libexpr-tests: add nix-expr-benchmarks

Provides focused microbenchmarks for expression evaluation hot paths (dynamic attrs, getDerivations attr scanning, and repeated builtins.match).

src/libexpr-tests/bench-main.cc

@@ -0,0 +1,14 @@
+#include <benchmark/benchmark.h>
+
+#include "nix/expr/eval-gc.hh"
+#include "nix/store/globals.hh"
+
+int main(int argc, char ** argv)
+{
+    nix::initLibStore(false);
+    nix::initGC();
+
+    ::benchmark::Initialize(&argc, argv);
+    ::benchmark::RunSpecifiedBenchmarks();
+    return 0;
+}

src/libexpr-tests/dynamic-attrs-bench.cc

@@ -0,0 +1,55 @@
+#include <benchmark/benchmark.h>
+
+#include "nix/expr/eval.hh"
+#include "nix/expr/eval-settings.hh"
+#include "nix/fetchers/fetch-settings.hh"
+#include "nix/store/store-open.hh"
+
+using namespace nix;
+
+static std::string mkDynamicAttrsExpr(size_t attrCount)
+{
+    std::string res;
+    res.reserve(attrCount * 24);
+    res += "{ ";
+    for (size_t i = 0; i < attrCount; ++i) {
+        res += "${\"a";
+        res += std::to_string(i);
+        res += "\"} = ";
+        res += std::to_string(i);
+        res += "; ";
+    }
+    res += "}";
+    return res;
+}
+
+static void BM_EvalDynamicAttrs(benchmark::State & state)
+{
+    const auto attrCount = static_cast<size_t>(state.range(0));
+    const auto exprStr = mkDynamicAttrsExpr(attrCount);
+
+    for (auto _ : state) {
+        state.PauseTiming();
+
+        auto store = openStore("dummy://");
+        fetchers::Settings fetchSettings{};
+        bool readOnlyMode = true;
+        EvalSettings evalSettings{readOnlyMode};
+        evalSettings.nixPath = {};
+
+        EvalState st({}, store, fetchSettings, evalSettings, nullptr);
+        Expr * expr = st.parseExprFromString(exprStr, st.rootPath(CanonPath::root));
+
+        Value v;
+
+        state.ResumeTiming();
+
+        st.eval(expr, v);
+        st.forceValue(v, noPos);
+        benchmark::DoNotOptimize(v);
+    }
+
+    state.SetItemsProcessed(state.iterations() * attrCount);
+}
+
+BENCHMARK(BM_EvalDynamicAttrs)->Arg(100)->Arg(500)->Arg(2'000);

src/libexpr-tests/get-drvs-bench.cc

@@ -0,0 +1,64 @@
+#include <benchmark/benchmark.h>
+
+#include "nix/expr/get-drvs.hh"
+#include "nix/expr/eval-settings.hh"
+#include "nix/fetchers/fetch-settings.hh"
+#include "nix/store/store-open.hh"
+#include "nix/util/fmt.hh"
+
+using namespace nix;
+
+namespace {
+
+struct GetDerivationsEnv
+{
+    ref<Store> store = openStore("dummy://");
+    fetchers::Settings fetchSettings{};
+    bool readOnlyMode = true;
+    EvalSettings evalSettings{readOnlyMode};
+    EvalState state;
+
+    Bindings * autoArgs = nullptr;
+    Value attrsValue;
+
+    explicit GetDerivationsEnv(size_t attrCount)
+        : evalSettings([&]() {
+            EvalSettings settings{readOnlyMode};
+            settings.nixPath = {};
+            return settings;
+        }())
+        , state({}, store, fetchSettings, evalSettings, nullptr)
+    {
+        autoArgs = state.buildBindings(0).finish();
+
+        auto attrs = state.buildBindings(attrCount);
+
+        for (size_t i = 0; i < attrCount; ++i) {
+            auto name = fmt("pkg%|1$06d|", i);
+            auto sym = state.symbols.create(name);
+            auto & v = attrs.alloc(sym);
+            v.mkInt(i);
+        }
+
+        attrsValue.mkAttrs(attrs.finish());
+    }
+};
+
+} // namespace
+
+static void BM_GetDerivationsAttrScan(benchmark::State & state)
+{
+    const auto attrCount = static_cast<size_t>(state.range(0));
+    GetDerivationsEnv env(attrCount);
+
+    for (auto _ : state) {
+        PackageInfos drvs;
+        getDerivations(
+            env.state, env.attrsValue, /*pathPrefix=*/"", *env.autoArgs, drvs, /*ignoreAssertionFailures=*/true);
+        benchmark::DoNotOptimize(drvs.size());
+    }
+
+    state.SetItemsProcessed(state.iterations() * attrCount);
+}
+
+BENCHMARK(BM_GetDerivationsAttrScan)->Arg(1'000)->Arg(5'000)->Arg(10'000);

