Add nightly benchmark regression detection pipeline

libeigen/eigen!2349

Co-authored-by: Rasmus Munk Larsen <rmlarsen@gmail.com>

ci/scripts/build.benchmark.sh

@@ -0,0 +1,45 @@
+#!/bin/bash
+# Build Eigen benchmarks for a given ISA target.
+#
+# Expected environment variables:
+#   EIGEN_CI_BUILDDIR         - build directory (default: .bench-build)
+#   EIGEN_CI_CXX_COMPILER     - C++ compiler
+#   EIGEN_CI_C_COMPILER        - C compiler
+#   EIGEN_BENCH_ISA_FLAGS     - ISA-specific compiler flags (e.g. "-mavx2 -mfma")
+
+set -ex
+
+rootdir=$(pwd)
+builddir=${EIGEN_CI_BUILDDIR:-.bench-build}
+mkdir -p "${builddir}"
+cd "${builddir}"
+
+# Install Google Benchmark from source if not already present.
+# The common before_script already installs cmake/ninja; we only need
+# git and ca-certificates for the clone.
+if ! pkg-config --exists benchmark 2>/dev/null; then
+  apt-get update -qq
+  apt-get install -y --no-install-recommends git ca-certificates
+  git clone --depth 1 --branch v1.9.1 https://github.com/google/benchmark.git /tmp/gbench
+  cmake -G Ninja -S /tmp/gbench -B /tmp/gbench-build \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DBENCHMARK_ENABLE_TESTING=OFF \
+    -DCMAKE_INSTALL_PREFIX=/usr/local
+  cmake --build /tmp/gbench-build --target install
+  rm -rf /tmp/gbench /tmp/gbench-build
+fi
+
+# Configure benchmarks.  ISA flags are passed via CMAKE_CXX_FLAGS so they
+# apply globally to all benchmark targets.
+cmake -G Ninja \
+  -DCMAKE_CXX_COMPILER="${EIGEN_CI_CXX_COMPILER}" \
+  -DCMAKE_C_COMPILER="${EIGEN_CI_C_COMPILER}" \
+  -DCMAKE_CXX_FLAGS="${EIGEN_BENCH_ISA_FLAGS}" \
+  -DCMAKE_BUILD_TYPE=Release \
+  "${rootdir}/benchmarks"
+
+# Build all benchmark targets.  The nightly/weekly scope filtering happens
+# at run time, not build time.
+cmake --build . -- -k0 || cmake --build . -- -k0 -j1
+
+cd "${rootdir}"