LTO=thin appears broken on nightly rustc?

[allight]

When building https://github.com/alacritty/alacritty with cargo build -r on nightly rust the resulting binary will segfault. This does not occur when built with stable rust.

This behavior change appears to be gated by lto="thin".

Code

I tried this code:

rustup default nightly
rustup update
git clone https://github.com/alacritty/alacritty
cd alacritty
cargo build -r
./target/release/alacritty

I expected to see this happen: alacritty to startup as occurs with rustup default stable

Instead, this happened: compiled program segfaults

Version it worked on

stable-x86_64-unknown-linux-gnu unchanged - rustc 1.68.2 (9eb3afe 2023-03-27)

Version with regression

nightly-x86_64-unknown-linux-gnu unchanged - rustc 1.70.0-nightly (9df3a39 2023-04-11)

rustc --version --verbose:

rustc 1.70.0-nightly (9df3a39fb 2023-04-11)
binary: rustc
commit-hash: 9df3a39fb30575d808e70800f9fad5362aac57a2
commit-date: 2023-04-11
host: x86_64-unknown-linux-gnu
release: 1.70.0-nightly
LLVM version: 16.0.2

This appears to be triggered by lto being set to "thin" in Cargo.toml

See alacritty issues: alacritty/alacritty#6818 and alacritty/alacritty#6854 for investigation.

[allight]

NB This also appears to require that optimization is turned on to reproduce.

[matthiaskrgr]

Hm fun, I can reproduce this with cargo build --release in alacritty git on x86

[matthiaskrgr]

I tried to bisect this

# 04.11:  bad
# 04.08 : bad

# 04.01: bad 
# 03.27: bad  
# 03.26: bad
# 03.25: good
# 03.20: good 

and t looks like this regressed in nightly-2023-03-26, so probably llvm upgrade? rip :(

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high

[dianqk]

Upstream candidate patch: D148456.
Here is a reproduction example.

fn foo(_b: &[u8]) {}

fn main() {
    let b = Vec::<u8>::new();
    let b = std::borrow::Cow::Borrowed(&b);
    foo(&b);
}

rustc +nightly ./main.rs -C opt-level=1 -C panic=abort -C codegen-units=1
./main
# [1]    619598 illegal hardware instruction (core dumped)  ./main

If you change from &Vec<u8> to &[u8] this will work fine.

diff --git a/alacritty/src/renderer/text/atlas.rs b/alacritty/src/renderer/text/atlas.rs
index 9d4cb401..f106c8be 100644
--- a/alacritty/src/renderer/text/atlas.rs
+++ b/alacritty/src/renderer/text/atlas.rs
@@ -157,6 +157,7 @@ impl Atlas {
             // Load data into OpenGL.
             let (format, buffer) = match &glyph.buffer {
                 BitmapBuffer::Rgb(buffer) => {
+                    let buffer: &[u8] = buffer;
                     multicolor = false;
                     // Gles context doesn't allow uploading RGB data into RGBA texture, so need
                     // explicit copy.
@@ -175,6 +176,7 @@ impl Atlas {
                 },
                 BitmapBuffer::Rgba(buffer) => {
                     multicolor = true;
+                    let buffer: &[u8] = buffer;
                     (gl::RGBA, Cow::Borrowed(buffer))
                 },
             };