Profile Guided Optimization
rustc supports doing profile-guided optimization (PGO).
This chapter describes what PGO is, what it is good for, and how it can be used.
The basic concept of PGO is to collect data about the typical execution of
a program (e.g. which branches it is likely to take) and then use this data
to inform optimizations such as inlining, machine-code layout,
register allocation, etc.
There are different ways of collecting data about a program’s execution.
One is to run the program inside a profiler (such as
perf) and another
is to create an instrumented binary, that is, a binary that has data
collection built into it, and run that.
The latter usually provides more accurate data and it is also what is
Generating a PGO-optimized program involves following a workflow with four steps:
- Compile the program with instrumentation enabled
rustc -Cprofile-generate=/tmp/pgo-data main.rs)
- Run the instrumented program (e.g.
./main) which generates a
- Convert the
.profrawfile into a
- Compile the program again, this time making use of the profiling data
rustc -Cprofile-use=merged.profdata main.rs)
An instrumented program will create one or more
.profraw files, one for each
instrumented binary. E.g. an instrumented executable that loads two instrumented
dynamic libraries at runtime will generate three
.profraw files. Running an
instrumented binary multiple times, on the other hand, will re-use the
.profraw files, updating them in place.
.profraw files have to be post-processed before they can be fed back
into the compiler. This is done by the
llvm-profdata tool. This tool
is most easily installed via
rustup component add llvm-tools-preview
Note that installing the
llvm-tools-preview component won’t add
llvm-profdata to the
PATH. Rather, the tool can be found in:
llvm-profdata coming with a recent LLVM or Clang
version usually works too.
llvm-profdata tool merges multiple
.profraw files into a single
.profdata file that can then be fed back into the compiler via
# STEP 1: Compile the binary with instrumentation rustc -Cprofile-generate=/tmp/pgo-data -O ./main.rs # STEP 2: Run the binary a few times, maybe with common sets of args. # Each run will create or update `.profraw` files in /tmp/pgo-data ./main mydata1.csv ./main mydata2.csv ./main mydata3.csv # STEP 3: Merge and post-process all the `.profraw` files in /tmp/pgo-data llvm-profdata merge -o ./merged.profdata /tmp/pgo-data # STEP 4: Use the merged `.profdata` file during optimization. All `rustc` # flags have to be the same. rustc -Cprofile-use=./merged.profdata -O ./main.rs
Using this feature with Cargo works very similar to using it with
directly. Again, we generate an instrumented binary, run it to produce data,
merge the data, and feed it back into the compiler. Some things of note:
We use the
RUSTFLAGSenvironment variable in order to pass the PGO compiler
flags to the compilation of all crates in the program.
We pass the
--targetflag to Cargo, which prevents the
arguments to be passed to Cargo build scripts. We don’t want the build
scripts to generate a bunch of
--releaseto Cargo because that’s where PGO makes the most sense.
In theory, PGO can also be done on debug builds but there is little reason
to do so.
It is recommended to use absolute paths for the argument of
-Cprofile-use. Cargo can invoke
varying working directories, meaning that
rustcwill not be able to find
.profdatafile. With absolute paths this is not an issue.
It is good practice to make sure that there is no left-over profiling data
from previous compilation sessions. Just deleting the directory is a simple
way of doing so (see
This is what the entire workflow looks like:
# STEP 0: Make sure there is no left-over profiling data from previous runs rm -rf /tmp/pgo-data # STEP 1: Build the instrumented binaries RUSTFLAGS="-Cprofile-generate=/tmp/pgo-data" \ cargo build --release --target=x86_64-unknown-linux-gnu # STEP 2: Run the instrumented binaries with some typical data ./target/x86_64-unknown-linux-gnu/release/myprogram mydata1.csv ./target/x86_64-unknown-linux-gnu/release/myprogram mydata2.csv ./target/x86_64-unknown-linux-gnu/release/myprogram mydata3.csv # STEP 3: Merge the `.profraw` files into a `.profdata` file llvm-profdata merge -o /tmp/pgo-data/merged.profdata /tmp/pgo-data # STEP 4: Use the `.profdata` file for guiding optimizations RUSTFLAGS="-Cprofile-use=/tmp/pgo-data/merged.profdata" \ cargo build --release --target=x86_64-unknown-linux-gnu
It is recommended to pass
-Cprofile-usephase. 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 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.
rustc‘s PGO support relies entirely on LLVM’s implementation of the feature
and is equivalent to what Clang offers via the
-fprofile-use flags. The Profile Guided Optimization section
in Clang’s documentation is therefore an interesting read for anyone who wants
to use PGO with Rust.