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Add troubleshooting section to PGO chapter in rustc book. #65402

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Oct 16, 2019
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Add troubleshooting section to PGO chapter in rustc book. #65402

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11 changes: 11 additions & 0 deletions src/doc/rustc/src/profile-guided-optimization.md
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Expand Up @@ -125,6 +125,17 @@ RUSTFLAGS="-Cprofile-use=/tmp/pgo-data/merged.profdata" \
cargo build --release --target=x86_64-unknown-linux-gnu
```

### Troubleshooting

- It is recommended to pass `-Cllvm-args=-pgo-warn-missing-function` during the
`-Cprofile-use` phase. LLVM by default does not warn if it cannot find
profiling data for a given function. Enabling this warning will make it
easier to spot errors in your setup.

- There is a [known issue](https://github.com/rust-lang/cargo/issues/7416) in
Cargo prior to version 1.39 that will prevent PGO from working correctly. Be
sure to use Cargo 1.39 or newer when doing PGO.

## Further Reading

`rustc`'s PGO support relies entirely on LLVM's implementation of the feature
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