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Clarify RUSTFLAGS caveats #63

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Jan 24, 2025
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Clarify RUSTFLAGS caveats #63

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3 changes: 3 additions & 0 deletions README.md
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Expand Up @@ -228,6 +228,9 @@ Here's a short guide how to compile LLVM with BOLT manually. You will need a rec
The built files should be located at `<llvm-dir>/llvm-install/bin`. You should add this directory
to `$PATH` to make BOLT usable with `cargo-pgo`.

## Caveats
- `cargo-pgo` needs to set RUSTFLAGS for the crate being compiled. If you pass your own RUSTFLAGS using `config.toml` file, please make sure to use the `[target.<...>] rustflags = ...` section, instead of the `[build] rustflags = ...` section. With `target`, your flags will be combined with the PGO flags. If you use `build`, your flags will be overridden instead. See [#49](https://github.com/Kobzol/cargo-pgo/issues/49) for more context.

# Related work
- [cargo-pgo](https://github.com/vadimcn/cargo-pgo) I basically independently reimplemented this
crate. It uses an almost identical approach, but doesn't support BOLT. It's not maintained
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