + callee->size;
for (int i = 0; i < numArgs; i++)
size += arguments[i]->size;

ast_expression_new* ast_new = (ast_expression_new*)malloc(size);
ast_new->type.id = AST_NEW_EXPR;
ast_new->type.size = size;

CL(uint) offset = sizeof(ast_expression_new);
char* dest = (char*)ast_new;

// copy callee
memcpy(dest + offset, callee, callee->size);
ast_new->callee = offset;
offset += callee->size;

// copy arguments
if (numArgs) {
ast_new->arguments = offset;
for (int i = 0; i < numArgs; i++) {
ast_type* arg = arguments[i];
memcpy(dest + offset, arg, arg->size);
offset += arg->size;
}
} else
ast_new->arguments = 0;
ast_new->numArguments = numArgs;

return ast_new;
}

Lateral AST structs
new Array(10)
offset ﬁeld value
0 type.id AST_NEW_EXPR (0x308)
4 type.size 52
8 callee 20 (offset)
12 arguments 40 (offset)
16 numArguments 1
20 callee node ast_identiﬁer (“Array”)
arguments
40 ast_literal_number (10)
node

Lateral AST structs
Shared across the Host and the OpenCL runtime
Host writes, Lateral reads
Constructed on Host as contiguous blobs
Easy to send to GPU: memcpy(gpu, ast, ast->size);
Fast to send to GPU, single buffer write
Simple to traverse w/ pointer arithmetic

Building Blocks
JS Type Structs

AST Traverse Stack Lateral State

Call/Exec Stack Heap Symbol/Ref Table

Return Stack Scope Stack

AST Traverse Loop Interpret Loop

Kernels
#include "state.h"
#include "jsvm/asttraverse.h"
#include "jsvm/interpreter.h"

// Setup VM structures
kernel void lateral_init(GPTR(uchar) lateral_heap) {
LATERAL_STATE_INIT
}

// Interpret the AST
kernel void lateral(GPTR(uchar) lateral_heap, GPTR(ast_type) lateral_ast) {
LATERAL_STATE

ast_push(lateral_ast);
while (!Q_EMPTY(lateral_state->ast_stack, ast_q) || !Q_EMPTY(lateral_state->call_stack,
call_q)) {
while (!Q_EMPTY(lateral_state->ast_stack, ast_q))
traverse();
if (!Q_EMPTY(lateral_state->call_stack, call_q))
interpret();
}
}

Let’s interpret...

var x = 1 + 2;

var x = 1 + 2;
{
"type": "VariableDeclaration", AST Call Return
"declarations": [
{
"type": "VariableDeclarator",
"id": {
"name": "x"
},
"init": {
"type": "BinaryExpression",
"operator": "+",
"left": {
"type": "Literal",
"value": 1
},
"right": {
"type": "Literal",
"value": 2
}
}
}
],
"kind": "var"
}

var x = 1 + 2;
{
"declarations": [
{
"type": "VariableDeclarator", VarDecl
"id": {
"name": "x"
},
"init": {
"operator": "+",
"left": {
"type": "Literal",
"value": 1
},
"right": {
"type": "Literal",
"value": 2
}
}
}
],
"kind": "var"
}

var x = 1 + 2;
{
"declarations": [
{
"type": "VariableDeclarator", VarDtor
"id": {
"name": "x"
},
"init": {
"operator": "+",
"left": {
"type": "Literal",
"value": 1
},
"right": {
"type": "Literal",
"value": 2
}
}
}
],
"kind": "var"
}

var x = 1 + 2;
{
"declarations": [
{
"type": "VariableDeclarator", Ident VarDtor
"id": {
"type": "Identiﬁer", Binary
"name": "x"
},
"init": {
"operator": "+",
"left": {
"type": "Literal",
"value": 1
},
"right": {
"type": "Literal",
"value": 2
}
}
}
],
"kind": "var"
}

var x = 1 + 2;
{
"declarations": [
{
"id": {
"type": "Identiﬁer", Literal Binary
},
"name": "x"
Literal
"init": {
"operator": "+",
"left": {
"type": "Literal",
"value": 1
},
"right": {
"type": "Literal",
"value": 2
}
}
}
],
"kind": "var"
}

