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共 14451 条

Emscripten: An LLVM-to-JavaScript Compiler

Alon Zakai

Mozilla

azakai@mozilla.com

Abstract

We present Emscripten, a compiler from LLVM (Low Level

Virtual Machine) assembly to JavaScript. This opens up

two avenues for running code written in languages other

than JavaScript on the web: (1) Compile code directly into

LLVM assembly, and then compile that into JavaScript using

Emscripten, or (2) Compile a language’s entire runtime into

LLVM and then JavaScript, as in the previous approach, and

then use the compiled runtime to run code written in that

language. For example, the former approach can work for

C and C++, while the latter can work for Python; all three

examples open up new opportunities for running code on the

web.

Emscripten itself is written in JavaScript and is avail-

able under the MIT license (a permissive open source li-

cense), at http://www.emscripten.org. As a compiler

from LLVM to JavaScript, the challenges in designing Em-

scripten are somewhat the reverse of the norm – one must go

from a low-level assembly into a high-level language, and

recreate parts of the original high-level structure of the code

that were lost in the compilation to low-level LLVM. We

detail the methods used in Emscripten to deal with those

challenges, and in particular present and prove the validity

of Emscripten’s Relooper algorithm, which recreates high-

level loop structures from low-level branching data.

1. Introduction

Since the mid 1990’s, JavaScript [5] has been present in

most web browsers (sometimes with minor variations and

under slightly different names, e.g., JScript in Internet Ex-

plorer), and today it is well-supported on essentially all

web browsers, from desktop browsers like Internet Ex-

plorer, Firefox, Chrome and Safari, to mobile browsers on

[Copyright notice will appear here once ’preprint’ option is removed.]

smartphones and tablets. Together with HTML and CSS,

JavaScript forms the standards-based foundation of the web.

Running other programming languages on the web has

been suggested many times, and browser plugins have al-

lowed doing so, e.g., via the Java and Flash plugins. How-

ever, plugins must be manually installed and do not integrate

in a perfect way with the outside HTML. Perhaps more prob-

lematic is that they cannot run at all on some platforms, for

example, Java and Flash cannot run on iOS devices such as

the iPhone and iPad. For those reasons, JavaScript remains

the primary programming language of the web.

There are, however, reasonable motivations for running

code from other programming languages on the web, for ex-

ample, if one has a large amount of existing code already

written in another language, or if one simply has a strong

preference for another language and perhaps is more pro-

ductive in it. As a consequence, there has been work on

tools to compile languages into JavaScript. Since JavaScript

is present in essentially all web browsers, by compiling one’s

language of choice into JavaScript, one can still generate

content that will run practically everywhere.

Examples of the approach of compiling into JavaScript

include the Google Web Toolkit [8], which compiles Java

into JavaScript; Pyjamas

, which compiles Python into

JavaScript; SCM2JS [6], which compiles Scheme to JavaScript,

Links [3], which compiles an ML-like language into JavaScript;

and AFAX [7], which compiles F# to JavaScript; see also

[1] for additional examples. While useful, such tools usually

only allow a subset of the original language to be compiled.

For example, multithreaded code (with shared memory) is

not possible on the web, so compiling code of that sort is

not directly possible. There are also often limitations of the

conversion process, for example, Pyjamas compiles Python

to JavaScript in a nearly 1-to-1 manner, and as a conse-

quence the underlying semantics are those of JavaScript, not

Python, so for example division of integers can yield unex-

pected results (it should yield an integer in Python 2.x, but

in JavaScript and in Pyjamas a ﬂoating-point number can be

generated).

In this paper we present another project along those lines:

Emscripten, which compiles LLVM (Low Level Virtual

http://pyjs.org/

1 2013/5/14

Machine

) assembly into JavaScript. LLVM is a compiler

project primarily focused on C, C++ and Objective-C. It

compiles those languages through a frontend (the main ones

of which are Clang and LLVM-GCC) into the LLVM in-

termediary representation (which can be machine-readable

bitcode, or human-readable assembly), and then passes it

through a backend which generates actual machine code for

a particular architecture. Emscripten plays the role of a back-

end which targets JavaScript.

