Rollup merge of #65402 - michaelwoerister:pgo-troubleshooting-docs, r=alexcrichton
Add troubleshooting section to PGO chapter in rustc book.
- Adds a note about using `-pgo-warn-missing-function` in order to spot mistakes in PGO setup.
- Mentions cargo symbol name issue fixed in 1.39.
Nominating for backport.
r? @alexcrichton
diff --git a/src/doc/rustc/src/profile-guided-optimization.md b/src/doc/rustc/src/profile-guided-optimization.md
index 38be07a..d066f4a 100644
--- a/src/doc/rustc/src/profile-guided-optimization.md
+++ b/src/doc/rustc/src/profile-guided-optimization.md
@@ -125,6 +125,17 @@
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
