I wanted to try writing wasm manually (wat format).
1 Lox test code#
I decided to try manually translating my Lox test code into wasm
print "# Testing scope";
var a = 5;
print "outer a";
print a;
{
var a = 10;
print "inner a";
print a;
}
print "outer a";
print a;
print "# Testing expressions";
print true;
print 2 + 1;
print !((3 + 5 / 10) == 3.5);
print "# Testing if statements";
if (false) {
print "N true";
} else {
print "Y false";
}
if (true) {
print "Y true";
}
print "# Testing logical operators";
print "hi" or 2; // "hi"
print "hi" and 2; // 2
print nil or "yeh"; // "yeh"
print nil or nil; // nil
print "# Testing while loops";
var i = 3;
while (i > 0) {
print i;
i = i - 1;
}
print "# Testing for loops, Fibonacci sequence";
var a = 0;
var temp;
for (var b = 1; a < 10000; b = temp + b) {
print a;
temp = a;
a = b;
}
- https://developer.mozilla.org/en-US/docs/WebAssembly/Guides/Understanding_the_text_format[1] was useful
- https://stanford-cs242.github.io/f18/lectures/04-1-webassembly-practice.html[2] too
- https://webassembly.github.io/spec/core/text/instructions.html[3] has a list of all wasm instructions
- https://developer.mozilla.org/en-US/docs/WebAssembly/Reference/Control_flow[4]
Some things I don’t understand:
- why (local.get 0) vs local.get 0, and similarly (i32.add) vs i32.add?
- https://nishtahir.com/the-wasm-text-format/[5] shows parentheses and then shows without parentheses
- some pages like https://developer.mozilla.org/en-US/docs/WebAssembly/Guides/Understanding_the_text_format[6] show parentheses only sometimes — look at the
logAllMemoryfunction which uses both in the same function
other:
- wasmtime can compile wasm into x86 assembly; I tried this on godbolt[7]
- the loop construct in wasm[8] is more like do-while, so I need to turn while(a) { b } into if(a) do { b } while (a), or block Y { loop X { if (!a) break Y; b; branch X; } . After seeing the output of assemblyscript and wasm-opt, I see that it could instead be loop X { if (a) { b ; branch X; } }
Source: learning-wasm.js
2 JS vs wasm#
I’m going to convert this js code to wasm:
function main(seed, N) { const m = 0x7fff; const a = 1103515245; const c = 12345; for (let i = 0; i < N; i++) { seed = (a * seed + c) & m; } return seed; }
(func $loop (param $seed i32) (param $N i32) (result i32)
(local $i i32)
(local.set $i (i32.const 0))
(block $for_2_end
(loop $for_2_start
(local.get $i) ;; test !(i < N)
(local.get $N)
(i32.ge_s)
(br_if $for_2_end)
(local.get $seed)
(i32.const 1103515245)
(i32.mul)
(i32.const 12345)
(i32.add)
(i32.const 0x7fff)
(i32.and)
(local.set $seed) ;; seed = (a * seed + c) & m
(local.get $i)
(i32.const 1)
(i32.add)
(local.set $i) ;; i++
(br $for_2_start)
)
)
(local.get $seed)
)
I also tried AssemblyScript[9]:
export function main(seed: i32, N: i32): i32 {
const m: i32 = 0x7fff;
const a: i32 = 1103515245;
const c: i32 = 12345;
for (let i = 0; i < N; i++) {
seed = (a * seed + c) & m;
}
return seed;
}
which generated this wasm:
(func $assembly/index/main (param $seed i32) (param $N i32) (result i32)
(local $i i32)
i32.const 0
local.set $i
loop $for-loop|0
local.get $i
local.get $N
i32.lt_s
if
i32.const 1103515245
local.get $seed
i32.mul
i32.const 12345
i32.add
i32.const 32767
i32.and
local.set $seed
local.get $i
i32.const 1
i32.add
local.set $i
br $for-loop|0
end
end
local.get $seed
return
)
and that ran in the same speed as my hand-written wasm, 725ms for js and 145ms for wasm in Firefox; 660ms for js and 145ms for wasm in Chrome. My test code doens’t work in safari yet.
I also tried wasm2c and ran the c code (with optimizer on) on my machine, and it took 116ms, so wasm wasn’t much slower than native code.