ch2-05-em-asm.md

2.5 Using EM_ASM

Many compilers support embedding assembly code directly in C/C++ code. We can write "inline JavaScript" in Emscripten with a simular way by using EM_ASM and related macros.

2.5.1 EM_ASM

EM_ASM is very simple to use, look at the example bellow:

#include <emscripten.h>

int main() {
	EM_ASM(console.log('你好，Emscripten！'));
	return 0;
}

The code above will call the JavaScript function console.log() to output "你好，Emscripten！".

We can write multiple JavaScript statements in one EM_ASM(), separated by ';', for example:

    EM_ASM(var k = 42;console.log('The answer is:', k););

The code above will output:

The answer is:42

It is also feasible to write multiple lines of statements and end-of-line comments (the statements must be separated by semicolons when they are written, and cannot be omitted), for example:

	EM_ASM(
		var k = 42;  //define k
		console.log('The answer is:', k);
	);

2.5.2 EM_ASM_/EM_ASM_DOUBLE

EM_ASM_ macro can be used if parameters and return values are needed in the inline JavaScript code. The inline JavaScript code in the EM_ASM_ macro must be placed in the code block enclosed by {} (to separate the code and parameters) and must contain at least one parameter. The inline JavaScript code accesses the n+1th parameter via $n. The following example calls JavaScript to calculate the sum of the three parameters and returns the result:

	int sum = EM_ASM_({return $0 + $1 + $2;}, 1, 2, 3);
	printf("sum(1, 2, 3): %d\n", sum);

Variables can be used as parameters while using EM_ASM_, for example:

	char buf[32];
	double pi = 3.14159;
	EM_ASM_(
		{
			console.log('addr of buf:', $0);
			console.log('sqrt(pi):', $1);
		},
		buf, sqrt(pi)
    );

The above code will output the address of buf and the square root of pi.

EM_ASM_DOUBLE usage is basically the same as EM_ASM_, the difference is that EM_ASM_DOUBLE returns a value of double instead of int. For example:

	double pi2 = EM_ASM_DOUBLE(
		{
			return $0 * $1;
		},
		pi, 2.0
	);
	printf("pi2: %lf\n", pi2);

The inline JavaScript code in the EM_ASM_/EM_ASM_DOUBLE macro will be expanded into a JavaScript function, so in addition to $n, the built-in arguments object can be used to access parameters. For example:

	EM_ASM_(
		{
            console.log('arguments count:', arguments.length);
			for(var i = 0; i < arguments.length; i++) {
				console.log('$', i, ':', arguments[i]);
			}
		},
		42, 13
	);

The code above will output:

arguments count: 2
$ 0 : 42
$ 1 : 13

2.5.3 EM_ASM_INT_V/EM_ASM_DOUBLE_V

If the inline JavaScript code doesn't have parameters, you can use the EM_ASM_INT_V/EM_ASM_DOUBLE_V macro. Since there are no parameters, the inline code doesn't need to be surrounded by {}, for example:

	int answer = EM_ASM_INT_V(return 42);
	printf("The answer is:%d\n", answer);
	double pi_js = EM_ASM_DOUBLE_V(return 3.14159);
	printf("PI:%lf\n", pi_js);

The output of the examples in this section is as follows: