Emscripten optimizations are not used when building with the --release option

[davidMcneil]

Emscripten optimizations are not used when building a project with:

cargo build --release --target asmjs-unknown-emscripten

The Emscripten optimization level can be forced by placing the following in a .cargo/config

[target.asmjs-unknown-emscripten]
rustflags = [
    "-Clink-args=-O3 --memory-init-file 0",
]

For example when building the following code...

use std::time::Instant;

fn main() {
    let start_time = Instant::now();
    let duration = start_time.elapsed();
    println!("Runtime {:.5} ms",
             (duration.as_secs() as f64) * 1000.0 +
             (duration.subsec_nanos() as f64) / 1_000_000.0);
}

and then running it using node target/asmjs-unknown-emscripten/release/example.js with and without the optimizations forced there is a noticeable difference between the two runtimes.

Meta

rustc 1.15.0 (10893a9 2017-01-19)
binary: rustc
commit-hash: 10893a9
commit-date: 2017-01-19
host: x86_64-unknown-linux-gnu
release: 1.15.0
LLVM version: 3.9

[RReverser]

Looks like duplicate of #36899 which is implemented in nightly

[alexcrichton]

Yay!