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Speed up passing ASCII-only strings to WASM

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Some speed up numbers from my string-heavy WASM benchmarks:
 - Firefox + encodeInto: +45%
 - Chrome + encodeInto: +80%
 - Firefox + encode: +29%
 - Chrome + encode: +62%

Note that this helps specifically with case of lots of small ASCII strings, in case of large strings there is no measurable difference in either direction.
This commit is contained in:
Ingvar Stepanyan 2019-04-17 18:14:53 +01:00
parent befefe0da6
commit 57b1a57c5e
1 changed files with 43 additions and 18 deletions
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61
crates/cli-support/src/js/mod.rs
View File

@ -1445,18 +1445,51 @@ impl<'a> Context<'a> {
self.expose_text_encoder();
self.expose_uint8_memory();
// A fast path that directly writes char codes into WASM memory as long
// as it finds only ASCII characters.
//
// This is much faster for common ASCII strings because it can avoid
// calling out into C++ TextEncoder code.
//
// This might be not very intuitive, but such calls are usually more
// expensive in mainstream engines than staying in the JS, and
// charCodeAt on ASCII strings is usually optimised to raw bytes.
let start_encoding_as_ascii = format!(
"
{}
let size = arg.length;
let ptr = wasm.__wbindgen_malloc(size);
let offset = 0;
{{
const mem = getUint8Memory();
for (; offset < arg.length; offset++) {{
const code = arg.charCodeAt(offset);
if (code > 0x7F) {{
arg = arg.slice(offset);
break;
}}
mem[ptr + offset] = code;
}}
}}
",
debug
);
// The first implementation we have for this is to use
// `TextEncoder#encode` which has been around for quite some time.
let use_encode = format!(
"
{}
const buf = cachedTextEncoder.encode(arg);
const ptr = wasm.__wbindgen_malloc(buf.length);
getUint8Memory().set(buf, ptr);
WASM_VECTOR_LEN = buf.length;
if (offset !== arg.length) {{
const buf = cachedTextEncoder.encode(arg);
ptr = wasm.__wbindgen_realloc(ptr, size, size += buf.length);
getUint8Memory().set(buf, ptr + offset);
offset += buf.length;
}}
WASM_VECTOR_LEN = offset;
return ptr;
",
debug
start_encoding_as_ascii
);
// Another possibility is to use `TextEncoder#encodeInto` which is much
@ -1465,23 +1498,15 @@ impl<'a> Context<'a> {
let use_encode_into = format!(
"
{}
let size = arg.length;
let ptr = wasm.__wbindgen_malloc(size);
let writeOffset = 0;
while (true) {{
const view = getUint8Memory().subarray(ptr + writeOffset, ptr + size);
const {{ read, written }} = cachedTextEncoder.encodeInto(arg, view);
writeOffset += written;
if (read === arg.length) {{
break;
}}
arg = arg.substring(read);
if (offset !== arg.length) {{
ptr = wasm.__wbindgen_realloc(ptr, size, size += arg.length * 3);
const view = getUint8Memory().subarray(ptr + offset, ptr + size);
offset += cachedTextEncoder.encodeInto(arg, view).written;
}}
WASM_VECTOR_LEN = writeOffset;
WASM_VECTOR_LEN = offset;
return ptr;
",
debug
start_encoding_as_ascii
);
// Looks like `encodeInto` doesn't currently work when the memory passed