From f81b1548ddf886449d8ea95f82b56274b60fbac7 Mon Sep 17 00:00:00 2001 From: Michael Woerister Date: Mon, 14 Oct 2019 10:34:21 +0200 Subject: [PATCH] Add troubleshooting section to PGO chapter in rustc book. --- src/doc/rustc/src/profile-guided-optimization.md | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/src/doc/rustc/src/profile-guided-optimization.md b/src/doc/rustc/src/profile-guided-optimization.md index 38be07a6440da..d066f4a9cf59c 100644 --- a/src/doc/rustc/src/profile-guided-optimization.md +++ b/src/doc/rustc/src/profile-guided-optimization.md @@ -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