Ask questionsLTO triggers apparent miscompilation on Windows 10 x64

I tried this code:


name = "bmp-minimal"
version = "0.1.0"
authors = ["Malloc Voidstar <>"]
edition = "2018"


lto = true
codegen-units = 1

use std::io::Write;

fn main() {
    let mut out = Vec::with_capacity(51200);
    for val in 0u8..=255 {
        out.write_all(&[val, val, val, 0]).unwrap();

    for _ in (0..1).rev() {
        for _ in 0..1 {
    println!("{} bytes written", out.len());

(extremely minimized form of grayscale BMP encoding from image crate)

Command: cargo run --release

I expected to see this happen: 1025 bytes written

Instead, this happened:

memory allocation of 26843545600 bytes failederror: process didn't exit successfully: `target\release\bmp-minimal.exe` (exit code: 0xc0000409, STATUS_STACK_BUFFER_OVERRUN)


rustc --version --verbose:

rustc 1.45.0 (5c1f21c3b 2020-07-13)
binary: rustc
commit-hash: 5c1f21c3b82297671ad3ae1e8c942d2ca92e84f2
commit-date: 2020-07-13
host: x86_64-pc-windows-msvc
release: 1.45.0
LLVM version: 10.0

It appears to work fine on ARM Linux in my limited testing.

This minimized form works/doesn't work on the following versions:

Rust version Works?
1.45 (LLVM 10.0) No
1.44.1 No
1.43.1 Yes
1.42.0 No
1.41.1 No
1.40.0 No
1.39.0 No
1.38.0 (LLVM 9.0) No
1.37.0 Yes
1.36.0 Yes
1.35.0 Yes
1.34.0 Yes

If the LTO and codegen-units settings are removed it works on all above versions.

I originally noticed this when a project that worked on 1.44.1 suddenly started failing with huge memory allocations on 1.45, so this table specifically applies to this repro, not whatever the underlying bug is.


Answer questions luqmana

Another datapoint: adding -C llvm-args=-cvp-dont-add-nowrap-flags makes it work.

That flag changed from default true to false about a year ago which kinda lines up with the repro chart above (minus 1.43.1). There was an LLVM bug about that causing some miscompilations last I checked.

cc @nikic who I believe worked on that upstream.


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