Added: Tests for PGO with Cross Compilation

And various corrections in terms of how PGO works.

.github/workflows/test-build-workflow.yml

@@ -133,6 +133,37 @@ jobs:
           build-library: true
           upload-artifacts: false
 
+  # This tests gathering PGO data on a host, and building for a different target.
+  test-pgo-with-cross-compile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Test Repository
+        uses: actions/checkout@v4
+        with:
+          repository: Sewer56/prs-rs
+          ref: d08599ed5473616f57d57a0966939e1a5dbda9b4
+      - name: Test PGO with Cross-Compilation (Binary)
+        uses: Reloaded-Project/devops-rust-lightweight-binary@v1
+        with:
+          rust-project-path: tools/cli/
+          pgo-project-path: .
+          crate-name: prs-rs-cli
+          target: aarch64-unknown-linux-gnu # x64 host to aarch64 simulated guest
+          use-pgo: true
+          use-cross: true
+          pgo-benchmark-name: my_benchmark
+          upload-artifacts: false
+      - name: Test PGO with Cross-Compilation (Library)
+        uses: Reloaded-Project/devops-rust-lightweight-binary@v1
+        with:
+          crate-name: prs-rs
+          target: aarch64-unknown-linux-gnu # x64 host to to aarch64 simulated guest
+          use-pgo: true
+          use-cross: true
+          pgo-benchmark-name: my_benchmark
+          upload-artifacts: false
+          build-library: true
+
   test-library-feature-build:
     runs-on: ubuntu-latest
     steps:

README.MD

@@ -124,10 +124,17 @@ to keep the benchmark as close to the final usage of your application as possibl
 
 Then said profile is used to build the final version of your application.
 
-Setting `use-pgo` should work for any target technically speaking, as the profiling data is collected
-on the host machine. However, I personally am not sure if it's a valid use case to use PGO data
-when cross compiling code to a different CPU architecture. I haven't ran into issues trying, but
-just be careful.
+For PGO to work, the `target` platform must be the same as the host that generated the profiling
+data. Sometimes you can get away with using `cross` to achieve this, for example:
+
+```yaml
+# Host is x86-linux but this seems to work just a-ok!
+target: aarch64-unknown-linux-gnu # x64 host to to aarch64 simulated guest
+use-pgo: true
+use-cross: true
+```
+
+If the process fails, your CI will fail, so do experiment.
 
 ## Building Libraries
 

action.yml

@@ -135,7 +135,8 @@ runs:
           fi
 
           # Generate PGO profiling data
-          RUSTFLAGS="$RUSTFLAGS -Cprofile-generate=${GITHUB_WORKSPACE}/${{ inputs.rust-project-path }}/target/pgo-profiles" cargo +nightly test --bench ${{ inputs.pgo-benchmark-name }} --release --target ${{ inputs.target }} --features $PGO_FEATURES_ARGS
+          RUSTFLAGS_BACKUP="$RUSTFLAGS"
+          RUSTFLAGS="$RUSTFLAGS -Cprofile-generate=${GITHUB_WORKSPACE}/${{ inputs.rust-project-path }}/target/pgo-profiles" $BUILD_CMD +nightly test --bench ${{ inputs.pgo-benchmark-name }} --release --target ${{ inputs.target }} --features $PGO_FEATURES_ARGS
 
           # Merge the PGO profiles
           # This is tricky, llvm-profdata isn't in $PATH, so we need to find it ourselves.
@@ -156,7 +157,7 @@ runs:
           sanitized_path="${sanitized_path%/}"
 
           # Use the generated PGO profiling data to perform an optimized build
-          RUSTFLAGS="$RUSTFLAGS -C llvm-args=-pgo-warn-missing-function -C profile-use=${sanitized_path}/target/pgo-profiles/merged.profdata"
+          RUSTFLAGS="$RUSTFLAGS_BACKUP -C llvm-args=-pgo-warn-missing-function -C profile-use=${sanitized_path}/target/pgo-profiles/merged.profdata"
         fi
 
         # Build the project