From 0c3d08e9676a7defd16b88307838f7294d28c3e5 Mon Sep 17 00:00:00 2001 From: Alex Crichton Date: Sun, 28 Oct 2018 08:26:47 -0700 Subject: std: Improve codegen size of accessing TLS Some code in the TLS implementation in libstd stores `Some(val)` into an `&mut Option` (effectively) and then pulls out `&T`, but it currently uses `.unwrap()` which can codegen into a panic even though it can never panic. With sufficient optimizations enabled (like LTO) the compiler can see through this but this commit helps it along in normal mode (`--release` with Cargo by default) to avoid codegen'ing the panic path. This ends up improving the optimized codegen on wasm by ensuring that a call to panic pulling in more file size doesn't stick around. --- src/libstd/thread/local.rs | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) (limited to 'src/libstd/thread') diff --git a/src/libstd/thread/local.rs b/src/libstd/thread/local.rs index 59f100fad1b..ccbead7cc2f 100644 --- a/src/libstd/thread/local.rs +++ b/src/libstd/thread/local.rs @@ -14,6 +14,7 @@ use cell::UnsafeCell; use fmt; +use hint; use mem; /// A thread local storage key which owns its contents. @@ -275,7 +276,15 @@ impl LocalKey { // operations a little differently and make this safe to call. mem::replace(&mut *ptr, Some(value)); - (*ptr).as_ref().unwrap() + // After storing `Some` we want to get a reference to the contents of + // what we just stored. While we could use `unwrap` here and it should + // always work it empirically doesn't seem to always get optimized away, + // which means that using something like `try_with` can pull in + // panicking code and cause a large size bloat. + match *ptr { + Some(ref x) => x, + None => hint::unreachable_unchecked(), + } } /// Acquires a reference to the value in this TLS key. -- cgit 1.4.1-3-g733a5