Add caching for downloading compiler-rt

.github/workflows/main.yml

@@ -4,6 +4,8 @@ on: [push, pull_request]
 env:
   RUSTDOCFLAGS: -Dwarnings
   RUSTFLAGS: -Dwarnings
+  RUST_LLVM_VERSION: 18.0-2024-02-13
+  RUST_COMPILER_RT_ROOT: ./compiler-rt
 
 jobs:
   test:
@@ -115,12 +117,18 @@ jobs:
         path: /tmp/.buildx-cache
         key: ${{ matrix.target }}-buildx-${{ github.sha }}
         restore-keys: ${{ matrix.target }}-buildx-
-
+
+    - name: Cache compiler-rt
+      id: cache-compiler-rt
+      uses: actions/cache@v4
+      with:
+        path: compiler-rt
+        key: ${{ runner.os }}-compiler-rt-${{ env.RUST_LLVM_VERSION }}
     - name: Download compiler-rt reference sources
+      if: steps.cache-compiler-rt.outputs.cache-hit != 'true'
       run: |
-        curl -L -o code.tar.gz https://github.com/rust-lang/llvm-project/archive/rustc/18.0-2024-02-13.tar.gz
-        tar xzf code.tar.gz --strip-components 1 llvm-project-rustc-18.0-2024-02-13/compiler-rt
-        echo RUST_COMPILER_RT_ROOT=./compiler-rt >> $GITHUB_ENV
+        curl -L -o code.tar.gz "https://github.com/rust-lang/llvm-project/archive/rustc/${RUST_LLVM_VERSION}.tar.gz"
+        tar xzf code.tar.gz --strip-components 1 llvm-project-rustc-${RUST_LLVM_VERSION}/compiler-rt
       shell: bash
 
     # Non-linux tests just use our raw script

ci/download-rt.sh

@@ -0,0 +1,32 @@
+#!/bin/bash
+
+# Download LLVM's compiler-rt only if it does not exist or is out of date
+
+set -eauxo pipefail
+
+url="https://github.com/rust-lang/llvm-project/archive/rustc/18.0-2024-02-13.tar.gz"
+rt_path="llvm-project-rustc-18.0-2024-02-13/compiler-rt"
+expected_sha=""
+
+download () {
+  curl -L -o code.tar.gz "$url"
+  tar xzf code.tar.gz --strip-components 1 "$rt_path"
+}
+
+existing_hash=$(
+  find compiler-rt -type f -print0 | 
+    sort -z |
+    xargs -0 shasum -a 256 |
+    shasum -U -a 256 |
+    cut -d' '' -f1 |
+    tr -d '\n'
+)
+
+if [ "$existing_hash" = "$expected_sha" ]; then
+  echo "Got expected sha '$expected_sha'; nothing to do"
+  exit
+fi
+
+echo "Got $existing_hash; expected $expected_sha. Downloading"
+
+download()