By using Emscripten, potentially many languages can be

run on the web, using one of the following methods:

•

Compile code in a language recognized by one of the

existing LLVM frontends into LLVM, and then compile

that into JavaScript using Emscripten. Frontends for var-

ious languages exist, including many of the most popular

programming languages such as C and C++, and also var-

ious new and emerging languages (e.g., Rust

•

Compile the runtime used to parse and execute code in a

particular language into LLVM, then compile that into

JavaScript using Emscripten. It is then possible to run

code in that runtime on the web. This is a useful approach

if a language’s runtime is written in a language for which

an LLVM frontend exists, but the language itself has no

such frontend. For example, there is currently no frontend

for Python, however it is possible to compile CPython –

the standard implementation of Python, written in C –

into JavaScript, and run Python code on that (see Sec-

tion 4).

From a technical standpoint, one challenge in design-

ing and implementing Emscripten is that it compiles a low-

level language – LLVM assembly – into a high-level one –

JavaScript. This is somewhat the reverse of the usual situa-

tion one is in when building a compiler, and leads to some

unique difﬁculties. For example, to get good performance in

JavaScript one must use natural JavaScript code ﬂow struc-

tures, like loops and ifs, but those structures do not exist in

LLVM assembly (instead, what is present there is a ‘soup of

code fragments’: blocks of code with branching information

but no high-level structure). Emscripten must therefore re-

construct a high-level representation from the low-level data

it receives.

In theory that issue could have been avoided by compiling

a higher-level language into JavaScript. For example, if com-

piling Java into JavaScript (as the Google Web Toolkit does),

then one can beneﬁt from the fact that Java’s loops, ifs and so

forth generally have a very direct parallel in JavaScript. But

of course the downside in that approach is it yields a com-

piler only for Java. In Section 3.2 we present the ‘Relooper’

algorithm, which generates high-level loop structures from

the low-level branching data present in LLVM assembly. It

is similar to loop recovery algorithms used in decompilation

http://llvm.org/

https://github.com/graydon/rust/

(see, for example, [2], [9]). The main difference between the

Relooper and standard loop recovery algorithms is that the

Relooper generates loops in a different language than that

which was compiled originally, whereas decompilers gen-

erally assume they are returning to the original language.

The Relooper’s goal is not to accurately recreate the original

source code, but rather to generate native JavaScript control

ﬂow structures, which can then be implemented efﬁciently

in modern JavaScript engines.

Another challenge in Emscripten is to maintain accuracy

(that is, to keep the results of the compiled code the same

as the original) while not sacriﬁcing performance. LLVM

assembly is an abstraction of how modern CPUs are pro-

grammed for, and its basic operations are not all directly

possible in JavaScript. For example, if in LLVM we are to

add two unsigned 8-bit numbers x and y, with overﬂowing

(e.g., 255 plus 1 should give 0), then there is no single oper-

ation in JavaScript which can do this – we cannot just write

x + y, as that would use the normal JavaScript semantics. It

is possible to emulate a CPU in JavaScript, however doing

so is very slow. Emscripten’s approach is to allow such emu-

lation, but to try to use it as little as possible, and to provide

tools that help one ﬁnd out which parts of the compiled code

actually need such full emulation.

We conclude this introduction with a list of this paper’s

main contributions:

•

We describe Emscripten itself, during which we detail its

approach in compiling LLVM into JavaScript.

•

We give details of Emscripten’s Relooper algorithm,

mentioned earlier, which generates high-level loop struc-

tures from low-level branching data, and prove its valid-

ity.

In addition, the following are the main contributions of Em-

scripten itself, that to our knowledge were not previously

possible:

•

It allows compiling a very large subset of C and C++ code

into JavaScript, which can then be run on the web.

•

By compiling their runtimes, it allows running languages

such as Python on the web (with their normal semantics).

The remainder of this paper is structured as follows. In

Section 2 we describe the approach Emscripten takes to

compiling LLVM assembly into JavaScript, and show some

benchmark data. In Section 3 we describe Emscripten’s in-

ternal design and in particular elaborate on the Relooper al-

gorithm. In Section 4 we give several example uses of Em-

scripten. In Section 5 we summarize and give directions for

future work.

