Open ceautery opened 8 years ago
kripken
commented
8 years ago The problem is that wasm % can trap (on corner cases) while asm.js % cannot. But if you build with asm2wasm --imprecise it will ignore the corner cases, and do what you want here.
I am surprised about that browser error you saw, though. Do you have a testcase + full steps to reproduce?
ceautery
commented
8 years ago @kripken - I'm running 64 bit Chrome Canary 54.0.2827.0 with chrome://flags/#enable-webassembly enabled, and a local web server. To reproduce what I'm seeing...
Create test.asm.js with the following content:
function TestModule() {
"use asm";
function test(n) {
n = n | 0;
return n % 2
}
return { test: test }
}Convert to S-expr syntax, and assemble with:
binaryen/bin/asm2wasm test.asm.js -o test.wast
binaryen/bin/wasm-as test.wast -o test.wasmPlace test.wasm in web server's root directory. Create test.html with this content:
<!doctype html>
<html>
<head>
<script>
var module;
fetch("test.wasm")
.then(resp => resp.arrayBuffer())
.then(buffer => module = Wasm.instantiateModule(new Uint8Array(buffer)));
</script>
</head>
<body>
</body>
</html>Navigate to localhost(:port)/test.html, then open the JavaScript console. The complete error I'm seeing is:
Uncaught (in promise) WasmModule::Instantiate(): Import #0 module="asm2wasm" function="i32s-rem" error: FFI is not an objectRunning asm2wasm with -i, as you suggested, corrected the import error. Thanks for the reply and the suggestion!
kripken
commented
8 years ago That looks like invalid asm.js. You must coerce on a % because otherwise it can't tell if it's a signed or unsigned operation.
I didn't realize you were using handwritten asm.js here. asm2wasm is only tested on emscripten output, so it's possible you'll find bugs, although in this case, I'm not sure yet.
ceautery
commented
8 years ago Ah. I'll conjecture emscripten would have also created the $i32s-rem import... somewhere, so this likely isn't a bug, as you said.
asm2wasm handles handwritten asm.js code liberally. I've had success using much fewer type coercions than the spec would suggest you need. In general, once a variable's type has been referenced in a function, you seem pretty free to operate on it willy-nilly with variables of the same type. Here's an example, from my (amateur - I've been playing with asm.js for just a few days) attempts at writing various prime number calculators, where I only needed to include the -i flag on asm2wasm, and wasm-as didn't complain about at all, which ran fine in a browser:
function PrimeModule() {
"use asm";
var HEAP8 = new global.Int8Array(buffer);
var HEAP32 = new global.Int32Array(buffer);
var sqrt = global.Math.sqrt;
function sieve(max) {
max = max | 0;
var sieveOffset = 0, upperLimit = 0, i = 0, outputOffset = 0;
upperLimit = ~~sqrt(+max);
for (sieveOffset = 2; sieveOffset <= upperLimit; sieveOffset = sieveOffset + 1) {
if (HEAP8[sieveOffset >> 0] == 0) {
for (i = sieveOffset * sieveOffset; i <= max; i = i + sieveOffset) {
HEAP8[i >> 0] = 1;
}
}
}
outputOffset = max + ((4 - max % 4) % 4);
for (i = 2; i <= max; i = i + 1) {
if (HEAP8[i >> 0] == 0) {
HEAP32[outputOffset >> 2] = i;
outputOffset = outputOffset + 4;
}
}
return (outputOffset - max - ((4 - max % 4) % 4)) >> 2;
}
return { getPrimes: sieve };
}...and...
function PrimeModule() {
"use asm";
var HEAP32 = new global.Int32Array(buffer);
function getPrimes(max) {
max = max | 0;
var byteOffset = 0, candidate = 0, limit = 0, square = 0;
candidate = 3;
limit = 2;
square = 4;
HEAP32[byteOffset >> 2] = 2; // First prime
while (candidate < max) {
if ((isPrime(candidate, byteOffset, limit) | 0) == 1) {
byteOffset = byteOffset + 4;
HEAP32[byteOffset >> 2] = candidate;
}
candidate = candidate + 2;
if (candidate >= square) {
square = square + (limit * 2) + 1;
limit = limit + 1;
}
}
return (byteOffset >> 2) + 1;
}
function isPrime(candidate, stopByteOffset, limit) {
candidate = candidate | 0;
stopByteOffset = stopByteOffset | 0;
limit = limit | 0;
var primeDivisor = 0, byteOffset = 0;
byteOffset = 4;
while (byteOffset <= stopByteOffset) {
primeDivisor = HEAP32[byteOffset >> 2];
if (primeDivisor > limit) break;
if (candidate % primeDivisor == 0) return 0;
byteOffset = byteOffset + 4;
}
return 1;
}
return { getPrimes: getPrimes };
}Note the general lack of any coercions after variables are declared.
Anyway, thanks for your time. I didn't realize on the outset that these tools were specifically meant to be used in conjunction with emscripten... and they've turned out to be quite usable directly.
kripken
commented
8 years ago It would be great if it did work on all asm.js, just I've only had time for emscripten output ;) Fixes for non-emscripten output would be welcome of course.
The asm.js line:
...where "val" and "i" are both ints, gets translated as an external import in the header and a call to that import:
and then
This compiles fine with wasm-as, but fails in the browser with an instantiation error of
If I edit the asm2wasm output to remove the imports and swap the call_import line with:
...wasm-as compiles it fine, and the module works as expected in the browser. Can asm2wasm use the built-in S expression i32.rem_s for integer modulus operations, or is there an overriding reason to stick with the external call? If so, how do I reference the import from JavaScript?
Thanks!