src/libexpr-tests/meson.build

@@ -87,3 +87,33 @@ test(
   },
   protocol : 'gtest',
 )
+
+# Build benchmarks if enabled
+if get_option('benchmarks')
+  gbenchmark = dependency('benchmark', required : true)
+
+  benchmark_sources = files(
+    'bench-main.cc',
+    'dynamic-attrs-bench.cc',
+    'get-drvs-bench.cc',
+    'regex-cache-bench.cc',
+  )
+
+  benchmark_exe = executable(
+    'nix-expr-benchmarks',
+    benchmark_sources,
+    config_priv_h,
+    dependencies : deps_private_subproject + deps_private + deps_other + [
+      gbenchmark,
+    ],
+    include_directories : include_dirs,
+    link_args : linker_export_flags,
+    install : true,
+    cpp_pch : do_pch ? [ 'pch/precompiled-headers.hh' ] : [],
+  )
+
+  benchmark(
+    'nix-expr-benchmarks',
+    benchmark_exe,
+  )
+endif

src/libexpr-tests/meson.options

@@ -0,0 +1,9 @@
+# vim: filetype=meson
+
+option(
+  'benchmarks',
+  type : 'boolean',
+  value : false,
+  description : 'Build benchmarks (requires gbenchmark)',
+  yield : true,
+)

src/libexpr-tests/package.nix

@@ -33,7 +33,7 @@ mkMesonExecutable (finalAttrs: {
     ../../.version
     ./.version
     ./meson.build
-    # ./meson.options
+    ./meson.options
     (fileset.fileFilter (file: file.hasExt "cc") ./.)
     (fileset.fileFilter (file: file.hasExt "hh") ./.)
   ];

src/libexpr-tests/regex-cache-bench.cc

@@ -0,0 +1,45 @@
+#include <benchmark/benchmark.h>
+
+#include "nix/expr/eval.hh"
+#include "nix/expr/eval-settings.hh"
+#include "nix/fetchers/fetch-settings.hh"
+#include "nix/store/store-open.hh"
+
+using namespace nix;
+
+static void BM_EvalManyBuiltinsMatchSameRegex(benchmark::State & state)
+{
+    static constexpr int iterations = 5'000;
+
+    static constexpr std::string_view exprStr =
+        "builtins.foldl' "
+        "(acc: _: acc + builtins.length (builtins.match \"a\" \"a\")) "
+        "0 "
+        "(builtins.genList (x: x) "
+        "5000)";
+
+    for (auto _ : state) {
+        state.PauseTiming();
+
+        auto store = openStore("dummy://");
+        fetchers::Settings fetchSettings{};
+        bool readOnlyMode = true;
+        EvalSettings evalSettings{readOnlyMode};
+        evalSettings.nixPath = {};
+
+        EvalState st({}, store, fetchSettings, evalSettings, nullptr);
+        Expr * expr = st.parseExprFromString(std::string(exprStr), st.rootPath(CanonPath::root));
+
+        Value v;
+
+        state.ResumeTiming();
+
+        st.eval(expr, v);
+        st.forceValue(v, noPos);
+        benchmark::DoNotOptimize(v);
+    }
+
+    state.SetItemsProcessed(state.iterations() * iterations);
+}
+
+BENCHMARK(BM_EvalManyBuiltinsMatchSameRegex);