2. Compilation Approach

Let us begin by considering what the challenge is, when we

want to compile LLVM assembly into JavaScript. Assume

we are given the following simple example of a C program:

2 2013/5/14

#include <stdio.h>

int main()

{

int sum = 0;

for (int i = 1; i <= 100; i++)

sum += i;

printf("1+...+100=%d\n", sum);

return 0;

}

This program calculates the sum of the integers from 1

to 100. When compiled by Clang, the generated LLVM

assembly code includes the following:

@.str = private constant [14 x i8]

c"1+...+100=%d\0A\00"

define i32 @main() {

%1 = alloca i32, align 4

%sum = alloca i32, align 4

%i = alloca i32, align 4

store i32 0, i32* %1

store i32 0, i32* %sum, align 4

store i32 1, i32* %i, align 4

br label %2

; <label>:2

%3 = load i32* %i, align 4

%4 = icmp sle i32 %3, 100

br i1 %4, label %5, label %12

; <label>:5

%6 = load i32* %i, align 4

%7 = load i32* %sum, align 4

%8 = add nsw i32 %7, %6

store i32 %8, i32* %sum, align 4

br label %9

; <label>:9

%10 = load i32* %i, align 4

%11 = add nsw i32 %10, 1

store i32 %11, i32* %i, align 4

br label %2

; <label>:12

%13 = load i32* %sum, align 4

%14 = call i32 (i8*, ...)*

@printf(i8* getelementptr inbounds

([14 x i8]* @.str, i32 0, i32 0),

i32 %13)

ret i32 0

}

At ﬁrst glance, this may look more difﬁcult to translate into

JavaScript than the original C++. However, compiling C++

in general would require writing code to handle preprocess-

ing, classes, templates, and all the idiosyncrasies and com-

plexities of C++. LLVM assembly, while more verbose in

this example, is lower-level and simpler to work on. Com-

piling it also has the beneﬁt we mentioned earlier, which is

one of the main goals of Emscripten, that it allows many

languages can be compiled into LLVM and not just C++.

A detailed overview of LLVM assembly is beyond our

scope here (see http://llvm.org/docs/LangRef.html).

Brieﬂy, though, the example assembly above can be seen to

deﬁne a function main(), then allocate some values on the

stack (alloca), then load and store various values (load and

store). We do not have the high-level code structure as we

had in C++ (with a loop), instead we have labeled code

fragments, called LLVM basic blocks, and code ﬂow moves

from one to another by branch (br) instructions. (Label 2 is

the condition check in the loop; label 5 is the body, label 9

is the increment, and label 12 is the ﬁnal part of the func-

tion, outside of the loop). Conditional branches can depend

on calculations, for example the results of comparing two

values (icmp). Other numerical operations include addition

(add). Finally, printf is called (call). The challenge, then, is

to convert this and things like it into JavaScript.

In general, Emscripten’s main approach is to translate

each line of LLVM assembly into JavaScript, 1 to 1, into

‘normal’ JavaScript as much as possible. So, for example,

an add operation becomes a normal JavaScript addition, a

function call becomes a JavaScript function call, etc. This

1 to 1 translation generates JavaScript that resembles the

original assembly code, for example, the LLVM assembly

code shown before for main() would be compiled into the

following:

function _main() {

var __stackBase__ = STACKTOP;

STACKTOP += 12;

var __label__ = -1;

while(1) switch(__label__) {

case -1:

var $1 = __stackBase__;

var $sum = __stackBase__+4;

var $i = __stackBase__+8;

HEAP[$1] = 0;

HEAP[$sum] = 0;

HEAP[$i] = 0;

__label__ = 0; break;

case 0:

var $3 = HEAP[$i];

var $4 = $3 <= 100;

if ($4) { __label__ = 1; break; }

else { __label__ = 2; break; }

case 1:

var $6 = HEAP[$i];

var $7 = HEAP[$sum];

var $8 = $7 + $6;

HEAP[$sum] = $8;

3 2013/5/14

__label__ = 3; break;

case 3:

var $10 = HEAP[$i];

var $11 = $10 + 1;

HEAP[$i] = $11;

__label__ = 0; break;

case 2:

var $13 = HEAP[$sum];

var $14 = _printf(__str, $13);

STACKTOP = __stackBase__;

return 0;

}

Some things to take notice of:

•

A switch-in-a-loop construction is used in order to let the

ﬂow of execution move between basic blocks of code in

an arbitrary manner: We set label to the (numerical

representation of the) label of the basic block we want

to reach, and do a break, which leads to the proper basic

block being reached. Inside each basic block, every line

of code corresponds to a line of LLVM assembly, gener-

ally in a very straightforward manner.