var x = 1 + 2;
{
"declarations": [
{
"id": {
},
"name": "x"
Literal
"init": {
Literal
"operator": "+",
"left": {
"type": "Literal",
"value": 1
},
"right": {
"type": "Literal",
"value": 2
}
}
}
],
"kind": "var"
}

var x = 1 + 2;
{
"declarations": [
{
"type": "VariableDeclarator", VarDtor
"id": {
},
"name": "x"
Literal
"init": {
Literal
"operator": "+",
"left": {
Ident
"type": "Literal",
"value": 1
},
"right": {
"type": "Literal",
"value": 2
}
}
}
],
"kind": "var"
}

var x = 1 + 2;
{
"declarations": [
{
"type": "VariableDeclarator", VarDtor “x”
"id": {
},
"name": "x"
Literal
"init": {
Literal
"operator": "+",
"left": {
"type": "Literal",
"value": 1
},
"right": {
"type": "Literal",
"value": 2
}
}
}
],
"kind": "var"
}

var x = 1 + 2;
{
"declarations": [
{
"id": {
"type": "Identiﬁer", Binary 1
},
"name": "x"
Literal
"init": {
"operator": "+",
"left": {
"type": "Literal",
"value": 1
},
"right": {
"type": "Literal",
"value": 2
}
}
}
],
"kind": "var"
}

var x = 1 + 2;
{
"declarations": [
{
"id": {
"type": "Identiﬁer", Binary 1
},
"name": "x"
2
"init": {
"operator": "+",
"left": {
"type": "Literal",
"value": 1
},
"right": {
"type": "Literal",
"value": 2
}
}
}
],
"kind": "var"
}

var x = 1 + 2;
{
"declarations": [
{
"id": {
"type": "Identiﬁer", 3
"name": "x"
},
"init": {
"operator": "+",
"left": {
"type": "Literal",
"value": 1
},
"right": {
"type": "Literal",
"value": 2
}
}
}
],
"kind": "var"
}

Benchmark

Small loop of FLOPs
var input = new Array(10);
for (var i = 0; i < input.length; i++) {
input[i] = Math.pow((i + 1) / 1.23, 3);
}

Execution Time
Lateral
GPU CL CPU CL V8
ATI Radeon 6770m Intel Core i7 4x2.4Ghz Intel Core i7 4x2.4Ghz

116.571533ms 0.226007ms 0.090664ms

What went wrong?
Everything
Stack-based AST Interpreter, no optimizations
Heavy global memory access, no optimizations
No data or task parallelism

Stack-based Interpreter
Slow as molasses
Memory hog Eclipse style
Heavy memory access
“var x = 1 + 2;” == 30 stack hits alone!
Too much dynamic allocation
No inline optimizations, just following the yellow brick AST
Straight up lazy

Replace with something better!
Bytecode compiler on Host
Bytecode register-based interpreter on Device

Too much global access
Everything is dynamically allocated to global memory
Register based interpreter & bytecode compiler can
make better use of local and private memory
// 11.1207 seconds
size_t tid = get_global_id(0);
c[tid] = a[tid];
while(b[tid] > 0) { // touch global memory on each loop
b[tid]--; // touch global memory on each loop
c[tid]++; // touch global memory on each loop Optimizing memory access
}

// 0.0445558 seconds!! HOLY SHIT!
yields crazy results
size_t tid = get_global_id(0);
int tmp = a[tid]; // temp private variable
for(int i=b[tid]; i > 0; i--) tmp++; // touch private variables on each loop
c[tid] = tmp; // touch global memory one time

No data or task parallelism
Everything being interpreted in a single “thread”
We have hundreds of cores available to us!
Build in heuristics
Identify side-effect free statements
Break into parallel tasks - very magical

input[0] = Math.pow((0 + 1) / 1.23, 3);
var input = new Array(10);
for (var i = 0; i < input.length; i++) { input[1] = Math.pow((1 + 1) / 1.23, 3);

}
input[i] = Math.pow((i + 1) / 1.23, 3);
...
input[9] = Math.pow((9 + 1) / 1.23, 3);

What’s in store
Acceptable performance on all CL devices
V8/Node extension to launch Lateral tasks
High-level API to perform map-reduce, etc.
Lateral-cluster...mmmmm

Thanks!

Jarred Nicholls
@jarrednicholls
jarred@webkit.org

JavaScript on the GPU

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