Enabling LTO causes spurious rebuilds #8750
Description
Problem
With LTO set to thin or fat, doing a cargo build --target riscv32imac-unknown-none-elf followed by cargo run followed by cargo build --target riscv32imac-unknown-none-elf causes Cargo to rebuild some packages even when nothing has changed.
Setting lto="off" prevents this behavior.
Steps
- Ensure you have the
riscv32imac-unknown-none-elftoolchain - Clone the
xous-corerepository - Build an example such as
shell - Run a program
- Rebuild
shell
This series of steps should demonstrate the problem:
rustup target add riscv32imac-unknown-none-elf
git clone https://github.com/betrusted-io/xous-core.git
cd xous-core
git checkout 6cb1aad93ca908bb394b9e7c13274b9f55d5aa8b
cargo build --release --target riscv32imac-unknown-none-elf -p shell
cargo run --release -p tools --bin copy-object
cargo build --release --target riscv32imac-unknown-none-elf -p shell # This will cause a partial rebuild
cargo run --release -p tools --bin copy-object # This will also cause a partial rebuild
Notes
If you modify Cargo.toml to remove lto="thin", or change it to lto="off", then the issue goes away.
Output of cargo version: cargo 1.46.0 (149022b 2020-07-17)
This may be due to the fact that cargo run and cargo build are targeting different architectures.