•

Memory is implemented by HEAP, a JavaScript array.

Reading from memory is a read from that array, and

writing to memory is a write. STACKTOP is the current

position of the stack. (Note that we allocate 4 memory

locations for 32-bit integers on the stack, but only write

to 1 of them. See Section 2.1.1 for why.)

•

LLVM assembly functions become JavaScript functions,

and function calls are normal JavaScript function calls. In

general, we attempt to generate as ‘normal’ JavaScript as

possible.

•

We implemented the LLVM add operation using simple

addition in JavaScript. As mentioned earlier, the seman-

tics of that code are not entirely identical to those of the

original LLVM assembly code (in this case, overﬂows

will have very different effects). We will explain Em-

scripten’s approach to that problem in Section 2.1.2.

2.1 Performance

In this section we will deal with several topics regard-

ing Emscripten’s approach to generating high-performance

JavaScript code.

2.1.1 Load-Store Consistency (LSC)

We saw before that Emscripten’s memory usage allocates the

usual number of bytes on the stack for variables (4 bytes for

a 32-bit integer, etc.). However, we only wrote values into

the ﬁrst location, which appeared odd. We will now see the

reason for that.

To get there, we must ﬁrst step back, and note that Em-

scripten does not aim to achieve perfect compatibility with

all possible LLVM assembly (and correspondingly, with all

possible C or C++ code, etc.); instead, Emscripten targets

a large subset of LLVM assembly code, which is portable

and does not make crucial assumptions about the underlying

CPU architecture on which the code is meant to run. That

subset is meant to encompass the vast majority of real-world

code that would be compiled into LLVM, while also being

compilable into very performant JavaScript.

More speciﬁcally, Emscripten assumes that the LLVM

assembly code it is compiling has Load-Store Consistency

(LSC), which is the requirement that after a value with a

speciﬁc type is written to a memory location, loads from

that memory location will be of the same type (until a value

with a different type is written there). Normal C and C++

code generally does so: If x is a variable containing a 32-bit

ﬂoating point number, then both loads and stores of x will

be of 32-bit ﬂoating point values, and not 16-bit unsigned

integers or anything else.

To see why this is important for performance, consider

the following C code fragment, which does not have LSC:

int x = 12345;

printf("first byte: %d\n", *((char*)&x));

Assuming an architecture with more than 8 bits, this code

will read the ﬁrst byte of x. (This might, for example, be

used to detect the endianness of the CPU.) To compile this

into JavaScript in a way that will run properly, we must do

more than a single operation for either the read or the write,

for example we could do this:

var x_value = 12345;

var x_addr = stackAlloc(4);

HEAP[x_addr] = (x_value >> 0) & 255;

HEAP[x_addr+1] = (x_value >> 8) & 255;

HEAP[x_addr+2] = (x_value >> 16) & 255;

HEAP[x_addr+3] = (x_value >> 24) & 255;

[...]

printf("first byte: %d\n", HEAP[x_addr]);

Here we allocate space for the value of x on the stack, and

store that address in x

addr. The stack itself is part of the

‘memory space’, which is the array HEAP. In order for the

read on the ﬁnal line to give the proper value, we must go

to the effort of doing 4 store operations, each of the value

of a particular byte. In other words, HEAP is an array of

bytes, and for each store into memory, we must deconstruct

the value into bytes.

Alternatively, we can store the value in a single operation,

and deconstruct into bytes as we load. This will be faster in

some cases and slower in others, but is still more overhead

than we would like, generally speaking – for if the code does

Note that we can use JavaScript typed arrays with a shared memory buffer,

which would work as expected, assuming (1) we are running in a JavaScript

engine which supports typed arrays, and (2) we are running on a CPU

with the same architecture as we expect. This is therefore dangerous as

the generated code may run differently on different JavaScript engines and

different CPUs. Emscripten currently has optional experimental support for

typed arrays.

4 2013/5/14

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